Accurate speed and slip measurement of induction motors

Two alternative hardware circuits, for the accurate measurement of low slip in cage induction motors, are discussed. Both circuits compare the periods of the fundamental of the supply frequency and pulses from a shaft-connected toothed-wheel. The better of the two achieves accuracy to 0.5% of slip over the range 0.1 to 0.005, or better than 0.001% of speed over the range. This method is considered useful for slip measurement of motors supplied by either constant frequency mains or variable speed controllers with PWM waveforms. It is demonstrated that accurate slip measurement supports the conclusions of work previously done on the detection of broken rotor bars.