Induction motor slip frequency controller

Abstract

A slip frequency controller for an induction motor. The controller includes an oscillator for providing a high frequency (f.sub.o) pulse stream which is counted down by first counter having modulo M and a second counter with modulo M+K, where M is a positive integer, K is a positive or negative integer, and K is small compared with M. An incremental shaft encoder provides a relatively low frequency pulse stream having a pulse repetition frequency equal to M times the shaft rotation frequency for the motor times the number of pole pairs in the motor. Each pulse produced by the encoder represents an angular increment in position of the motor shaft. The output frequency of the first counter is slightly modified from the nominal f.sub.o /M by a pulse adder/subtracter network which either adds or subtracts a pulse from the input pulse stream applied to the first counter each time a pulse is received from the shaft encoder. The output frequency of the first counter is then f.sub.o /M plus or minus the shaft frequency times the number of pole pairs. The output frequency of the second downcounter is approximately equal to ##EQU1## The output pulse streams from both counters are mixed to obtain a low frequency beat note which has a waveform that approximates a sinewave and which corresponds to the desired stator frequency. The slip frequency may be discretely varied by digitally controlling the modulus of the second counter. Alternatively, the slip frequency may be continuously varied by controlling the frequency of the high frequency oscillator.