SolutionsEngineering tor is the single phase AC induction motor. This is the workhorse found in most household air-conditioning units, refrigerators, washers and dryers, although it does not lend itself to the more advanced control because the stator windings cannot be individually controlled. With this overview behind us, the following sections provide more information on each of these four motor types.
Step Motors: Do We Have a Pulse?
Hall Inputs
6-step (Trapezoidal Commutation)
Sinusoidal Commutation
Step motors are self-positioning and thus do not require an encoder, although many applications often add an encoder so that a “stall” can be detected during motion. Step motors are also “brushless,” meaning there is no direct electrical/mechanical contact to the rotor. That eliminates problems that can occur with mechanical commutation such as arcing or physical degradation. Finally, step motors I II III IV V VI produce a high torque for a given size and weight. A Despite these advantages, step motors have drawbacks. B The most significant is that C step motors create noise and induce vibrations that can disturb the load. Vibration can be A reduced using microstepping drive techniques or even mechanical dampers, but these solutions seldom eliminate the B problem completely. C Another significant limitation of step motors is that they have low high-end speeds. For A most systems 5,000 RPM is the most that can be expected. And the torque that is available from B a step motor drops significantly C at higher velocities. Finally, step motors are generally not 0 60 120 180 240 300 available in power ranges above several hundred watts. The Figure 2 The excitation schemes of a brushless most common National ElectriDC motor showing the relationship cal Manufacturers Association between the drive signals and the (NEMA) motor sizes for step feedback Hall sensor signals. motors are 17, 23 and 34. Desired Torque
Q +_ Loop
Q PI Command Filter
D Loop
D Command
+_
Zero
Park Transfer
PI Filter
Clarke Transfer
Phase A Inverse Park Transform
Phase B
Phase A
PWM Generator Phase B
Motor Encoder
Phase C Measured Current A Measured Current B Encoder
Figure 3
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Control of an AC induction motor involves much more compute-intensive processing, but the declining cost of the electronics is making this an increasingly popular choice.
March 2007
DC Brush Motors: Old Faithful
DC brush motors are used in a wide variety of applications that require positioning, and often in simpler applications such as speed or torque control. A special kind of DC servo motor known as a universal motor has windings in both the rotor and stator, and is used in high-volume everyday items such as hand drills and kitchen appliances. By itself, however, a DC brush motor has no sense of position. This means it must be connected to an encoder for use in positioning applications. The encoder provides the feedback, which is connected to a controller, which in turn generates an output command using a PID algorithm or similar servo scheme. The servo controller’s “job” is to minimize the difference between the desired motor position and the actual measured position. DC brush motors are available in a large variety of sizes, up to a kilowatt and beyond. They can operate at speeds of 10,000 RPM and even higher. DC servo motors run smoothly, and relatively quietly. DC servo motors have two primary disadvantages however. The first is that DC brush motors require a mechanical device for commutation. These brushes can wear out, or cause electrical arcing. The second disadvantage is that DC servo motors have a relatively low torque output for a given size and weight. This is due to the fact that DC brush motors drive current through coils located on the rotor. From a thermodynamic standpoint the rotor is not well-connected to the motor frame, and therefore the total amount of energy that can be removed from the coil is limited. This in turn limits the available torque output of DC servo motors.
Brushless DC Motors: The Overachiever
Brushless DC motors are rapidly becoming the overall motor of choice because for many applications they provide a “no compromise” solution to positioning control. Brushless DC motors are relatively smooth and quiet, and do not require mechanical brushes for commutation. In addition, brushless DC motors locate heat-generating coils in the stator, which is directly connected to