Electrical Control for Machines 7th Edition Lobsiger Solutions Manual

Instructor’s Resource Guide

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I n s t r u c t o r ’s R e s o u r c e G u i d e t o A c c o m p a n y

Electrical Control for Machines 7th Edition

Diane Lobsiger Peter Giuliani Kenneth Rexford

Australia • Brazil • Mexico • Singapore • United Kingdom • United States

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Instructor Resource Guide to Accompany Electrical Control for Machines, 7E Diane Lobsiger, Peter Giuliani, Kenneth ­Rexford Vice President, GM Skills & Product Planning: Dawn Gerrain Product Team Manager: James DeVoe Senior Director Development: Marah ­Bellegarde Senior Product Development Manager: Larry Main Senior Content Developer: John Fisher Editorial Assistant: Andrew Ouimet Vice President Marketing Services: Jennifer Baker

© 2016, 2004 Cengage Learning WCN: 01-100-101 ALL RIGHTS RESERVED. No part of this work covered by the copyright herein may be reproduced, transmitted, stored, or used in any form or by any means graphic, electronic, or mechanical, including but not limited to photocopying, recording, scanning, digitizing, taping, Web distribution, information networks, or information storage and retrieval systems, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without the prior written permission of the publisher.

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ISBN-13: 978-1-133-69374-1 Cengage Learning 20 Channel Center Street Boston, MA 02210 USA Cengage Learning is a leading provider of customized learning solutions with office locations around the globe, including Singapore, the United Kingdom, Australia, Mexico, Brazil, and Japan. Locate your local office at: international.cengage.com/region Cengage Learning products are represented in Canada by Nelson Education, Ltd. To learn more about Delmar, visit www.cengage.com/delmar Purchase any of our products at your local college store or at our preferred online store www.cengagebrain.com Notice to the Reader Publisher does not warrant or guarantee any of the products described herein or perform any independent analysis in connection with any of the product information contained herein. Publisher does not assume, and expressly disclaims, any obligation to obtain and include information other than that provided to it by the manufacturer. The reader is expressly warned to consider and adopt all safety precautions that might be indicated by the activities described herein and to avoid all potential hazards. By following the instructions contained herein, the reader willingly assumes all risks in connection with such instructions. The publisher makes no representations or warranties of any kind, including but not limited to, the warranties of fitness for particular purpose or merchantability, nor are any such representations implied with respect to the material set forth herein, and the publisher takes no responsibility with respect to such material. The publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or part, from the readers’ use of, or reliance upon, this material.

Printed in the United States of America Print Number: 01 Print Year: 2014

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Contents Chapter Outlines and Achievement Review Answers Chapter 1 Transformers and Power Supplies.................................................................. 1 Chapter 2 Fuses, Disconnect Switches, and Circuit Breakers........................................... 5 Chapter 3 Control Units for Switching and Communication............................................ 7 Chapter 4 Relays........................................................................................................... 11 Chapter 5 Solenoids..................................................................................................... 15 Chapter 6 Types of Control........................................................................................... 19 Chapter 7 Motion Control Devices................................................................................ 21 Chapter 8 Pressure Control........................................................................................... 25 Chapter 9 Temperature Control.................................................................................... 29 Chapter 10 Time Control................................................................................................. 33 Chapter 11 Count Control.............................................................................................. 37 Chapter 12 Control Circuits............................................................................................. 39 Chapter 13 Motors......................................................................................................... 43 Chapter 14 Motor Starters.............................................................................................. 47 Chapter 15 Introduction to Programmable Controllers................................................... 51 Chapter 16 Industrial Data Communications................................................................... 63 Chapter 17 Quality Control............................................................................................. 65 Chapter 18 Safety........................................................................................................... 69 Chapter 19 Troubleshooting........................................................................................... 73 Chapter 20 Designing Control Systems for Easy Maintenance.......................................... 77

Appendix........................................................................................................................ 81

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Preface In the foreword in the textbook, it is stated that the material requires the student’s understanding of ­basic elementary electricity. However, in some areas or under some conditions, the student may not have had a ­sufficient background. It, therefore, may be advisable for the instructor to have a brief review of elementary electricity.

For the instructor to determine if such a review is indicated, one effective method is suggested. This is a pretest given to the students on the first class day. From the results of this test, the instructor can determine how rapidly the textbook material can be started. See the Appendix in this manual for the Pre-test and Pre-test answers.

Even if the instructor has determined that the elementary electricity is not necessary for a given class, it will not be time wasted to briefly review some of the high spots of this material. Such items as the DC generator, electromagnetism, generation of AC, single and three phase, and the transformer are of particular importance.

The ideas given thus far are offered only as suggestions. They are not intended to disrupt or replace any given and proven class instruction method.

For all the chapters in the textbook, a brief summary is given of the material to be covered and suggestions are made on how to cover the material.

At the close of each class session, assignments should be made to study the next chapter. Also the student should answer the Achievement Review questions and hand them in at the next class session.

As a suggestion, all the Achievement Review questions plus those added in the Instructor’s Manual can be typed, one question each of 4" 3 5" cards. At the start of each class period, these cards with questions from the last class period material, can be passed out at random. This will give each student a different question for a 10 minute quiz. For each chapter the instructor can select from approximately 30 questions.

One of the problems that an instructor often faces is in determining if the material presented in class and studied by the student is thoroughly understood. There may be a few students who have problems but will not ask questions. There may be at least two avenues open to the instructor: 1. Observe the student in the classroom.

2. Check home assignments and daily quiz results.

In cases where a problem is evident, the instructor can: 1. Devote extra time explaining the material.

2. Provide individual counseling.

Many of the questions can be answered directly from the material in the textbook. However, there may be a few open-ended questions that call on the students to express their conclusions.

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Chapter Outlines and Achievement Review Answers Chapter 1

Transformers and Power Supplies

As transformers are used in practically every electrical control system, the instructor may find it advisable to provide a brief review of transformers. For example: An alternating current flowing through a coil generates an alternating magnetic field around the coil. If this alternating magnetic field cuts through the turns of a second coil, an AC voltage is induced in the second coil, just as voltage is induced in a coil cut by its own magnetic field. This action of generating a voltage is called transformer action. In transformer action, electrical energy is transferred from one coil, called the primary to another coil, called the secondary, by means of a varying magnetic field.

1. Control Transformers. Be sure the students understand “turns ratio” and series-parallel connections.

2. Transformer Regulation. Go through several examples of calculations for transformer regulation.

3. Temperature Rise in a Transformer. Bring out the cause of temperature rise and the problem with frequency.

4. Operating Transformers in Parallel. Note the importance of voltage and frequency when operating transformers in parallel.

5. Constant Voltage Regulators (CVR). Discuss the general design of a constant voltage regulator.

6. Uninterruptible Power Systems (UPS). Note the importance of UPS in industry where the utility power supply is often less than perfect.

ACHIEVEMENT REVIEW ANSWERS 1. Alternating current (AC) 2. a. Safety b. Use of standard designed components 3. 120 VAC is available on equipment to provide power for receptacles, components that operate using 120 VAC, and to power the 24 VDC power supply.

4. 4:1 5. 2:1 6. 90 VAC 7. a. 2:1 b. 15 A 8. 456/4 5 114 V 1

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Chapter Outlines and Achievement Review Answers

9.

10.

480 VAC Three Phase 60 Hz

11. Step-down transformer (c) 12.

1202110 10 3 100 5 3 100 5 9% 110 110

13. Total current draw 16 3 .2A 5 3.2A 10 3 .1A 5 1A 6 3 .5A 5 3A Total 5 7.2A 30% additional capacity: 7.2 3.3 5 2.16 A Total current 5 2.16 A 1 7.2 A 1 9.36 A kVA rating 5 230 V 3 9.36 A 5 2152.8 VA Select 2.5 kVA 14. Copper winding and iron core (b and c) 15. A transformer of 1 kVA or larger, designed for a 60-Hz supply, will not operate satisfactorily at 50 Hz due to higher losses and resultant heat rise.

16. The components can drop out (de-energize).

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Chapter 1

Transformers and Power Supplies

3

17.

18. Backup is provided by an internal battery that is automatically connected to the DC portion of the UPS when input power fails.

19. Electrical storms, noisy and largely varying loads, and accidents all lead to a less than perfect supply emerging from the utility power supply.

20. Electrical storms, noisy and largely varying loads, and accidents all lead to a less than perfect supply emerging from the utility power supply.

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Chapter 2

Fuses, Disconnect Switches, and Circuit Breakers

1. Protective Factors. Point out the two important factors involving disconnecting and protection.

2. Fuse Construction and Operation. Show the general construction of a one-time fuse. (See Textbook Figure 2-2.) Point out the three types of fuses: one-time, time-delay, and current-limiting. Discuss the applications where time-delay and current-limiting fuses are used.

3. Fuse Types. Show the physical changes in fuse design. Point out that they must be used with a fuse block designed to accept them. (See Textbook Figure 2-4.)

4. Let-thru Current and I2t. a. Limiting action b. I2t vs. Ip and total clearing time c. I2t and characteristic of fuse Be sure to devote sufficient time to discussing I2t as a measure of the degree of current limitation provided by fuses. This is extremely important in selecting fuses. Use the diagrams provided to emphasize the importance. (See Textbook Figures 2-7, 2-8, 2-9, 2-10.)

Be sure the student knows how to read a graph showing melting time-current data vs. time graphs for a given fuse. (See Textbook Figure 2-6.)

5. Voltage and Frequency Surges a. From lightning b. From switching Be sure the student knows how a lightning arrester operates. Also be sure the student understands the relative damage caused by a direct hit, induction on the line, and surges caused by switching.

6. Circuit Breaker Types. Point out the four different types of circuit breakers, their uses, and how they differ in operating characteristics.

7. Programmable Motor Protection. Discuss some of the protective features.

8. Electrical Metering and Voltage Protection. Discuss some of the protective features.

9. Selecting Protective Devices. Discuss the important factors to consider when selecting protective devices. Point out why they are important. Discuss the importance of impedance in a transformer when calculating the interrupting capacity of a protective device. Go through several examples using various values of transformer kVA, voltage, and impedance. Be sure the student understands that impedance is the current-limiting characteristic of a transformer.

ACHIEVEMENT REVIEW ANSWERS 1. a. Means of disconnecting electrical energy from the circuits. b. Protection against sustained overloads and short-circuit current.

2. Where a heavy load can exist for a short period of time. An example is motor starting.

5

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Chapter Outlines and Achievement Review Answers

3. a. Nonautomatic (circuit interrupter) b. Thermal c. Magnetic d. Thermal magnetic 4. To clear a circuit in case of short circuit.

5. a. Size

e. Voltage rating

b. If a time lag is required

f. Number of poles

c. Interrupting capacity

g. Mounting, operator, and if an enclosure

d. Ambient temperature

is required

6. Yes. One, three, and four poles are also available.

7.

8. The highest current at rated voltage that the fuse can interrupt.

9. Effective heat transfer (b) 10. It parallels the characteristics of conductors, motors, transformers, and other electrical apparatus (c) 11. Ip (peak let-thru current) and I2t (t is the total clearing time) are two measures of the degree of current limitations provided by a fuse.

12. I2t values of a fuse are derived from oscillograms of fuses operating within their current-limiting range and are calculated.

13. The highest overvoltage will be present when there is a ground fault in the system.

14. A lightning arrester will limit the crest of the surge by breaking down and conducting to ground.

15. Normal full load current 5 5

15000 5 31.25 amps 480 31.25 5 781 amps 0.04

he circuit breaker or fuse would have a minimum interrupting capacity of 781 amps at 480 volts. You T would use an 800-amp circuit breaker or fuse, as it is the next larger commercial size.

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Chapter 3

Control Units for Switching and Communication

1. Oil-Tight Units, Push-Button Switches, Selector Switches, Pilot or Indicating Lights. Show and explain the electrical symbols for a selector switch. Show and explain the circuit for a push-to-test pilot light. (See Textbook Figure 3-20A.) 2. Circuit Applications. This is the start of schematic circuit diagrams so the student should get a good picture of general format that will continue through most of the textbook.

3. Annunciators. Explain the need for annunciation.

4. Light-Emitting Diodes (LEDs). Explain their use in visual communication and troubleshooting.

(See Textbook Figure 3-32.)

Emphasis in this chapter should be placed on communication. Communication can be accomplished in several ways: Conditions of electrical signals (present or absent), through the opening or closing of switch contacts.

The transfer of signals through solid-state elements.

The indication of the conditions of a circuit through the use of pilot lights, LEDs, and annunciators.

Membrane switches have many uses. This not only includes industry but many uses in home appliances.

The student should have a clear picture of their functions.

Visual communication is accomplished through the use of pilot lights or indicating lights, annunciators, and LEDs. The LED (light-emitting diode) is used extensively. The basic design with the use of a current-limiting resistor should be explained.

Although only a brief reference to circuit numbers is made in Chapter 3, the instructor should impress on the class the importance of these numbers. From this point on, as each new chapter is studied, a new component will be added, so the follow-through with circuit numbers should be stressed.

Be sure they understand that the complete drawing line for the return of control power for a push-to-test pilot light is not required, only indicated by 1 A. This will be noted both at the lamp and the fuse. If many pilot lights are used, drawing all the connecting lines would only clutter the drawing.

ACHIEVEMENT REVIEW ANSWERS 1. The base of the operator, with the contact block attached, is mounted through an opening in the panel.

It is then secured in place by a threaded ring installed from the front of the panel.

2. Will withstand vibration (b) 3.

4. Yes. Such colors as red, amber, green, white, or clear are available. The different colors can be used to increase the safety factor in operating the machine.

5. Provides space for terminal block installation (a) 6.

7

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Chapter Outlines and Achievement Review Answers

7.

1SSW

×OO OO×

×OO OO×

8. When the pilot light lens is depressed (operated), it mechanically operates the contact block. The n­ ormally closed contacts open and the normally open contacts close. The normally closed contacts are connected to the control circuit. The normally open contacts are connected directly to a source of 120 volts. Thus, when the lens is depressed (operated), the pilot light transformer primary is connected directly across 120 volts. Six volts are now applied to the bulb.

9.

OFF

ON

G

O×

1SSW

10. They are used as indicators of both incoming and outgoing signals.

11. It displays the plant or process status by lighting individual windows, identifying process functions. This may include operating status or alarm of pumps, valves, hoppers, tank levels, and so on.

12.

ON START 1PB

OFF

MANUAL

STOP

G

2PB 1SSW OO×

13.

1PB

R

2PB

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Chapter 3

14.

STA 1

1SSW

15.

OFF

1SSW

Control Units for Switching and Communication

9

STA 2

×O

G

O×

A

ON

G

O×

W

16.

1SB

2SB

17.

1SB

2SB

18.

1SB

3SB

2SB

19.

1SB

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Chapter 4

Relays

At this point in the electrical control course, it is important for the instructor to start stressing elementary control circuits. The ability of the student to both read and draw elementary control circuits can be of tremendous help. Although circuit format will change with solid state and PLCs, the concept of logically following through a circuit and knowing why each step is taken is important to a student’s understanding of control.

The student should now know from the first three chapters that the source of control power comes from a three-phase power line through a single-phase, isolated, secondary control transformer for 120 VAC control.

A power supply is required to obtain 24 VDC control. Therefore, in drawing electromechanical control circuits from this point on, the student can omit the power circuit and transformer and show only the two vertically spaced lines to represent the control power source. Each circuit should be neatly drawn with proper circuit numbers in place.

Also be sure that the student knows that this type of circuit is known as a ladder diagram. Ladder diagrams will be used extensively throughout the course. In some cases, the instructor may choose to pass over these simple ladder diagrams as shown in Chapter 4. However, do not let the student become complacent about ladder diagrams. Later, as the diagrams become more complex, the student who has not mastered the simple circuits will be lost in the more complex circuits. This is generally a matter of self-discipline.

1. Control Relays and Their Uses. Be sure that the student understands the difference between in rush current and sealed current in a relay. Also pick-up and drop-out voltage.

2. Timing Relays. The contact symbols for the timing relay can be confusing. Use one of each symbol in a circuit to demonstrate its function.

3. Latching Relays. Develop a circuit showing the use of the latching relay. Explain how the contacts ­remain in their operated condition after the latch coil is de-energized.

4. Plug-In Relays. Go through the advantages of using the plug-in relay where changes are anticipated or down-time is at a premium.

5. Contactors. Using a contactor, design and show a circuit that could be used in controlling a heating load.

ACHIEVEMENT REVIEW ANSWERS 1. When the relay coil is energized, the plunger is in an out position. Due to the open gap in the magnetic path or circuit, the initial current in the coil is high. The current level at this time is known as inrush ­current. As the plunger moves into the coil, closing the gap, the current drops to a lower value. This lower value is known as holding or sealed current.

2. a. Inrush or make contact capacity b. Normal or continuous carrying capacity c. The opening or break capacity 3. Contact wipe results from relative motion of the two contact surfaces after they make contact. When contacts bounce, they open momentarily. This may cause the loss of seal-in with a memory circuit.

4.

1CR

1CR

1CR

COIL

NO CONTACT

NC CONTACT

11

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Chapter Outlines and Achievement Review Answers

5.

START 1PB

1CR

STOP 2PB

1CR-1 R

1LT

6. A timing contact as well as instantaneous contacts is available from the same energizing relay coil. The timing contact can be arranged to delay after energizing the coil or after de-energizing the coil.

7. a. Normally open; delay after energizing

c. Normally open; delay after de-energizing

b. Normally closed; delay after energizing

d. Normally closed; delay after de-energizing

8.

START 1PB

1TR

STOP 2PB

1TR-1

1TR

R

1LT

9. The latching relay is electromechanically operated and is held operated by means of a mechanical latch.

By energizing the coil of the relay (called the latch coil), the relay operates. This results in the normally open contacts closing and the normally closed contacts opening.

The electrical energy can now be removed from the coil of the relay and the contacts remain in their operated condition. It now requires that a second coil on the relay (called the unlatch coil) be energized to return the contacts to their unoperated condition.

10. Generally, the contactor is supplied in 2-, 3-, or 4-pole arrangements. Current-carrying capacity is in the range of 9 A through approximately 2250 A. 11. Relays can be changed without disturbing the circuit wiring (c).

12. 85% (b) 13.

Timer Coil

Contact A

Contact B

off on (initial) on (time delay expired)

open open closed

closed closed open

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Chapter 4

14.

Timer Coil

Contact A

Contact B

on off (initial) off (time delay expired)

closed closed open

open open closed

15.

1SB

2SB

1KA

2SB

1KA

Relays

13

1KA

1KA

16.

1SB

1KA

1KA 2KA

1KA

17.

1SA

2SB

1KT

1KA

1KA

1KA

1KA

93741_All_Chapters_ptg01_00i-096.indd 13

1KT

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14

Chapter Outlines and Achievement Review Answers

18.

1SA

2SB

1KT

1KA

1KA 1KA

1KT

2KT

1KT

2KT

2KA

2KA

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Chapter 5

Solenoids

1. Solenoid Action. Be sure the student knows the difference between inrush and holding current. Cover pull of solenoids and duty cycle of the work load.

2. Circuit Applications. Go through a circuit with the students, using solenoids and fluid power diagrams.

3. Proportional Valves. Using the diagram shown in Textbook Figure 5-14, explain how current varies to determine solenoid force with spool movement in one direction only.

4. Servo Valves. Using the diagram in Textbook Figure 5-16, explain to students how they are used for high-performance, closed-loop control. Explain how the force motor operates, linear force/current relationship, and reversing direction of current.

Additional and more detailed information is supplied to the instructor in explaining the construction and operation of solenoids. As solenoids are used extensively in industry, it is important that the student has a thorough understanding of how and why the plunger moves within the solenoid coil. With present advanced technology and the increasing use of DC solenoids, it is important to know the difference between AC and DC solenoids. The following information is supplied through the courtesy of Vickers, Inc. Solenoid Operation. In an electrically operated valve, the electrical energy fed into the valve is converted into mechanical energy in order to move the valve spool. The component that carries out this conversion is known as a solenoid and can be operated by either an AC or DC supply. Two types of solenoid construction have been used in recent years. The older air-gap type, however, is now being replaced by the wet-armature arrangement.

Air-gap Solenoids. An air-gap solenoid consists of a coil wound onto a former and surrounded by a laminated iron frame. The purpose of the frame is to concentrate and hence strengthen the magnetic field produced by the coil, the laminations reducing losses caused by eddy currents.

The solenoid is fitted with a plunger or armature in the center of the coil, which connects to the end of the spool by means of a push pin. When the solenoid is de-energized, the plunger is pushed out of the coil slightly (by the spool springs), creating an air gap between the plunger and frame. Energizing the coil creates a magnetic field that is concentrated in the frame and plunger. Since iron is a considerably better magnetic conductor than air, the magnetic field pulls the armature into the coil, thereby closing the air gap and completing the magnetic circuit. As the plunger is pulled into the coil, the push pin moves the spool across the valve body to operate the valve.

The major drawback with the air-gap solenoid is that an O-ring seal is required on the push pin to seal the tank chamber of the valve from the solenoid. Being a dynamic seal, the O-ring is prone to wear or damage, which will then allow hydraulic fluid to leak from the valve.

Wet-Armature Solenoids. To overcome the problem of push pin seal leakage, later types of solenoids have the armature and push pin immersed in fluid and enclosed inside a core tube.

As with the air-gap solenoid, when the coil is energized, the armature is pulled into the coil and makes contact with the pole piece to complete the magnetic circuit. Since the armature and push pin are immersed in fluid, there are now no dynamic seals to cause leakage.

The core tube is actually manufactured from two different types of material such that the magnetic field passes freely through the core tube at the ends but is concentrated in the armature and pole piece in the center.

DC Solenoids. A solenoid coil will possess both resistance and inductance. The effect of inductance in a DC circuit is to delay the rise of current when the supply is switched on. Energizing a DC solenoid, therefore, produces a gradual rise in solenoid current up to its maximum value.

The maximum current is determined purely from the applied voltage and coil resistance (Imax 5 V/R), which means that even if the valve spool has stuck and the armature does not pull in, the solenoid current will not exceed its normal maximum value.

15

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16

Chapter Outlines and Achievement Review Answers

AC Solenoids. Applying an AC voltage to a solenoid produces a varying current and magnetic field and hence a varying solenoid force. When the solenoid current is at a maximum (in either direction), the solenoid force will also be at a maximum. As the current passes through zero, the solenoid force will drop, which tends to cause a chattering effect as the spool springs attempt to push the armature back. To reduce the chatter, a copper ring, known as a shading ring is incorporated in the face of the pole piece.

The magnetic force produced by the coil passes through the shading ring and induces a secondary current in the ring. The shading ring is in effect a single turn coil of very low resistance so the induced current will be very high and almost exactly 90 degrees out of phase with the solenoid coil current. This secondary current will therefore tend to sustain the magnetic field as the current in the main coil passes through zero and thereby reduces solenoid chatter.

The value of the current in an AC coil is determined by the applied voltage and the coil impedance (I 5 V/ Z). The coil impedance is predominantly made up of two components, resistance and inductive reactance.

While the resistance of the coil may change a certain amount as the coil heats up, the inductance will vary quite considerably depending upon whether the solenoid is in the energized or de-energized position. In the deenergized position, the inductance and hence impedance, will be low since the magnetic circuit is incomplete, whereas in the energized position, inductance and impedance will be high since a complete magnetic circuit is now established.

The implication of this is that as an AC solenoid is energized, the low impedance initially causes a high current that subsequently drops as the armature pulls in and the impedance increases.

The initial peak current is known as the inrush and the subsequent lower value, the holding current.

In practice, therefore, this is the effect that may cause a solenoid coil to burn out, since if the valve spool sticks and the armature does not pull in, the solenoid current remains at high inrush value, creating excessive heat and coil burnout.

Under normal circumstances, the extra heating effect created by the inrush current will only be significant on rapidly cycled solenoids. The inrush current could be reduced by, for example, shortening the stroke of the solenoid or using lighter spool springs, but both of these modifications may be detrimental to the hydraulic performance of the valve.

If the solenoid supply voltage is higher than the rated voltage of the coil, this will create a higher inrush and holding current and hence more heat. Also if the supply voltage is lower than rated, this will reduce the solenoid force such that the armature may not fully close, which again results in higher holding current and more heat. For satisfactory performance, therefore, the voltage supply should be maintained between ±10% of the rated voltage.

Comparison of AC and DC Solenoids. Although AC and DC solenoids perform the same function, they have different characteristics that may affect the choice of voltage supply. For example: Response Times. Generally speaking, AC solenoids are faster acting than DC solenoids. For example, on energizing, the time for an AC solenoid (center to offset position) would be 25 ms as compared to 50 ms for the DC equivalent. De-energizing times are not so easily determined since they depend upon the type of switching used with a DC solenoid and the point on the AC curve where switching occurs for an AC solenoid. In certain applications, however, a slow response time is desirable to reduce shocks in the hydraulic system. This can be achieved hydraulically by adding an orifice plug. This feature would only be practical on DC solenoid valves since the longer solenoid pull-in time achieved would mean that an AC coil would remain on inrush current for a longer period of time and hence overheat.

Reliability. AC solenoids are more prone to burn out than DC versions owing to the fact that a malfunction of the valve (spool sticking) or an incorrect voltage supply may cause the solenoid to remain on inrush current and overheat. However, the very slight chattering effect that will remain on an AC solenoid means that the spool is constantly vibrated very slightly and thus makes the valve spool less prone to stick.

Practical Considerations. Obviously in certain cases, such as mobile applications, the only supply available may be DC, in which case the choice is already made. Also, in industrial applications, the increasing use of proportional valves, which use a DC solenoid, means that DC-operated conventional valves will probably become more popular.

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Chapter 5

Solenoids

17

ACHIEVEMENT REVIEW ANSWERS 1.

2. If the plunger does not complete its stroke, the current in the coil will be high, resulting in ­damage to the coil.

3. Inrush current appears when the plunger is in the out position.

4. If the pull is a little less than the load, the solenoid action will be sluggish and may not complete its stroke.

5. The operation of a solenoid above its maximum cycling rate will result in excessive heating and mechanical damage.

6. The spring force returns the valve spool to its original de-energized position.

7.

START 1PB

STOP

1CR

2PB

1CR-1

1CR-2

SOL 1PA

8. Sealed or holding current. (c) 9. is depressed (loaded). (b) 10. A magnetic field that is produced about the coil. (c) 11. Proportional solenoids operate in much the same way as ON/OFF DC solenoids. The main difference is that the solenoid current is now varied to determine the solenoid force.

12. The spool can be moved to a greater or lesser amount in the valve body by varying the solenoid current.

13. Reversing the direction of current through the coil will reverse the armature polarity and hence the force direction.

14. Force motors give a more linear force/current relationship than proportional solenoids.

15. Solenoid temperature will increase rapidly.

16. The inrush and holding currents are the same.

17. Shading coils keep the solenoid from buzzing when the sine wave goes through zero.

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18

Chapter Outlines and Achievement Review Answers

18. The temperature rise from one condition to another can be computed if the resistance at these two ­conditions is known. These two conditions are usually termed hot and cold resistance.

19.

SOL 1PA P

T

20. Both the spring return valve and a detented valve contain a valve spool that shifts when the solenoid coil is energized. The difference is that the spring return valve will return to its unoperated (or normal) position when the solenoid is de-energized. A detented valve will remain shifted when the solenoid is de-energized. An additional solenoid is required to be energized in order to return the detented valve to its unoperated (or normal) position.

21.

ADV SLIDE 1

1CR

2CR-2

ADVANCE SLIDE 1 ADV SLIDE 2

2CR

1CR-2

ADVANCE SLIDE 2 1CR-1

SOL 1PA

2CR-1

SOL 2PA

22.

SLIDE 1

SOL 1PA

SOL 2PA P

23.

1SB

SLIDE 2

2SB

T

P

T

1KA

1KA

1KA

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1YV

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Chapter 6

Types of Control

This chapter may prove to be more difficult for the student to understand. As a result, the instructor may choose to devote more time to this material. Items to cover carefully are: 1. Open Loop Control a. Accuracy of results and causes 2. Closed Loop Control a. Transducers b. Input signal c. Feedback signal d. Gain e. Proportional control f. Error signal g. Effect of increased gain h. The derivative term i. Error rate j. Vertical load situation k. Steady state error 1. Integral term m. Stability in fast-acting systems n. Combination of derivative and integral terms o. PID

ACHIEVEMENT REVIEW ANSWERS 1. To close the loop in closed-loop control system, a position transducer is connected to the load and provides a feedback signal proportional to the load position.

2. Starting with the statement, control signal (input signal 2 feedback signal) 3 Gain, the control signal can be thought of as the error between input and output. All the time the error exists, the servo valve operates to move the load. When the load reaches the desired position, the error signal becomes zero. The servo valve centers and the load stops.

3. When the amplifier gain is increased, the amplifier produces a larger control signal. Although the output arrives at the desired position faster, it now overshoots because the load is traveling at such a speed that it cannot be stopped quickly enough when it reaches the required position.

4. To compensate for leakage across the valve spool, the servo valve spool must be displaced slightly away from center to prevent movement of the load. To offset the valve spool, a small control signal is required.

In order for there to be a control signal, an error must exist between input and output. That is, the load can never be exactly in the required position. This is known as the Steady State Error.

19

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20

Chapter Outlines and Achievement Review Answers

5. The error 3 time component is known as the integral term and is effectively equal to the error multiplied by the length of time the error has existed. That is, the longer the time, the larger the term becomes.

6. Three-term control consists of Proportional, Integral, and Derivative action. This is generally referred to as PID. 7. 1

CYCLE START

2 3

2LS

4

REVERSE

1LS

5

6

1CR ADVANCE PISTON

1CR-1

1CR-2

SOL 1HA

7

2LS

1LS

SOL 1HA P

T

8. If the feedback loop was disconnected, the error would be large. In this case, the error would be equal to the desired output command signal.

9. A flow meter could be installed upstream of the pump to ensure that there is an adequate flow of water to the pump.

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Chapter 7

Motion Control Devices

As this chapter contains more material than the preceding chapters, the instructor may want to scan through a brief summary of the topics covered. It will then be easier to allot time for each section.

1. Importance of Position Indication and Control a. Accuracy b. Troubleshooting 2. Limit Switches a. Mechanical (1) Precision (2) Linear motion (3) Rotary motion (4) Operators (5) Symbols b. Circuit applications 3. Proximity a. Hall effect b. Inductive c. Capacitive d. Vane switches 4. Linear Position Displacement Transducers a. Linear variable differential transducers b. Measuring principle c. Angular position displacement transducers d. Rotary potentiometers 5. Rotary Encoders a. Incremental b. Absolute c. Optical sensing 6. AC Synchronous and DC Stepping Motors a. Examples and problems 7. Photoelectric Transducers a. Through-beam sensors b. Retroreflective sensors c. Diffuse reflection sensors d. Fiber-optic 8. Flow Monitors a. Principles of thermal conductivity 21

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Chapter Outlines and Achievement Review Answers

ACHIEVEMENT REVIEW ANSWERS 1. Mechanical, proximity, vane 2. It is a control circuit device used with machinery having a repetitive cycle of operation where motion can be correlated to shaft rotation.

3. a. Current rating of the contacts b. Slow or snap action c. Isolated or common connections d. Spring return or maintained contacts e. Number of normally open and normally closed contacts f. The operator: length of travel, speed, force available, accuracy, and type of mounting 4. a. Operating force is the amount of force applied to the switch to cause the “snap over” of the contacts. b. Release force is the amount of force still applied to the switch at the instant of snap back of the contacts to the unoperated condition.

5.

6. There is no mechanical contact with the operator, and it can be used in nearly all environments. The vane switch offers excellent accuracy and response time. Repeatability is constant within ±0.0025 inch.

Response time is less than a millisecond.

7.

8. Capacitive fields. (a) Light rays. (b) 9. Dependent upon the oscillator frequency. (c) 10.

START 1PB

1CR

STOP 2PB

1 LS

1CR-1

1CR-2

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SOL 1PA

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Chapter 7

Motion Control Devices 23

11. Fiber optics are used for sensing objects in positions where, owing to limited space or high temperatures, it is not possible to mount a transducer.

12. Thermistors are used to convert temperature differences into electrical signals, which are further p­rocessed in the control monitor.

13. Thermal conductivity. (c) 14. The pulse modulated photoelectric sensor operates with two units: the emitter and the receiver. Each unit is mounted some distance from each other, but with the lens of the emitter in direct line with the lens of the receiver. The emitter’s role is to turn on and off an LED at a high rate of speed. The light of the LED is focused by a lens (like a flashlight) to the lens of the receiver. The receiver’s role is to detect a change in the pulsed light of the LED coming into its lens. If the light changes, the receiver will change a set of electrical contacts or switch on an external load.

If the emitter is moved further away from the receiver, the intensity of the light will decrease. A decrease in light intensity will cause the receiver to increase its sensitivity to changes in the received light. This could cause a false triggering of the receiver’s load circuit.

15. Ultrasonic sensors emit a high frequency sound wave while a photoelectric sensor emits a flash of light.

Sound waves are best for environments where light could be interrupted or distorted, such as dusty conditions or reflective surfaces. Light is best in environments that are noisy with high frequency sounds and strong electromagnetic fields.

16. The role of the position decoder is to sense the change in movement of the structure to which the decoder is attached. The position decoder must be calibrated to a known starting point. After the structure moves, the decoder will provide a signal (analog or digital) equal to the amount of change in the movement. For multiple axis devices (i.e., robots) each axis member requires its own position decoder.

The role of the controller is to interpret the signals received by the decoders, compare the value to a programmed position, and provide an output voltage equal to the difference. The important element of each articulated machine is the end effecter (i.e., welder, picker arm, machining tool). Therefore, to control the simultaneous movement of all axes, to accurately position the end effecter, the controller’s program will translate the desired movement of the end effecter into independent changes to each axis.

The role of the program is to allow the operator to enter the desired path of the end effecter. The program will calculate the amount of movement of each axis required to move the end effecter.

17. The motion profile describes the change in control element (electrical or fluid) to control the speed and acceleration of the resulting actuator (i.e., cylinder rod). This profile can be equated to our automotive driving pattern during a trip. We accelerate from a stopped position. We dwell while waiting for a stop light. We maintain constant speeds through streets. We decelerate when we approach a stoplight or our destination. When traveling away from our starting point, we move in a positive direction, and as we travel back to out starting point, we move in a negative direction.

Therefore, in Figure 7-38(a) the cylinder rod would start at a beginning point, such as completely retracted. From this point the rod would accelerate at some rate A until it reaches a point where the rod would maintain a constant speed A. After some distance of travel, the rod will decelerate (at the rate it accelerated) to a stop. The rod is held at this position for some dwell time. After the dwell time is over, the rod would again accelerate, move at a constant speed, and decelerate to another point. The rod is held at this position for some dwell time. After the dwell time is over, the rod would extend again, accelerating, moving, and decelerating to its final extended position.

While retracting, the rod will accelerate until it reaches a constant speed. At some point, it will begin to reduce speed until it stops retracting at its original starting point.

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Chapter Outlines and Achievement Review Answers

18. The motion profile describes the change in the control element (electrical or fluid) to control the speed and acceleration of the resulting actuator (cylinder rod). Therefore, in Figure 7-38(b) the cylinder rod would start at a beginning point such as completely retracted. From this point the rod would accelerate at some rate A until it reaches a speed of A. It keeps this speed until some distance of travel the rod will accelerate to a higher speed B. Again, after some distance the rod will begin to decrease its speed (­decelerate A) until it reaches a point where its speed (D) is constant. Finally, at another point, the rod will reduce its speed (decelerate B) until it comes to a stop at its maximum extended point.

While retracting, the rod will accelerate at a rate of D until it reaches a constant speed D. At some point, it will begin to reduce speed (decelerate C) until it stops retracting at its original starting point.

19.

2SB

1SB

1SQ

1KA

1KA 1YV

1KA

20.

2SQ

1SB

2SB

1SQ

1KA

1KA 1YV

1KA

21.

1SB

1SB

2SB

1SQ

1KA

1KA

1KA

1KA

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1YV

2YV

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Chapter 8

Pressure Control

1. Importance of Pressure Indication and Control. The instructor can use the pressure chart shown in Figure 8-2 in the textbook. Devote some time in explaining the difference between “actuation and ­deactuation points.” Three more important items are differential, accuracy, and tolerance. The student should understand the importance of communicating physical action on a machine and electrical signals.

2. Types of Pressure Switches. Explain the difference between the various types of pressure switches. It is important that the student knows where each type can be used and their limitations. Be sure that the student understands the Wheatstone Bridge and its application in the Pressure Transducer.

3. Circuit Applications. The instructor should review several circuits using the pressure switch. Additional circuits can be developed using both the NO and NC contacts. Note that control circuits continue to build on the original material that included oil-tight units, relays, solenoids, and limit switches.

ACHIEVEMENT REVIEW ANSWERS 1.

2. Differential is the range between the actuation point and the deactuation point.

3. Sealed piston; Bourdon tube; diaphragm; solid state 4. An O-ring is used to prevent oil leakage past the piston.

5. The Bourdon-tube pressure switch is extremely sensitive; it senses peak pressures. Consequently, it may be necessary to use some form of dampening or snubbing on the pressure entering the tube.

6. The range is from vacuum to 150 psi.

7.

START 1PB

STOP

IPS

1CR

2PB 1CR-1

1CR-2

SOL 1PA

8. Semiconductor strain gauges. (c) 9. Switching is accomplished with solid-state triacs.

10. Four different switching configurations are: single pole, single throw (NC); single pole, single throw (NO); single pole, double throw; double make, double break.

25

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26

Chapter Outlines and Achievement Review Answers

11. Applying a constant voltage to the bridge. (a) 12.

START

STOP

2LS

1PB

IPS

3CR-2 4CR-1

1CR ADVANCE

2PB

1CR-1

2CR

1CR-2 2CR-1

CONDITIONS TO ADVANCE

3CR RETURN

3CR-1

13.

2LS

IPS

4CR

1CR-3

SOL 1HA

3CR-3

SOL 1HB

SLIDE ADVANCED 1LS

1CR

3CR

STOP 2PB

CONDITIONS TO RETURN

CYCLED MEMORY

3CR-1

14. If limit switch 3 LS was moved to the right, piston B would not begin to advance until piston A was all the way advanced. There may be a situation in which the circuit will not work at all. If 1PS is actuated first, 1CR will de-energize and 3 LS may never be actuated causing the circuit to not function properly.

15.

2CR-2

3CR

START 2PS

CYCLED MEMORY

3CR-1

16.

1SB

2SB

1SP

1KA

P 1KA

1KA

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Chapter 8

17.

1SB

2SB

2SQ

1SP

Pressure Control

27

1KA

P 1KA 2KA

P

2KA 1YV

1KA

2YV

2KA

18.

1SB

2SB

2SQ

1SP

1KA

P 1KA 3SQ

1KA

2SP

2KA

P 2KA 1KA

2KA

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1YV

2YV

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Chapter 9

Temperature Control

1. Importance of Temperature Indication and Control Three areas of concern are: a. Safety b. Troubleshooting c. Means of processing material In these three areas, the instructor should stress the importance of overheated components and conductors.

This one area probably causes more problems in malfunctions of both power and control than any other single item. The quality of product also depends on accurate control of temperature during the process period.

2. Temperature Controllers Factors to consider are: a. Temperature range available b. Type of sensing element c. Response time d. Sensitivity e. Operating differential 3. Sensing Elements a. Thermocouple b. Thermistor c. Resistance temperature detector (RTD) 4. Types of Electronic Temperature Controllers a. Millivolt b. Potentiometric c. Stepless or proportional As the stepless or proportional controller is in the largest current use, the instructor should go through its operation with the students, covering: a. The sensor b. Controller c. Firing package 5. Temperature Switches a. Mechanical link b. Bimetal c. Liquid filled

29

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Chapter Outlines and Achievement Review Answers

6. Circuit Applications Using the circuits included in this chapter, go through several examples involving both power and control.

ACHIEVEMENT REVIEW ANSWERS 1. a. Electrical b. Differential expansion of metal c. Expansion of fluid, gas, or vapor 2. Speed of response is a measure of elapsed time from the instant the temperature change occurs at the sensing element until it is converted into controller action.

3. Resolution sensitivity is the amount of temperature change necessary to actuate the controller. Controller operating differential is the difference in temperature at the sensing element between make and break of the controller’s contacts when the controller is cycled.

4. The thermocouple operates on the principle of joining two dissimilar metals A and B at their extremities.

When a temperature difference exists between the two extremity points, a potential is generated. This potential is in direct proportion to the temperature difference.

5. Time proportioning turns the heating elements on and off in accordance with the demands of the temperature set point. There is a point below the set point where the power to the heating elements is on continuously. Likewise, there is a point above the set point where the power is off. Between these two points the power is on at a proportional rate.

6. The relay output controller can handle approximately 500 watts to 1000 watts at 120 volts. The load capacity can be increased by using the relay to energize a contactor.

7. Band width: 800 3 0.02 5 16 degrees 8. The mechanical-link temperature switch has one metal piece directly connected or subjected to the part where the temperature is to be detected. The metal expands or contracts due to temperature change. This produces a mechanical action that operates a switch.

9. The tube connecting the sensing bulb to the switching mechanism is easily damaged by mechanical abuse. The response is slower with long lengths of connecting tube.

10.

11. 15 (b) 12. Uneven expansion of two different metals when heated. (b)

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Chapter 9

13.

START

STOP

1PB

2CR-1

1TS

Temperature Control

31

1CR HEAT

2PB

1CR-1 G 2TS

3TS

4TS

2CR-2

14.

2CR-3

2CR

5TS

BEARING TEMP OK

2TS - 5TS OPEN @ 130 °F

FAULT ACKNOWLEDGE 3PB

HEAT ON

R

HOT BEARING

3CR HOT BEARING

3CR-1 HORN

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Chapter 10

Time Control

1. Selected Operations Explain to the students that while the timer can be a very useful tool, it must be applied properly. They must know and use what is important for a specific control function. Is it time, position, pressure, or temperature? In many cases, it may be a combination of two or more. Be sure they understand the difference between a timer and time-delay relay. Review basic timer functions as shown in Textbook Figure 10-2.

2. Types of Timers a. Synchronous Motor Driven (1) Reset (2) Repeat cycle (3) Manual set Understand the reset timer contact symbols in their three different conditions: reset, timing, and timed out. Go through the circuit diagrams shown in Textbook showing different arrangements in energizing the clutch and motor. b. Solid State (1) Microprocessor based (2) Digital set and optional digital readout (3) Provisions for external set 3. Circuit Applications Using the circuits included in this chapter, go through several examples.

ACHIEVEMENT REVIEW ANSWERS 1. Time-delay relays are components that have their timing functions after the coil has been energized or de-energized. Timing contacts can be NO or NC. Instantaneous contacts both NO and NC are available.

When reference is made to timers, the time function can start on one or more of the contacts on energization of the timer or any time after energization during a preset time cycle. One or more of the contacts can operate at the end of a preset time cycle or remain operated until the timer is de-energized.

2. Synchronous motor driven; solid state.

3. Clutch and synchronous motor.

4. Reset or de-energized—clutch and motor de-energized. Timing—Clutch energized; motor can or cannot be energized, depending on its circuit application. Time out—motor de-energized; clutch can or cannot be de-energized, depending on its circuit application.

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Chapter Outlines and Achievement Review Answers

5.

START 1PB

1CR

STOP 2PB

1CR-1

1T 1CR-2

M

1T OXO 1T

2CR

OXO 2CR-1

SOL 1PA

6. It is used extensively in programming type of control.

7. Adjustable cams determine the point of closing and opening a switch.

8. The programmed outputs can be programmed to operate in one of four sequences: oox, oxo, oox with pulse output, and oox pulse output with repeat cycle operation.

9. Pressure builds, operating pressure switch IPS. (c) 10. Pressure builds, operating pressure switch 2PS. (c) 11.

START 1PB

2CR -1

REV 2PB

1CR 1T

1CR-1

1LS

1T OXO

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Chapter 10

Time Control

35

12. The circuit will function the same.

START

2LS

2CR-1

REV

1PB

1CR

2PB

1CR-1 1PS 1T OXO

1T

13.

START 1LS 1PS

CYCLED MEMORY

1PB

OXO 3CR-1

14.

3CR

START

1T

2LS

1CR

REV 2PB

1PS

ADVANCE SLIDE 1

1CR-1

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Chapter 11

Count Control

1. Preset Electrical Impulses. Be sure the student understands the difference between an electromechanical counter and an electromechanical timer. Also point out the contact operating sequences.

2. Circuit Applications. Show and discuss the circuit shown in Textbook Figure 11-3. Show and discuss how the piston can be reversed by position rather than by pressure.

3. Solid-State Counters. Discuss the features in the solid-state counter as shown in Textbook Figure 11-4.

ACHIEVEMENT REVIEW ANSWERS 1. The major difference is that the synchronous motor in the timer is replaced by a stepping motor in the counter.

2.

3. High-speed pulse operation with 100% accuracy is available in solid-state counters.

4.

Contact open in the de-energized condition; contact closed in the counting condition; contact open in the counted out condition.

5. Contacts are in the reset condition.

6. 100% (c) 7. The programmable timer/counter has the following features: One to six circuits Single or dual outputs per circuit One cycle Automatic repeat cycle Visual readout Internal or external cycle time base 1200 counts per minute Frequency counts up to 30 kilohertz (kHz) 8. With xox sequence on 1C-1, operate START push-button switch: Relay coil 1CR and counter clutch are energized. Relay contact 1CR-1 closes, sealing around the START push-button. Relay contact 1CR-2 closes, energizing solenoid A. Counter contact 1C-1 opens.

When the piston actuates limit switch 1LS, relay coil 2CR cannot be energized. Therefore, the piston continues to the workpiece, builds pressure to the setting on 1PS. 1PS contact opens de-energizing relay coil 1CR. Relay contact 1CR-1 opens, de-energizing the counter clutch. Relay contact 1CR-2 opens, de-energizing solenoid A. The piston now returns to its initial start position.

37

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Chapter Outlines and Achievement Review Answers

9. It is used to de-energize solenoid A each time that relay coil 2CR is energized. This energizing and de-energizing of solenoid A continues until the counter is counted out.

10. De-energized. (b) 11. Counter output action occurs when the count total indicated by the front thumbwheel switches is reached.

12. a. Reset condition b. Counting; x—Switch closed c. Counted out; o—Switch open 13.

1C G OOX

14.

1C

15.

3CR

START

OFF

CYCLED MEMORY

1PB

OOX 3CR-1

1LT

G

ENABLE

COUNTER

1SSW CAR PRESENT

MTR

1PS 1C 1C R

LOT FULL

OOX

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Chapter 12

Control Circuits

1. Placement of Components in a Control Circuit. The instructor has now completed detailed information on basic components providing input for position, pressure, temperature, time, and counting. In these chapters, a few simple circuit diagrams have been shown, using one or more of these components. It is now time to complete the circuit diagrams with line numbers. Note that the line numbers are enclosed in some geometric form to separate them from wire numbers. Later, in troubleshooting, an additional use of these line numbers will be explained.

2. Bar Chart. After an explanation in the use of a bar chart, the student will find reading a circuit diagram much easier and quicker than a lengthy contact-by-contact description.

At this point, the instructor should point out the division of control and power. The same division will be used in all control. See Textbook Figures 12-3 and 12-4. Explain the importance of showing line numbers in drawings and include a description of the function of each energized coil.

ACHIEVEMENT REVIEW ANSWERS 1. Push-button switches; limit switches; pressure switches; temperature switches; selector switches 2.

3. Output devices to be energized, such as relays, contactors, timers, and motor starters, are shown on the left-hand vertical column of a bar chart.

4. The fused disconnect switch is used to disconnect the power from the circuit and provide short-circuit protection.

5.

CYCLE START 1PB 1TR–1

REVERSE 2PB

1TR

2T–2 XXO

1PS

1LS

CYCLED OVERTIME TIMER

2T–1 OXO

ADVANCE SLIDES

2T

1TR–2

SOL 1HA

1TR–3

SOL 2HA

SLIDE 1 ADV SOL

SLIDE 2 ADV SOL 1TR

SOL 3HA

SLIDE 3 ADV SOL

39

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Chapter Outlines and Achievement Review Answers

6. De-energized (b) 7. To ensure that piston #2 is at the returned position before relay coil 4CR and solenoid B are energized.

Note that it is sealed through a contact on relay 4CR. 8. The counter clutch is energized when the timer clutch is energized. Sequence of operation: Press the ­CYCLE START push-button. Time-delay relay coil 1TR is energized. Inst. contact 1TR-1 closes, ­energizing relay coil 3CR. Relay contact 3CR-1 closes, energizing solenoid 1HA. Note at this point that the ON DELAY timing contact on 1TR is in the seal-in circuit around the CYCLE START push-button.

Therefore, the CYCLE START push-button must be held operated until 1TR times out and the timer clutch is energized.

The piston moves forward to the workpiece, builds pressure to the setting on pressure switch PS. PS contact closes, energizing relay coil 2CR. Relay contact 2CR-1 closes, completing a seal-in circuit around the 1PS contact. Relay contact 2CR-3 closes, energizing the counter motor (coil). Relay contact 2CR-2 opens, de-energizing relay coil 3CR. Relay contact 3CR-1 opens, de-energizing solenoid 1HA. The piston now retracts until it actuates limit switch 1LS. The NC limit switch contact 1LS opens, de-energizing relay coil 2CR. Relay contact 2CR-1 opens. Relay contact 2CR-3 opens, de-energizing the counter motor. This is now one count. Relay contact 2CR-2 closes, energizing relay coil 3CR. Relay contact 3CR-1 closes, energizing solenoid 1HA. The piston now again extends to the workpiece and builds pressure. Pressure switch contact IPS opens.

This oscillating between the workpiece and 1LS position continues until the counter has counted out.

When the counter counts out, counter contact 1C-2 opens so that the next pressure cycle, relay coil 2CR is not energized. Thus, relay coil 3CR is not de-energized, so solenoid 1HA remains energized. When the counter is counted out, counter contact 1C-1 closes, energizing the timer motor. Pressure will now hold on the workpiece until the timer times out. Once the timer times out, relay coil 1TR de-energizes. Contact 2T-1 opens de-energizing timer motor. Contact 2T-2 opens de-energizing 1TR. Contact 2T-3 opens de-energizing counter clutch, relay coil 3CR, and solenoid 1HA. With Solenoid 1HA de-energized, the piston retracts to its initial start position.

9. Following the de-energizing of relay coil 2CR by the opening of 3CR-1 contact at the close of a cycle, relay contact 2CR-2 is open. Unless both START push-button switches are returned to their unoperated condition, relay coil 2CR cannot be energized. It follows then that the relay contact 2CR-2 will remain open, preventing the start of a new cycle.

10. De-energized (a) 11. Similarities: Motors and solenoids are used Sequence of events Differences: Solenoid valves control other substances Sensors: Float switches 12.

2CR

3CR R

93741_All_Chapters_ptg01_00i-096.indd 40

BOTH TANKS LOW

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Chapter 12

Control Circuits

41

13.

2M–AUX BOX COUNTER 1CR

1C

14. To maintain the exact profile for extension and retraction, the potentiometer settings for P1, P2, and P3 would be exactly the same.

When the cylinder reaches 2LS, 2LS (NC) will unlatch 2CR. 2LS (NO) will activate 3CR which will close 3CR-2 and 3CR-3 which energizes ramp potentiometer P3. The negative voltage at the wiper of the P3 will be seen at the ramp 3 input. The negative voltage will cause the valve to shift in position causing the cylinder to retract. The amount of negative voltage will determine the slope of the change to the valve.

As the valve reaches its full open position, the speed of the cylinder is maintained until its reaches 3LS. 15.

MACHINE INCYCLE

1CR

FILL SOLENOID

SOL A

MIXER TIMER

1TR

RUN MIXER

2CR

MIXER MOTOR

MTR

RELEASE TIMER

2TR

RELEASE PRODUCT

SOL B

PROCESS START

16.

HEATER

1FS

1TR TO

2TR TO

1CON 1CR

PRIMARY LEVEL

2CR

AUX LEVEL

3CR

BYPASS PRIMARY

SOL A

RUN AUX

4CR

FLUID CONTROL

SOL B

1TS

93741_All_Chapters_ptg01_00i-096.indd 41

LOSE 1FS

1FS

LOSE 1FS and 2FS

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42

Chapter Outlines and Achievement Review Answers

17.

NONCONTROLLED MOTOR

2M

CONTROLLED MOTOR

1M 1TR

ADV PUSH ROD SOL 1PA FILL SOL 2PA BOX FULL

1CR START

1PED-1 OPENS

1LS 2LS PUSH LEVEL ROD ADVD FULL

LOSE 1LS

3LS PUSH ROD RET

1PED-1

1PED-1 OPENS

18.

ADVANCE

1CR

RETURN FAST

2CR

RETURN SLOW

3CR

NOT AT HOME

4CR

START

1LS DEACTUATED

2LS

3LS

1LS ACTUATED

19.

START

1TR

STOP

1PB

2PB

1TR-1 R 1LT 2TR

1TR

A 2LT 2TR G 3LT

20.

1TR 1LT 2TR 2LT 3LT START

93741_All_Chapters_ptg01_00i-096.indd 42

1TR TO

2TR TO

STOP

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Chapter 13

Motors

1. DC Motors a. Theory of operation. It is important to review the concept of electromagnetism and the basic operation of a single loop armature. b. Classifications c. Speed control d. Armature voltage e. Shunt field f. Speed regulation g. Heating h. Motor loads 2. AC Motors. Theory of Operation—Three-Phase Squirrel-Cage Induction Motors. The students must have a thorough understanding of elementary electricity. With this preparation, they should have no problems understanding this area of instruction. Basic principles are shown in Textbook Figures 13-7 through 13-9. It is important that the students understand how to calculate slip and synchronous speed.

3. Variable Frequency Drives a. Operation b. Rectifier circuit c. DC Bus circuit d. Inverter circuit e. Control circuit f. Volts per Hertz drive g. Vector drives 4. Single-Phase Motors. The instructor should cover: a. Starting methods b. Speed-torque characteristics c. Phase relationships d. Circuit diagrams e. Compare efficiencies Textbook Figures 13-4 through 13-20 will aid the instructor.

43

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44

Chapter Outlines and Achievement Review Answers

ACHIEVEMENT REVIEW ANSWERS 1. The bars and rings that go to make up the rotor give the appearance of a squirrel cage.

2. Synchronous speed 5

120f p

120 3 60 5 3600 rpm 2 Synchronous rpm 2 motor rpm 3 100 Synchronous rpm

1800 2 1740 60 3 100 5 3 100 5 3% 1800 1800

3. %Slip 5

4. With a single source of AC voltage connected to a single winding, a stationary flux field that pulsates in strength as the AC voltage varies, but does not rotate, is created. Consequently, if a stationary rotor is placed in the stationary stator field, it will not rotate.

5. The small diameter wire in the start winding causes a high current density, so it heats up very rapidly.

A centrifugal switch is used to remove the start winding from the circuit once the motor has reached an adequate speed to allow running on the main winding only.

6. Adding a capacitor results in the start winding current leading the line current, resulting in a time phase shift and a strong rotating field.

7. With the capacitor start motor, a centrifugal switch must be used to protect both the start winding and the capacitor from damage due to overheating. The permanent split capacitor motor uses a capacitor that is capable of running continuously and therefore does not have to be removed from the circuit.

8. The efficiency of the shaded pole motor is low due to the presence of winding harmonic content, particularly the third harmonic and losses in the shading coil.

9. 1. Shunt wound 2. Stabilized shunt 3. Compound wound 4. Series wound 10. Because excitation is entirely dependent on load, the application of series wound motors should be avoided where the load may drop below 25% rated torque or where the load is not solidly coupled.

11. T 5 KϕIa where T 5 Torque K 5 Machine constant ϕ 5 Main pole flux Ia 5 Armature current

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Chapter 13

12. HP 5

Motors

45

T 3 RPM 5250

where HP 5 Horsepower RPM 5 Revolutions per minute T 5 Torque in lb. ft.

13. Speed regulation is defined as: No load RPM 2 Full load RPM Full load RPM 14. It is accomplished by six diodes that may be in a single package or in small modules. The diodes are protected by input fuses, which are chosen for their speed and interrupting capacity. An input choke in the DC leg of the diode bridge protects against line transients and limits the rate at which input current may increase or decrease.

15. The rotor uses permanent magnets instead of shorted rotor bars.

16. The Hall effect switches are non-contact (electrically isolated) devices, which are mounted in positions 60° apart (on a four-pole Motor). It is not possible for these switches to be all on or all off at the same time. There will always be two on and one off or two off and one on at all times.

17. A brushless DC motor set to run at 1750 rpm (with a frequency reference) will run at 1750 rpm.

18. Speed regulation 5

19. HP 5

3550 rpm 2 3450 rpm 3 100% 5 2.9% 3450 rpm

1 25 ft lbs 2 1 1750 rpm 2 5250

20. Synchronous speed 5

93741_All_Chapters_ptg01_00i-096.indd 45

5 8.3 hp

120 3 60 5 1800 rpm 4

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Chapter 14

Motor Starters

As motor starters are used on practically every industrial machine, the student should have a thorough knowledge of the various types and how they operate. The instructor can use the following check list to aid in covering the subject.

1. Overload Relays — How and Why They Are Used a. Thermally responsive b. Ambient compensated c. Trip free d. Connecting the contacts 2. Across-the-line (Full-Voltage) Starters (See Textbook Figure 14-3B). a. Combination b. Reversing (See Textbook Figure 14-4B). c. Multispeed — Pole changing d. Understand the difference between no-voltage or low-voltage release and no-voltage or low-voltage protection. e. Jogging (See Textbook Figure 14-10.) f. Auxiliary contacts 3. Reduced-Voltage Motor Starters a. Autotransformer b. Primary resistor c. Wye (or star) delta d. Part winding As circuit diagrams for these starters will change from one manufacturer to another, it is generally not necessary to devote too much time to circuits. It is more important for the student to understand: (1) How reduced voltage affects torque; (2) Why multiple leads must be brought out from the motor for wye-delta and part winding starters.

ACHIEVEMENT REVIEW ANSWERS 1. To prevent the motor from being overloaded, which could result in serious damage to the motor.

2.

MOTOR START 1PB

MOTOR STOP

1MOLS

1M

2PB

1M-AUX

47

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48

Chapter Outlines and Achievement Review Answers

3. The overload heater elements will be affected by ambient temperature. A high ambient temperature will have an effect similar to excessive motor current.

4. The overload relay should be trip-free so that it will be impossible for anyone to hold or block a RESET button, which could damage a motor.

5.

480 VAC 3Ø 60 Hz

L1

1L1

L2

1L2

L3

1L3

1MF 1 MTR

1MR

CONTROL OFF

ON

1SSW MOTOR STOP 1 PB

H3

H1

FORWARD

X1

H2

H4

X2 1MR-AUX2

1MF

1MF-AUX2

1MR

1MOLS

2 PB 1MF-AUX1

REVERSE

1MR-AUX1

6. The reversing of a three-phase, squirrel-cage induction motor is accomplished by interchanging any two line connections to the motor. Interlocks are used to prevent both units on a reversing motor starter from closing their line contacts at the same time.

7. With a reduced-voltage starter set to apply 65% of full voltage to a motor for starting, the resulting ­starting torque will be 42% of normal.

8. The resistance is placed in series in each line with the motor on starting. This reduces the voltage ­available to the motor. When the timer times out, the resistance is shorted out, placing full voltage on the motor.

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Chapter 14

Motor Starters 49

The timer is set by the user to allow time between energizing of LC and AC. T2 is timed closing.

480 VAC

LC

L1

R

1MOLS AC

1 MTR

L2

3Ø 60 Hz

L3 CONTROL

OFF

ON

1SSW MOTOR STOP

H1

X1

H3

H2

H4

X2

MOTOR START

LC

1MOLS

LC CR

LC

T

AC

T CR

9. No-voltage or low-voltage release means that when there is a voltage failure, the starter will open its ­contacts or drop out. It will close again or pick up as soon as the voltage returns. No-voltage or ­low-voltage protection means that when there is a voltage failure, the starter contacts will open or drop out. The starter contacts will not reclose or pick up again automatically when the voltage returns.

10. Jogging provides for the energizing of a motor starter only as long as the JOG push-button is held ­operated. A relay is used in the circuit as a safety factor to prevent the starter from locking in during a jogging operation.

11. Thermal sensor (c) 12. Controlling the conduction of the silicon controlled rectifiers. (b) 13. With open transition the motor is momentarily disconnected from the line during the changing from the start contactor to the run contactor. With closed transition, the motor is not disconnected from the line during the changing from start condition to the run condition.

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Chapter 15

Introduction to Programmable Controllers

1. Primary concepts in logic control AND:

OR:

NOT:

2. Review division of control for PLCs 3. Definition of programmable controllers 4. Inputs/Outputs (I/O) 5. How isolation is accomplished 6. Signal indicators 7. The Processor 8. The Memory. Be sure the students understand the difference between volatile and nonvolatile memory.

9. Power supplies 10. Programming 11. Explain scanning.

12. Explain Examine On/Examine Off. 13. Peripheral and support devices

ACHIEVEMENT REVIEW ANSWERS 1. Pressure switch, push-button, limit switch, and temperature switch.

2. Virtual relay coil, timer, and counter.

3. Solenoid, pilot light, and relay.

4. Problem 1 A) L1

L2

REV

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

N

I1

I0

O0

I0

I2 O0

L1

H SOL 1PA L2 O0

START

O1 I1

1LS

I2

Figure 7-6

51

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52

Chapter Outlines and Achievement Review Answers B)

L2

INPUT MODULE

N

L1

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I0

H

I2

REV

SOL 1PA L2

O0 I0

L1

O0

O0

START I1 1LS

I2

Problem 2 A) L1

L2

INPUT MODULE

N

REV

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I3

I0

H

I2

SOL 1PA L2

O0 I0 START

L1

O0

O0

I1 1LS

I2

2LS

B)

I3

L2

INPUT MODULE

N

L1

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I3

I0

H

I2

REV

SOL 1PA L2

O0 I0

START

L1

O0

O0

I1 1LS

I2

2LS

I3

Figure 7-7 Problem 3 A) L1

L2

INPUT MODULE

REV

N

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I2

I0

H

I3

SOL 1PA L2

I000 I0 START

O0

1000

SOL 1PB I1

2LS

1LS

93741_All_Chapters_ptg01_00i-096.indd 52

L1

O1 1000 O0

I2

I3

I000

I2 O1

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Chapter 15 B)

L2

INPUT MODULE

N

L1

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I2

I0

L1

SOL 1PA L2

1000 I0

53

H

I4

REV

Introduction to Programmable Controllers

O0

1000

START

SOL 1PB O1

I1 1000 2LS

O0

I2

I3 1LS

1000

I3 O1

I4

Figure 8-11 Problem 4 A)

L2

L1

STOP

INPUT MODULE

N

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I4

I0

H

I2

SOL 1HA L2

1000 I0 START

L1

O0

1000

SOL 1HB I1

I4

O1

I2 1001

1PS

1001

I2 1000

O0 2LS

I4

1001 O1

B)

L2

INPUT MODULE

N

L1

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I4

I0

H

I2

STOP

1000 I0

1000

SOL 1HA L2 O0

START

SOL 1HB I1

1PS

L1

I5

O1

I3 1001

I2

1001 1000 O0

I3 1001 2LS

O1

I4 I5

Figure 8-11

93741_All_Chapters_ptg01_00i-096.indd 53

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54

Chapter Outlines and Achievement Review Answers

Problem 5 A)

L2

L1

STOP

INPUT MODULE

N

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I4

I0

H

I2

SOL 1HA L2

1000 I0 START

O0

1000

SOL 1HB I1

I6

I2

1001

O1

1000 I3

I4

1001

1PS

2PS

I3

2LS

I4

1000 O0 1001 O1

NOHC 3LS

B)

L2

L1

I6

INPUT MODULE

N

L1

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I4

I0

H

I2

STOP

SOL 1HA L2

1000 I0

L1

O0

1000

START

SOL 1HB

1PS

2PS

I1

I6

I2

1001

I3

2LS

I4

I5

I000

I3

O1 1001

1000 O0 1001 O1

I5 3LS

I6

Figure 8-12 Problem 6 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

H

N

L1 1TS

L1

L2

I1 I1

1000 O0

CLOSES AT 80°F

I2

O0

O0 HEATER CONTACTOR

I3 1000

STOP I2 START I3

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Chapter 15 B)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

L2

I1

1TS

I1

O0

1000 O0

OPENS AT 80°F

55

H

N

L1

Introduction to Programmable Controllers

I2

O0 HEATER CONTACTOR

I3 1000

STOP I2 START I3

Figure 9-23 Problem 7 A)

L2

L1

STOP

INPUT MODULE

N

OUTPUT MODULE

PROGRAMMED LOGIC

I1

I0

I001

L1

H

I2

L2

1000 I0 START I1

1000

1000

H

O0

1TS

G

O0

I2

R

O1

1001 O1 2TS

I3

H

13 1001 3TS

I4

14 15

4TS

I5 16

5TS

B) L1

I6

L2

INPUT MODULE

N STOP I0 START

OUTPUT MODULE

PROGRAMMED LOGIC

I1

10

1001

H

I2

L2

1000

I3

I2

I4

G

O0

1000

I1

H

1001

1TS

L1

R

O1

I5 2TS

I3

H

I6 3TS

I4

1000 O0

4TS

I5

1001 O1

5TS

I6

Figure 9-24

93741_All_Chapters_ptg01_00i-096.indd 55

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56

Chapter Outlines and Achievement Review Answers

Problem 8 A) L1

L2

INPUT MODULE

N

REV

OUTPUT MODULE

PROGRAMMED LOGIC

I1

L1

H

I0

SOL 1PA

O0 I0

START

O0

O0

I1

L2

O0

G

O1 TON

TON O1

B)

L2

INPUT MODULE

N

L1

I1

REV

OUTPUT MODULE

PROGRAMMED LOGIC

H

I0

SOL 1PA

O0 I0

O0

I1

L2

O0

O0

START

L1

G

O1 TON

TON O1

Figure 10-6A Problem 9 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

N

REV

I0

H

I1

SOL 1PA L2

1000 O0

I0 START

1000

PR: REGXX AR: REGXX ENABLE TB: XX

1000

I2

I1 1LS

I2

CONTROL

G

O1

TON

TON O1 1000 O0

B)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

REV

N

I0

H

I1

SOL 1PA L2

1000 START

O0

I0 1000 I1

1LS

I2

1000

PR: REGXX AR: REGXX ENABLE TB: XX

O1

G

I2 CONTROL

TON

TON O1 1000 O0

Figure 10-6B

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Chapter 15

Introduction to Programmable Controllers

57

Problem 10 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

N

REV

I0

H

I1

SOL 1PA L2

1000 O0

I0

START

1000

I2

I1 ENABLE 1LS

I2

PR: REGXX AR: REGXX TB: XX

G

O1

1000 CONTROL

TON

TON

1000

O1 1000 O0

B)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000

N

L1

I0

H

I1

REV

SOL 1PA L2

1000 O0

I0 START

1000

I2

I1 ENABLE 1LS

I2

PR: REGXX AR: REGXX TB: XX

G

O1

1000 CONTROL

TON

TON

1000

O1 1000 O0

Figure 10-6C Problem 11 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

REV

N

I0

I1

O0 1000

I1 1PS

SOL 1PA L2

1000

I0 START

H

1001

I2

PR: REGXX AR: REGXX TB: XX

I2 TON TON 1001 1001 O0

93741_All_Chapters_ptg01_00i-096.indd 57

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58

Chapter Outlines and Achievement Review Answers B)

INPUT MODULE

L2

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000

N

L1

I0

I1

H

1001

REV

SOL 1PA L2

1000 O0

I0 1000

START

I2

I1 1PS

PR: REGXX AR: REGXX TB:XX

TON

I2 TON

1001 1001 O0

Figure 10-12 Problem 12 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000

N

L1

I0

I1

H

I2 1000

REV

SOL 1PA L2 O0

1000

I0

SOL 2PA

O0

START

O1 1001

I1 1PS

I4 1001

I2

I2

PR: REGXX AR: REGXX TB: XX

1001 I3 2PS

TON TON

I4

1002 1002 O1

B)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

N REV

I0

I1

H

I2 1000

I0 START

1000

I2

SOL 2PA

O0

I1 1PS

SOL 1PA L2 O0

O1 1001

I4 1001

I3 I3

PR: REGXX AR: REGXX TB: XX

1001 2PS

TON I4

TON 1002 1002 O1

Figure 10-13

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Chapter 15

Introduction to Programmable Controllers

59

Problem 13 A)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000 L1

N

REV

I0

H

I2

I1

SOL 1HA L2

1000 O0

I0 1002

START 1002

I1

PR: REGXX AR: REGXX

RESET 1PS

I2

1001 CNT0

CONTROL 1LS

I3

2LS

I3

CNT0 1001

I4

1001

1000

1001

I4 O0

B)

L2

INPUT MODULE

OUTPUT MODULE

PROGRAMMED LOGIC

L1

1000

N

L1

I0

H

I2

I1

REV

SOL 1HA L2

1000 O0

I0 1002

START 1002 I1

PR: REGXX AR: REGXX

RESET 1PS

I2

1001 CONTROL

1LS

I3

I3

CNT0

CNT0 1001

I4

1001

1000

1001

2LS I4 O0

Figure 11-3 5. Can be expanded (modular) Control logic changes can be made easily (programmable) High reliability (solid state technology) Easy troubleshooting (built in diagnostics) Smaller space needed (solid state technology) 6. The function of the processor (Central Processing Unit or CPU) is to control all system activities by interpreting and executing programmed instructions. The processor scans the status of the input devices, changes the memory location that represents the device, evaluates the condition of the current rung of the program sequence, and appropriately updates the output devices.

Larger PLCs can control more peripheral devices such as printers, disk drives, operator monitors, and keyboard. The memory of the larger PLCs also have more memory chips to manage larger programs and a greater quantity of input and output devices.

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60

Chapter Outlines and Achievement Review Answers

7. The PLC I/O functions differ depending on the type of input devices that provide process status and the type of output devices that are processor actuators. The I/O functions are divided into input modules and output modules. Modules differ on operating voltage of the device and whether input signal conditioning is required.

8. Thermocouple input module Load cell input module Voltage isolation module Digital signals (TTL) module 9. PLC memory is logically constructed to represent the number of input and output devices connected to the PLC. For example, a typical memory word provides 16 individual input points. Another memory word would provide for 16 individual output points. Therefore, as the number of combined input and output points increases, the number of memory words will need to increase. If a PLC has a fixed number of memory words, then the number of input and output points will also be fixed.

10. Computer memory can be of two types: a. Random Access Memory (RAM) is used to store information and results needed as the PLC is operating. This memory is lost when power is removed. If memory must be retained, then a battery is used to keep power in the RAM chips while the PLC is turned off. b. Read Only Memory (ROM) is used to store information that will never change or will change occasionally.

11.

INPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 1 0 1

INPUT TERMINAL

REV I0 START I1 ILS I2

OUTPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 0 0 0

OUTPUT TERMINAL

SOL 1PA O0

Figure 7-6 INPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 1 1 0 1

INPUT TERMINAL

REV I0 START I1 2LS I2 ILS I3

OUTPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 0 0 0

OUTPUT TERMINAL

SOL 1PA O0

Figure 7-7

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Chapter 15

Introduction to Programmable Controllers

61

INPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 1 1 0 1

INPUT TERMINAL

REV I0 START I1 ILS I2 2LS I3

I4 OUTPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 0 0 0

OUTPUT TERMINAL

SOL 1PA SOL 1PB

O0 O1

Figure 7-8 INPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 1

3 2 1 0 0 1 0 1

INPUT TERMINAL

STOP I0 START I1 IPS I2

I3 2LS

I4 I5

OUTPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 0 0 0

OUTPUT TERMINAL

SOL 1HA O0 SOL 1HB

O1

Figure 8-11 INPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 1

3 2 1 0 1 1 0 1

INPUT TERMINAL

STOP I0 START I1 IPS I2 2PS I3 2LS

I4

I5 3LS

I6

OUTPUT TABLE 15 14 13 12 11 10 9 8 7 6 5 4 0 0 0 0 0 0 0 0 0 0 0 0

3 2 1 0 0 0 0 0

OUTPUT TERMINAL

SOL 1HA O0 SOL 2HA

O1

Figure 8-12

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Chapter Outlines and Achievement Review Answers

12. The role of the communication highway is to provide a communication link between all process equipment, such as sensors, actuators, controllers, printers, and operator control stations. In large processes, where the process equipment may be spread over a large physical plant (or several plants), the communication highway organizes and moves the digital signals from where the signal was created to where the signal is processed.

13. Normally open contact I:3/3 becomes a normally closed contact.

14. Normally open contact I:5/0 becomes a normally closed contact.

For the original design, if the wire to input I:5/0 was disconnected the motor would shut off. For the modified design, if the wire to input I:5/0 was disconnected the motor would remain running and cannot be shut off. The original design is failsafe.

15. Delay is 1 second.

16. When the REVERSE push-button is pressed the MCR coil will de-energize. The logic will no longer be solved. Coil B3:0/5 will turn off, coil B3:0/6 will turn off, coil O:2/0 will turn off, timer 1TON will reset, and counter CTR will reset.

17. Input I:4/1 will be energized all of the time. Coil B3:2/7 will not be energized. Therefore, the cycle ­cannot be started because coil B3:2/4 will not energize due to the normally open contact from B3:2/7.

18.

MOTORS START

1MOL 7

4

MOTORS STOP

1M

2MOL 5

2M

8 1M-AUX1 9 2M-AUX2 10 MOTORS START I:3/7

NO MOTORS STOP I:3/8 0:6/4

RUN 1M

0:6/5

RUN 2M

1M AUX 2M AUX I:3/9 I:3/10

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Chapter 16

Industrial Data Communications

1. This table will illustrate the differences: FACTORY WITH PROCESS CONTROL COMPUTERS 1. Production is composed of single step processes resulting in slow production and more worker involvement.

2. Since manufacturing steps are not controlled any attempt to increase production will result in more waste.

3. Workers are responsible for speed of production and monitoring quality.

4. Workers rely on human senses that can be slow and inaccurate.

DISTRIBUTED DATA FACTORY Production systems are integrated resulting in continuous manufacturing with less worker involvement.

Each process is dependent on previous processes resulting in higher quality and fewer delays.

Computers monitor the quality of the products and adjust the speed of production to maintain quality standards at the highest production rate.

High tech sensors can detect the minutest changes in physical dimensions (so minute a human could not detect).

2. The architecture approach focuses on the origination and destination of data throughout the industrial process, plant, and corporation. The approach creates a network where control devices are arranged to support the unique and specific data needs. Generally the architecture is documented in a network design that shows the type and function of the network equipment.

3. Connection—Transmission—Access 4. Provide data when and where it is needed.

5. Data originates from some source such as a sensor, moves through transmission lines such as twistedpair cable, is switched or routed to appropriate network segments, and terminates into a controller.

6. The error checking code is created so that it can be moved with the digital data bits over the t­ ransmission network. The code is checked at the destination of the transmission for any changes in the digital data.

If the error code changes, the data has changed values somewhere in the network and is considered ­erroneous.

7. (a) For a control device to be connected to a network it must be network compatible. (b) The string of digital data is structured in a manner that places the device identifier code, the destination code, an error checking code, and the operating value for the device all in the same digital bit string. (c) The process of determining the way in which the string fits together, how the communication is started and ended, whose turn it is to send or receive data, and how messages are confirmed is controlled by a protocol.

8. Parallel data transmission involves a group of wires, one for each bit of the data word as well as any synchronous control signals. Serial transmission uses two wires to conduct the series of pulses that represent each bit of the data word. Parallel transmission can be bidirectional over the same wires. However, serial transmission requires an additional set of wires (4 wire) if the data is bidirectional.

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Chapter Outlines and Achievement Review Answers

9. Synchronous data is generated at a specific timing event created by a controller. Asynchronous data is only generated when a change in the data occurs. Asynchronous data is applicable to devices such as sensors where data is only needed when there is a change in value. Synchronous data is applicable for controller devices, which need to evaluate the data in other controllers.

10. The industrial data highway is a concept that represents the electronics and cabling designed to move digital data between automation devices such as controllers, sensors, and operator interfaces.

11. This table will illustrate the differences:

Cabling

Reliability

BUS

STAR

RING

The devices on the network are connected together in a string (daisy chain). Usually with a terminator at the end of the bus.

If the central bus cable fails, the network will become unstable.

Each network device is connected to a central network controller. A star network generally requires more cable then a bus or ring.

If a device fails, it will not affect the entire network.

The devices on the network are connected together in a continuous loop so that the end is connected to the network controller.

A failure in any cable or device breaks the loop and can take down the entire network.

12. Ethernet is very stable and reliable in industrial network applications.

13. 1. Sensor to Actuator—typical devices may be a temperature, pressure, and displacement sensor. Actuators could be servo or stepper motors.

2. The controller program and operating system—typical devices or software would be the controller program specifically designed for an application. The operating system may be the proprietary software of the controller manufacturer or it may be a PC operating system such as Windows.

3. The network interface cards and switching electronics—typical devices are PLC network cards and network switching devices such as switches, hubs, and routers.

4. Network wires and connectors—typical devices such as Category 5, four-wire shielded cable. The cable is terminated with an RJ-45 connector.

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Chapter 17

Quality Control

• In class, encourage students to discuss what quality means to them.

• Review the alarm circuits that will identify when processes are not operating as intended.

• Discuss the normal distribution and the frequency distribution charts as a means of identifying the range of tolerance and control of a product.

• Have students describe how components and systems can become inaccurate.

ACHIEVEMENT REVIEW ANSWERS 1. The purpose of this question is to emphasize: a. the importance of quality (1) reduce repair costs (2) company’s image (3) competition b. value for the dollar (1) fewer problems (2) longer lasting c. image Early in their training, students should gain an appreciation of why quality should be constantly considered in every aspect of work. Quality should be part of everyone’s work.

2. The purpose of the question is to consider important criteria for quality in products they are most familiar with. In other words, as they consider quality from a consumer perspective, they can make better judgments on quality issues in their work. Students would expect quality, but they must realize that workers can control the degree of quality of a product.

3. Thermo switches can be used to control the curing rate of plastics and acrylics. If the switch is faulty, the product may be too soft or too hard.

Pressure switches can be used to control the compacting or force applied to an object. If the switch is faulty, the product may be too soft or too hard.

Limit switches (and their mounting structure) can be used to control the positioning of a part within an assembly. If the switch is faulty, the product may be out of alignment.

Flow switches can be used to monitor the flow of conditioned air in a public area (i.e., retail store or restaurant). When customers are comfortable, they will appreciate the quality of the environment and enjoy their stay in that area.

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Chapter Outlines and Achievement Review Answers

4.

HORN

5. The purpose of an analog-to-digital converter is to translate the changing analog output voltage from a transducer (i.e., thermocouple or load cell) into a digital word. The digital word is required of computerbased controllers to represent the physical state of the transducer (i.e., temperature or pressure).

The data acquisition system is a small computer that will sample multiple transducer channels and produce a digital word equal to the output of each transducer and the channel the transducer is connected to.

6. Sensors can become faulty, loose, dirty, too hot, too cold, experience changes in line voltage, not be calibrated to manufacturer’s specifications, and physically damaged.

To correct the problem before changes to quality can be detected, a thorough and effective preventative maintenance program must be instituted and followed.

7. To utilize technology in a manner that will allow process operators to maintain the quality standards as defined by the company.

8. Statistics 9. Environmental conditions, changes in the raw materials, and operate error.

10. Zero defects may be impossible to attain because of process and financial factors.

11. Rearrange data: 3, 4, 7, 9, 12, 15, 28, 28 a. mean 5

106 5 13.25 8 9 1 12 5 10.5 2

b. median 5

c. mode 5 28 12. Rearrange date: 5, 14, 16, 17, 17, 18, 28, 33, 33, 33 a. mean 5

214 5 21.4 10

b. median 5

17 1 18 5 17.5 2

c. mode 5 33

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Chapter 17

13. Length

# parts

1

5

2

10

3

10

4

20

5

2

6

10

Quality Control

67

a. Good parts 5 10 1 10 1 20 5 40 b. Rejected parts 5 5 1 2 1 10 5 17 c. Reworked parts 5 2 1 10 5 12 14.

1TR

1FS

R RESET

1TR

2CR

2CR

15.

1TR

1FS

R 1TR

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Chapter 18 • • • •

Safety

In class, encourage students to discuss the many aspects of safety.

Review several safety circuits. Emphasize machine safety versus worker safety.

Discuss how programmable controllers should and should not be used in safety circuits.

Have students identify the sequence of steps necessary for a control logic sequence to recover from a machine malfunction. For this, any circuit from Chapter 12 can be used to illustrate what would occur if: a. limit switches broke b. relays failed c. hydraulic valves failed d. motors malfunctioned

ACHIEVEMENT REVIEW ANSWERS 1. The purpose of this question is to emphasize: a. unsafe conditions can easily be overlooked b. unsafe conditions can take many forms involving moving and stationary objects c. safety can range from slight to life-threatening injuries 2.

START OPERATOR STOP SW A

OPERATOR STOP SW B

BEARING OVERHEATED 1CR

ZERO SPEED

1CR

3. The purpose of this question is to have students understand that codes and standards are important to safety because criteria for installation and use of electrical equipment will help (but not guarantee) to eliminate hazards to machines as well as operating and maintenance personnel.

Each chapter of the National Electrical Code handbook contains procedures and guidelines that not only provide uniformity of installation and operation but establish barriers to health and environmental hazards.

4. Proper emergency circuits should be designed and implemented to stop or temporarily disable the operation of the controller, machine or process. Emergency circuits should not be part of the programmed logic but hard-wired outside of the controller. This is necessary in case of total controller failure. For example, emergency stop devices should directly control motor starters and other actuators for immediate response.

5. The purpose of this question is to encourage the student to converse with a business person who has direct knowledge of safety problems and issues. This person may: a. show students how safety procedures are implemented and what their intended result is b. tell students about business issues that arise with safety: • higher insurance rates • lost employee time • role of management to ensure worker safety • cost of safety measures • inspections by local and federal inspectors 69

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Chapter Outlines and Achievement Review Answers

6. a. Photoelectric sensor—hands, arms, legs are in a dangerous zone b. Mechanical limit switch—guards are in place c. Pressure switch—worker is standing at a proper (or improper) location 7. The purpose of the question is to bring the awareness of safety to a higher level by observing and documenting the proper use of a machine. Critical thinking is needed when preparing a document that will inform the reader of potential hazards.

The student may choose any piece of equipment. Machines such as a saw, grinder, or lathe are good examples. A typical manual would have the following sections: • Purpose of the manual • Description of the machine • Points on the machine that are accessible and hazardous • Description of proper operations • Examples of improper operations 8. To better predict when fault events may take place before costly damage could occur.

9. See Section 18.5 in the textbook.

1

MASTER START

MASTER E-STOP

3

4

1PB

STATION 2 E-STOP

STATION 1 E-STOP 5

2PB

NO PLC FAULT 6

3PB

2 1CR

7

SAFETY CONTROL ON

4PB

1CR-1 ALARM OFF 1CR-2

R ON

8

1LT

9 1ALM

1SSW PLC POWER SUPPLY

1CR-3

10

10

TO PLC I/O WIRING

2

Figure 18-3

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Chapter 18

Safety

71

10. Insurance companies Industry-specific agencies Local and state government agencies 11.

LOCAL STOP

REMOTE STOP

START 4

1MOLS

1M

1M-1

12.

LOCAL STOP

REMOTE STOP 1

REMOTE STOP 2

START

1MOLS

1M

1M-1

13.

LOCAL STOP

REMOTE STOP

START

1TR

2CR 2CR

1TR

10 SECOND DELAY 1MOLS

1M MOTOR

1TR

14.

ALARM

1TR

1CR

1TR

1TR

2CR

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ACKNOWLEDGE

R 2CR

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Chapter 19

Troubleshooting

1. Go through the Safety First material very carefully, being sure the students understand all the safety requirements.

2. Analyzing the problem 3. Major trouble spots 4. Equipment for troubleshooting. Cover the use of multimeters (source voltage, voltage drop, voltage imbalance), current measurements, motor inrush current, contact bounce, and temperature measurements.

Devote extra time to the safety checks for meters.

5. Motors 6. Component coils. Cover the flow chart that can be used in troubleshooting. This extensive procedure may not always be required, but it develops discipline.

7. Troubleshooting a complete control circuit 8. Troubleshooting the programmable logic controller 9. Electronic Troubleshooting Hints. Devote extra time to this section.

ACHIEVEMENT REVIEW ANSWERS 1. a. When checking with the circuit de-energized, use an ohmmeter and check continuity of circuits and presence of grounds. It may be necessary to disconnect some circuits to eliminate measuring of alternate paths. b. When checking with the circuit energized, use a light bulb in a pigtail socket or a voltmeter. With this equipment, you can indicate electrical energy in the circuit. Either the series or half-split method may be used.

2.

a. Put lead A on L1; B on L2 should read line voltage. If not, the problem is in the supply. b. Put lead A on L1, B on 2. With line voltage reading, the #1 fuse is open. With no reading, #2 fuse is open or both fuses are open. c. Put lead A on 1, B on L2. With line voltage reading, the #2 fuse is open. With no reading now, both fuses are open.

3. Normally closed contacts can cause trouble and yet appear to be in good condition.

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Chapter Outlines and Achievement Review Answers

4. Filing a contact surface can remove a protective coating of silver cadmium. The contact will then have a short life.

5. A combination problem is that which has more than one source, such as electrical-mechanical, electricalpressure, or electrical-temperature.

6. a. All coils are tied to a common line that is grounded. The opposite side is protected by a device such as a fuse or circuit breaker. The presence of a ground will open the protective device. b. The common side is not grounded. Both sides are protected by a fuse or circuit breaker. A set of two ground detector lights is connected in series. A solid ground is placed between the two lights. With no grounds on the control system, both lights will glow at half brilliance. With a ground in the system, one light will go out and the other will glow at full brilliance.

7. When a common line connection such as a jumper is omitted, an open circuit is created. A more common location for this omission is on terminal blocks or components having more than one connection with the same circuit number.

Note that circuit connection #2 on relay coil 4CR has been missed in connecting to the terminal block.

A jumper could have been used for connection from #2 on 4CR coil to #2 on 3CR coil.

8. a. Connection points in solenoid valves, limit switches, and pressure switches. b. Pulling conductors through conduit and fittings. c. Loose strands of a conductor.

9. Dust, dirt, and grease on mechanical parts; loose pins and bolts on components such as large motor ­starters; overheated parts; dust, dirt, and fluids in enclosures.

10. Inadequate power supply; a conductor that is too small.

11. In a good preventive maintenance program, records are kept of each reported problem and the work done to correct it. These records are compiled periodically and are available to the supervisor. This not only leads to faster troubleshooting in the future, but it also gives the supervisor an indication of why production may be down in a given department.

12. Textbook Figure 19-19 shows the limit switch 1LS in an operated condition. This means that the NC contact will be held open. If 14-15 is wired to the NC contact, the circuit will be open; therefore, a cycle cannot be started.

13. 11. (b)

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Chapter 19

Troubleshooting

75

14. Neither (c) 15. The motor overloads for 1M have tripped. a. The bulb for 2LT is burned out. b. Wire number 6 has become disconnected. c. The common wire (wire number 2) has become disconnected for 2LT. 16. Contact 2CR-1 may be malfunctioning. a. The cylinder may be jammed or locked forward. b. The solenoid (SOL 1HB) may be damaged. c. Wire numbers 1 or 22 may have become disconnected from circuit shown on line #13.

17. Contact 1CR-3 is a NO contact and trying to energize both solenoids simultaneously.

18. I 5

E 120 5 5 12 amps R 10

19. An open circuit shows infinite resistance; a closed circuit shows low or zero resistance.

20. The centrifugal switch used in some single-phase motors is for the purpose of removing the start circuit from the line as soon as the motor reaches adequate speed.

21. a. Improper balance b. Uneven air gap, which can be caused by worn bearings c. Dirt in the air gap d. Single phasing 22. It is recommended that all circuits serving critical loads should be on dedicated circuits and neutrals.

23. Average 5 %5

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36 5 12 3 6 5 50% 12

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Chapter 20

Designing Control Systems for Easy Maintenance

1. Design Considerations Maintenance problems Planning the design Cooperation between user and builder Diagrams and layouts Locating, assembling, and installing equipment 2. Diagrams and Layouts Numbering Listing equipment Description of equipment Sequence of operations Drawing size Optional features 3. Locating, Assembling, and Installing Equipment Control panel Wiring duct and plastic ties Terminal blocks Check points Limit switch problems Accessibility of components Control cabinet Push-button stations The machine Numbering and identifying components Use of indicating lights Use of plug-in components

ACHIEVEMENT REVIEW ANSWERS 1. A numbering system should be used on the elementary circuit diagram, all terminal blocks, and ­components throughout the machine. This provides a great help in troubleshooting. Using a pattern of numbers on various machines in a plant helps the troubleshooter and maintenance worker. They can ­remember numbers in a common location on the various machines.

2. a. Evenly space circuit lines (approximately 1/2 inch). b. Space component symbols so they are clear and space is left for wire numbers. c. Use broken lines and arrows where two contacts (NO and NC) are used on limit switches, temperature switches, and pressure switches. This is of even greater importance where the contact symbols are widely spaced on the drawing. d. Number circuit lines and cross-reference relay contacts to line numbers.

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Chapter Outlines and Achievement Review Answers

3. The panel should be removable through the enclosure door opening. All line voltage devices (above 120) should be mounted above or to the side of control voltage devices.

4. Advantages: (a) Saves the maintenance workers’ time; (b) Faster job of troubleshooting. Disadvantages: (a) Higher initial cost; (b) Possibly more material.

5. Impact speed should not exceed 400 feet per minute. Designed overtravel should not exceed 10 degrees.

Do not use excessive overloads on contacts. Excessive voltage and high inductive loads also contribute to contact failure.

6. Contacts on the vane switch are rated at 0.75 ampere make and 0.2 ampere continuous. They are recommended for use in a dusty atmosphere.

7. a. Keep the components within an easy and safe range of working height. This is approximately 2 feet to 5 1/2 feet above the operating floor. b. Design the control panels so they can be removed through the cabinet door opening. c. Do not stack terminal blocks one on top of the other. d. Space components and terminal blocks so that the terminals are accessible to add or remove conductors. e. Use relays with such features as building block assembly for time delay, mechanical latch, and additional standard relay contacts. The changing of NO and NC contacts or the universal contact arrangement are useful.

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Chapter 20

Designing Control Systems for Easy Maintenance

79

8. These types of lights indicate: when a motor is energized, the type of operation being used, oil temperatures, if safety guards have been removed, and excessive loading or overtravel.

9. Does experience indicate a high rate of failure in a particular application? Will downtime for changing the component seriously affect production? Can repair work be accomplished at a bench easier and faster than on a machine? Is the machine a prototype or custom design that may require many changes before a final design is reached? Is the level of maintenance experience at a given user’s shop such that disconnecting and reconnecting circuits on a component may become a problem?

10. 8 1/2" × 11". (a) 11. Aid in checking circuit. (a) 12. 2 to 5 1/2 (b)

5FU 1M

2M

3FU

1FU

2FU

DISCONNECT

TRANSFORMER

6FU

4FU

13.

3M

1CR 1TR

14. #1 MOTOR START

#2 MOTOR START

#3 MOTOR START

CONTROL

MOTORS STOP

#3 MOTOR STOP

OFF

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ON

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Chapter Outlines and Achievement Review Answers

15.

2L3 5FU 1L2

2L1 4FU 1L1

L1

2L1

2L3

L2

L3

DISCONNECT

TRANSFORMER

6FU

X1

1L1 O

1A

X1

1L2 O

1L3 O

X2 3MAUX 2 4

1MAUX 1 1 6

1M

2MAUX 1 3

1

2

11

9

2M

4

5 3MAUX 1

8

1

2

14

12 3M

2

7

10

7

1CR

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2

4

1TR

2

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Appendix The pretest is general in content and includes elementary electricity. The test is designed to assist the instructor in evaluating the educational background and experience of the student in the electrical field.

PRETEST: ELECTRICAL CONTROL FOR MACHINES Circle the correct answer to Questions 1 through 10.

1. An electrician purchases a total of 553 boxes for a number of jobs to be performed. On seven of the jobs, 63, 56, 91, 57, 74, 79, and 68 boxes are used, respectively. How many boxes are left for one more job? a. 55

c. 71

b. 65

d. 165

2. A total load of 49,203 watts is distributed equally over 11 branch circuits. Find the average load per circuit. a. 4400 watts

c. 4556 watts

b. 4473 watts

d. 5467 watts

3. The total current in amperes in a lighting circuit is equal to the sum of the lamp currents. The following lamps are in a circuit: two 25-watt lamps (0.208 ampere each), three 100-watt lamps (0.834 ampere each), two 40-watt lamps (0.334 ampere each). Find the total current when all lamps are on. a. 1.376 amperes

c. 3.586 amperes

b. 1.584 amperes

d. 3.920 amperes

4. An armature shaft that is 1 3/8 inches in diameter is found to have an irregular surface. It is determined that 0.017 inch will be taken off the surface. Find the new diameter in decimal form. a. 1.258

c. 1.358

b. 1.341

d. 1.368

5. Find the number of watts in a circuit that has 120 volts, 54.3 ohms, and 2.2 amperes, if watts 5 I2 X R, where I is in amperes and R is in ohms. a. 262.81

c. 1087.4

b. 264.0

d. 14,335.2

6. If voltage, in volts, is equal to the square root of the number of watts times the square root of the number of ohms, find the total number of volts in a circuit containing 729 watts and 9 ohms. a. 81 volts

c. 2229 volts

b. 245.34 volts

d. 6687 volts

7. Solve for L if R 5 KL/d2. a. L 5 Rd2/K

c. L 5 d2K/R

b. L 5 Kd2/R

d. L 5 d2/RK

81

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82

Appendix

8. If R1 5 20 ohms, R2 5 40 ohms, and R3 5 50 ohms, find the total resistance, Rt, of a circuit using the expression Rt 5

1 .

1 1 1 1 1 R1 R2 R3

a. 1.053 ohms

c. 104.9 ohms

b. 10.53 ohms

d. 140 ohms

9. Impedance, Z, in a series equals "R2 1 X2. If R 5 80 ohms and X 5 60 ohms, find Z. a. 20 ohms

c. 104.9 ohms

b. 100 ohms

d. 140 ohms

10. In the right triangle shown, solve for E. a. 30 volts

c. 210 volts

b. 150 volts

d. 225 volts

11. How does a fuse prevent excess current from flowing in a circuit?

12. What is meant by an open circuit?

13. What is meant by a short circuit?

14. What is the unit and symbol for resistance?

15. In a circuit having a 5-k Ω resistor, how much voltage is necessary to produce a current of 20 mA? 16. If a circuit has a 15-volt battery, and a current of 0.3 milliampere is flowing, what is the circuit resistance?

17. How do you connect a voltmeter in a circuit?

18. How do you connect an ammeter in a circuit?

19. Is it necessary to remove the power from a circuit before resistance measurements can be made? Why? 20. Which is the more sensitive meter, a milliammeter or a microammeter?

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Appendix

83

21. Find the total resistance in the following circuit.

22. Determine the current in the circuit shown in Question 21.

23. Is the current through Rl the same as the current through R3? Explain your answer.

24. In the following circuit, find the source voltage and the voltage across R3 if the voltage across R5 is 15 volts.

25. If R2 in Question 24 is a 2-watt resistor, will its rating be exceeded? Show your work.

26. What is a solenoid?

27. An ammeter is used to measure

in a circuit.

28. What is the purpose of the zero-ohm set control on an ohmmeter?

29. What is the basic unit of measurement of capacitance?

30. What is meant by the turns ratio of a transformer?

31. The nameplate on an electric heater rates it at 120 volts ac, 10 amperes. The heating element of this device is almost pure resistance. With the current and voltage in phase, calculate (a) the power in watts consumed by the heater, and (b) the resistance of the heating element.

32. What is a bus duct? What is its use?

33. For what purpose are bus plugs used?

34. What is meant by the term combination starter?

35. What type of fuse is used on an induction motor load?

36. On what piece of electrical equipment would you be likely to find an overload relay?

37. Draw the symbol for a normally open limit switch.

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38. Draw the symbol for a normally closed temperature switch.

39. In the following circuit, will the light bulb be energized when the switch is closed? Explain.

40. In the following two battery arrangements, determine the resultant voltages Et in both examples, if Eb 5 15 V.

41. In the following battery arrangement, what is the resultant voltage Et?

42. What is expressed by Ohm’s Law? What three equations are used to state this law?

43. What is the difference between a step-up transformer and a step-down transformer?

44. Draw the symbol for a ferrule-type fuse.

45. Name at least two problems to look for in a defective extension cord.

46. Two examples of circuit lines crossing are shown in the following diagram. Describe the difference between these two examples.

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47. In the following electrical circuit, list three combinations of two switches each that must be closed to energize the light bulb.

48. Add a switch to the circuit shown in Question 47 that would require a combination of three switches to be closed to energize the light bulb.

49. What information can you obtain from the following drawing?

50. In general, what information would you be looking for if you were using a thermocouple?

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ANSWERS TO PRETEST 1. b. 63 553 56 2488 91 65 57 74 79 68 488 2. b. 49,203/11 5 4473 3. c. 0.208 3 2 5 0.416 0.834 3 3 5 2.502 0.334 3 2 5 0.668 3.586 4. b. 0.017 3 2 5 0.034

1.375 2 0.034 5 1.341

5. a. 2.2 3 2.2 5 4.84

4.84 3 54.3 5 262.81

6. a. 729 3 9 5 27 3 3 5 81 7. a. R 5 KL/d2 8. b. Rt 5

KL 5 Rd2

L 5 Rd2/K

1 1 1 5 5 5 10.53 1 1 1 10 1 5 1 4 19 1 1 20 40 50 200 200

9. b. "802 1 602 5 "6400 1 3600 5 "10,000 5 100 10. b. Et 5 "1202 1 902 5 "14,400 1 18,100 5 "22,500 5 150

11. The fuse opens the circuit due to the fuse element melting or vaporizing.

12. An open circuit is one that does not have continuity between the terminals of the circuit in question.

13. A short circuit is a closed circuit having minimum resistance.

14. The unit for resistance is the ohm. Symbol—Ω.

15. 20 mA 5 0.020 A 5 kΩ 5 5000 Ω E 5 IR 5 0.020 3 5000 5 100 V 16. 0.3 mA 5 0.0003 A R 5 E/I 5 15/0.0003 5 50,000 Ω or 50 kΩ 17. A voltmeter is connected in parallel with the energy source.

18. An ammeter is connected in series with the load.

19. Yes, the power must be removed for reasons of safety and more accurate measurements.

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20. A microammeter is the more sensitive meter.

21.

10 1000 2700 150 15 3875

22. I 5 E/R 5 30/3875 5 0.0077 A or 7.7 mA 23. Yes, the current is the same. In a series circuit, the current is the same throughout the circuit.

24.

I 5 E/R 5 15/75 5 0.2A E5

IR 5 0.2 3 10 5 2V (for R1) 0.2 3 25 5 5V (for R2) 0.2 3 50 5 10V (for R4) 0.2 3 25 5 15V (for R3) R5 given Total 5

15V 37V

25. W 5 EI 5 5 3 0.2 5 1 W. Therefore, the 2-watt resistor rating will not be exceeded.

26. A solenoid is an electromagnet that provides linear motion. (The motion is reciprocating within and along the axis of the coil.)

27. Current 28. The zero-ohm set control adjusts for voltage variations of the internal battery and inaccuracies of the multiple-range resistance.

29. The farad is the basic unit of measurement of capacitance.

30. The turns ratio is the number of conductor turns on the primary winding as compared to the number of turns on the secondary winding.

31. a. P 5 E 3 I 5 120 3 10 5 1200 watts b. R 5 E/I 5 120/10 5 12 ohms 32. A bus duct is an electrical conduit containing heavy conductors. It is used to distribute power (generally three phase) throughout an industrial plant.

33. Bus plugs are used to make connections between a bus duct and the load. They generally have a disconnecting means and short-circuit protection.

34. A combination starter is a complete motor-starter unit. It contains a disconnecting means, short circuit protection, a motor starter, START-STOP push-button switches, and, if the control voltage is lower than the line voltage, a control transformer is supplied. This equipment is housed in one enclosure.

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35. A dual-element or time-delay fuse is used.

36. An overload relay, a device that provides overload protection, is generally found on motor starters.

37. The symbol for a normally open limit switch is: 38. The symbol for a normally closed temperature switch is: 39. No. The circuit is not closed. There must be a return line from the light bulb to the battery in order for the light bulb to become energized.

40. It is given that the voltage of each battery is 15 V. When several batteries are connected in series with opposing polarities (1 to 2), the total sum is the sum of the individual batteries. Therefore, in the series connection, the voltage Et 5 60. When several batteries are connected in parallel, with like polarities connected together, the total voltage is the same as the individual battery voltage. Therefore, in this connection, the voltage Et 5 15.

41. When several batteries are connected in series, and two batteries with like polarities are connected together, the voltage of those two batteries will cancel each other. Therefore, in this circuit, the resultant voltage Et 5 20.

42. Ohm’s Law expresses the relationship of voltage, current, and resistance. It can be stated in three ways: E 5 IR

I5

E R

R5

E I

where E 5 voltage I 5 current R 5 resistance 43. A step-up transformer has a secondary voltage that is higher than the primary voltage. A step-down transformer has a secondary voltage that is lower than the primary voltage.

44. Symbol for a ferrule-type fuse: 45. A defective extension cord can be caused by a broken conductor, a loose connection in the plug or receptacle, or worn insulation.

46.

(A) Lines (conductors) connected

(B) Lines (conductors) not connected

47. The following combinations must be closed to energize the light bulb: A-D, B-D, C-D. 48.

49. The drawing shows a transformer regulation curve. You can determine the percent rated output voltage at any secondary load from 0 amperes to 60 amperes.

50. Temperature

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