1. Apparatus for determining the rotational speed of an AC induction motor, supplied from an AC power source with a line frequency, having a predetermined number of poles, and having a synchronous speed which is two times the line frequency divided by the number of poles, said apparatus comprising: a magnetic flux transducer for sensing magnetic flux generated by the motor during operation, producing magnetic flux signals; and a data processing system including: a spectrum analyzer receiving signals from said magnetic flux transducer for generating a flux frequency spectrum having a set of peaks each characterized by a frequency and an amplitude, means for searching the flux frequency spectrum for a plurality of sets of particularly defined families of peaks, each set potentially including at least one family of peaks satisfying the particular family definition, having a family score and indicative of a possible rotational speed, and selecting for each set that includes at least one family the possible speed having the highest family score as a candidate speed, and means for determining whether there is a candidate speed with an overall score which exceeds a predetermined threshold and, if so, determining the motor rotational speed as that candidate speed.