Speed and Torque Control of Polyphase Induction Motors

The induction motor is valuable in so many applications because it combines simplicity and ruggedness. Although a good number of industrial drives run at substantially constant speed, quite a few applications need variable speed. Speed-control capability is essential in such applications as conveyors, hoists, and elevators. Because the induction motor is essentially a constant-speed machine, designers have sought creative ways to easily and efficiently vary its speed continuously over a wide range of operating conditions. We only indicate the methods of speed control here.

The appropriate equation to be examined, based on Equation, is

n = (1 - S)n₁ = (1 - S) 120 f_s/P

where n is the actual speed of the machine in revolutions per minute, S is the per-unit slip, f_s is the supply frequency in hertz, P is the number of poles, and n₁ is the synchronous speed in revolutions per minute. Equation suggests that the speed of the induction motor can be varied by varying either the slip or the synchronous speed, which in turn can be varied by changing either the number of poles or the supply frequency. Any method of speed control that depends on the variation of slip is inherently inefficient because the efficiency of the induction motor is approximately equal to 1 - S. On the other hand, if the supply frequency is constant, varying the number of poles results only in discrete and stepped variation in motor speed. Indeed, all methods of speed control require some degree of sacrifice in performance, cost, and simplicity. These disadvantages must be weighed carefully against the advantages of speed variability.