ASHRAE Journal - October 2012 - 81

emerging technologies
Variable Speed Applications

induction or Pm motors
By Darrell J. King; Alissa cooperman; John Dieckmann, Member ASHRAE; and James Brodrick, Ph.D., Member ASHRAE

VAC&R systems rely on motors to run their integral components, blowers, fans, and compressors. Two main types of motors are

used: induction and permanent magnet, which can each be divided into a variety of configurations. The efficiency and efficacy of these motors play a major role in determining the overall system efficiency and power consumption.
If motor power consumption is reduced, while overall system capacity is maintained, system efficiency increases. Also, most AC and refrigeration equipment are used for variable loads and do not need to run continuously at full capacity. Instead of operating in on-off mode or throttling the relevant fluid flows to reduce the system’s capacity, variable speed control for motors reduces system capacity while reducing system energy consumption. For air and water flow, the associated pressure losses are proportional to the square of the flow rate. The flow rate is proportional to the speed of the motor. Therefore, the power drawn by the motor is proportional to the flow rate cubed. When an application can use less than 100% of the rated flow, be it of air or water, the relevant motor draws less power. Through variable speed control, compressor motor power consumption can be reduced by reducing or eliminating on-off cycling losses and reducing heat transfer temperature differences. In this article, the differences between induction and permanent magnet motors, and means to vary their speed of operation, thus their power consumption, will be discussed.

technologies
All motors, induction or permanent magnet, have the same basic components: a stator, a rotor, a shaft, and a motor housing (Figure 1). Also, they all operate via generated magnetic fields that create torque that spins the rotor to create usable mechanical work. The means by which the magnetic fields are generated varies by type and construction of motor. AC Induction Motor (IM) Single-phase or polyphase induction motors are widely used and available, as they are inexpensive to manufacture and reliable. They use various physical designs, such as wound or squirrel-cage rotors, and are categorized as asynchronous motors. Unlike synchronous motors, which run in tandem with the ac power frequency at a rotational speed inversely proportional to the number of poles, induction motors run at somewhat less than the ac power frequency. A rotating magnetic field in the stator is created by the applied ac power. The rotating speed of this field is synchronous with the ac power frequency. The rotor contains shorted conductors, into which currents are induced causing magnetic fields opposing those in the stator, thus causing the

rotor to turn. The rotor constantly tries to match the speed of the stator’s magnetic field. The difference in speed is called “slip.” As the load increases, so does the slip, since the required torque is produced in proportion to the amount of slip. Small single-phase induction motors, as seen in many HVAC&R applications, require some means of developing starting torque in the proper direction. The means include: • A copper band around a segment of each of the motor’s poles (shaded-pole motor configuration); • Start windings 90° to the main winding and of a different resistance wire; and • Introduction of a capacitor. All of these distort or phase-shift part of the magnetic field to create needed torque. After the motor has been started, the start windings and capacitors can either be electrically isolated from the motor circuit by a switch or remain connected, depending on the desired characteristics. Three-phase IMs, which are among those most commonly found in HVAC applications, are self-starting due to the sequential relationship of the voltage waveform phases. Though simple and reliable, IMs have a number of drawbacks. Single-phase IMs work well only for fixed-speed applications. The best power factor than can be achieved is about 85%.1 Below a few horsepower, induction motor efficiencies decrease compared to permanent magnet options. Many HVAC applications benefit from being able to vary motor speeds for fans and pumps to handle variable load conditions. Substantial energy savings can be realized since power drops off as the cube of the speed. Therefore, motors of all efASHRAE Journal 81

October 2012

ASHRAE Journal - October 2012

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