Using induction motor stator windings to extract speed information

A speed regulator based on a field-oriented control was constructed using speed information available from the moving flux wave in the air gap of an induction motor. The information takes the form of ripples in the flux caused by the motion of rotor bars through the flux wave. This information is accessed by using voltages available from taps taken off the stator windings. It is then isolated using a switched capacitor filter and captured using a frequency slaved loop. Although the speed regulator is limited in its operating range and shows only moderate performance, such a system could be used in applications where high performance is not required. It is also found that some improvements can be made by using a machine with unskewed rotor bars instead of the conventional skewed rotor.<>