x

By Grant Laidlaw

Problem Page

Experiencing problems of a technical nature, drop us a line at acra@netactive.co.za with problem page in the subject line.

Welcome to the PROBLEM PAGE. We find many people request assistance in the practical aspects of the industry. In response we will investigate the problem and endeavour to present the best possible solution. Thank you for all your questions sent in. Send your problems (and sometimes your creative solutions) to acra@netactive.co.za with problem page in the subject line. You may include pictures.

The part â&#x20AC;&#x201C; wind motor has 6 terminals. As can be seen from the diagram, the voltage across each winding is 380 â&#x20AC;&#x201C; 400 volts. In the part-wind motor 1, 2 & 3 indicates the one portion of the motor and the 7, 8 & 9 indicates that it is the second portion of the motor. The following is a typical power diagram control diagram for a part-wind starter.

Motorcar Nepalani asked for a simple explanation with regards to direct on line and three phase star delta starting. To complete the explanation we will look at a part wind motor connection. In order to understand part-wind starting you first have to understand what a part-wind motor consists of. In simple terms a part-wind motor is basically two motors in one casing. It can be said that within the part-wind motor, each of the two motors are started direct on line and, as such, will individually behave like direct on line motors. As the two motors each form a portion of the part-wind motor, I will, for the purposes of explanation, refer to the first and second portions respectively. When starting we apply current to firstly one portion of the motor. When the motor has started current is applied to the second portion. At this point therefore, current is supplied to both portions and the motor is in the run state. When the first portion of the motor is connected to the power supply it will start direct on line but being only half of the part wind motor only half the starting current is required. The torque produced will also be halved on starting. This is done to reduce stress created by the high torque and amperages drawn by a direct on line motor. (See July RACA Star-Delta starter) The following diagram illustrates the windings found in a part wind motor.

It is very important to note that the time delay between energising the first contactor and the second one is very short, usually only 0,1 to 0,3 seconds. Because the part-wind motor has 6 terminals, like the star-delta motor, it is very often incorrectly identified and treated as a star-delta started motor resulting incorrect connection of the motor. A further complication is that the time delay can be set too long. Should this be the case the result is that the motor will start on the first portion but when it comes up to the speed where the second portion motor is required, it could stall. The run down timer then energises the second portion of the motor at the set timing. The result is that both portions of the motor receive current and the motor will start in the same manner as a direct on line motor. This means that it will draw the full starting current and the advantage of part-wind starting is lost. Considerable stress is placed on both electrical and mechanical components due to this situation. It is therefore imperative to understand and be able to identify the various motor connections.

for resistance between Windings and Earth. Test terminals 1, 2, 3, 7, 8, 9, to Earth. Resistance must be above 0.5 Meg Ohm. I hope, Motorcar, that this has shed some light on the subject. I have had many requests to give more detail with regards to R410A piping schedules. It is become clear that when installing R410A systems you will have to stipulate your requirements when ordering copper tubing. In the case of the charts below, pay particular attention to the W.T. (Wall Thickness). Part wind motor testing Mechanical checks. Proceed as follows: Check: Name plate: Frame:

Information readable. Not damaged, Mounting feet not broken etc.

The information below is taken from a suppliers tubing schedule. The RC and RL schedules include the thinner wall thickness table and the thicker R410A compatible tables. “RC” Soft drawn copper tubing (R410A Compatible on far right)

Checking for continuity of windings.

Sizes RC soft drawn copper tubing Imperial

O.D.

W.T

(R410A) W.T.

¼”

6.35mm

0.71mm

0.91mm

Part-wind one: Terminals 1 to 2 (Ohm reading) Terminals 2 to 3 (Ohm reading) Terminals 3 to 1 (Ohm reading)

3/8”

9.53mm

0.71mm

0.91mm

½”

12.7mm

0.71mm

0.91mm

5/8”

15.88mm

0.71mm

0.91mm

All windings must give the same readings

¾”

19.08mm

0.81mm

0.91mm

Using a Multi-meter or Insulation tester set on Ohm scale. Identify the windings.

Part-wind two: Terminals 7 to 8 (Ohm reading) Terminals 8 to 9 (Ohm reading) Terminals 9 to 7 (Ohm reading)

“RL” Hard drawn copper tubing (R410A Compatible on far right)

All windings must give the same readings Part-wind one and Part-wind two may be the same value (50 - 50% split) or the values may differ to represent a 40% and 60% split. (In which case the 40% side represents the start winding) Using a Multi-meter or Insulation tester set on Ohm scale. Test for Earth-continuity from Earth-stud to all metal parts. Resistance must be below 0.2 Ohm’s. Using an insulation tester. Set on double the working voltage and using the Meg Ohm scale. Test for resistance between Part-one and Part-2 windings. Part-wind one, terminal “1” to Part-wind two, terminal “7” (Ohm reading) Part-wind one, terminal “2” to Part-wind two, terminal “8” (Ohm reading) Part-wind one, terminal “3” to Part-wind two, terminal “9” (Ohm reading) Resistance for all readings must be above 0.5 Meg Ohm.

Continue using the insulation tester. Set on double the working voltage and using the Meg Ohm scale and test

Sizes RL hard drawn copper tubing Imperial

O.D.

W.T.

(R410A) W.T.

3/8”

9.53mm

0.61mm

0.91mm

½”

12.7mm

0.61mm

0.91mm

5/8”

15.88mm

0.71mm

0.91mm

¾”

19.08mm

0.71mm

0.91mm

7/8”

22.23mm

0.81mm

1.22mm

1 1/8”

28.58mm

0.91mm

1.22mm

1 3/8”

34.93mm

1.02mm

1.22mm

REFERENCES: RECO Thanks to: Johan Gouws O.L. Gustavo Thanks to all for writing in with interesting questions. Looking forward to hearing from you – Grant Laidlaw

direct on line and three phase star delta starting

simple explanation with regards to direct on line and three phase star delta starting. To complete the explanation we will look at a part wi...