External search coil as a means of measuring rotor speed of an induction motor

The study in this paper investigates whether the air gap harmonics can be identified by using an external search coil placed on the frame of an induction motor. For this purpose the study focuses on the prediction of rotor speed of an induction motor from external search coil emf, using spectral analysis techniques. In the first stage of the study the type of search coil which maximizes the induced emf is sought, also the position of the coil on the induced emf is investigated. Several motors are tested with a sinusoidal supply as well as a PWM inverter power supply and the search coil voltage is sampled. FFT analysis of the recorded data is performed in Matlab environment. An algorithm is developed for identifying the rotor speed using the FFT analysis results. It is shown that the developed algorithm is capable of predicting the motor speed under various load conditions and motor speeds. The speed estimation accuracy of this approach is also discussed. In short, this paper illustrates that the induced emf in an external search coil contains information on air gap harmonics, and these harmonics can be identified even when the motors are supplied with a PWM inverter.