A study on the transient behaviour of induction machines with the help of measurement coils

Sensorless field oriented control of induction machines has been of high interest in recent years since the mechanical sensor increases the cost of a drive and decreases its reliability considerably. Methods which do not fail at zero fundamental frequency have to rely on nonfundamental effects of the motor. These effects are caused by saliencies, which are mainly due to slotting and saturation of the iron core and become visible if the machine is excited with high frequency. From the machines response, the desired information about fluxor rotor position can be evaluated. In this paper, voltage pulses applied by the inverter serve as the high frequency excitation. The change of the armature currents during these pulses is modulated with the machines saliencies and therefore a control signal can be extracted. Measurements were made with 3 different induction machines and the analysis of the measured signals focuses on the effect of stator and rotor slotting. One machine is the reference machine while the second one has a distinct number of stator and rotor slots and the third machine differs in the stator winding scheme. It is seen that these two design parameters influence the signal component related to rotor slotting to a high extent. Maximizing this signal component is advantageous for sensorless position control. If the flux position is to be detected, the machine design should be chosen to minimize the appropriate signal component