Evaluation of inductance measurement methods for PM machines

For simulation purposes and developing controllers, accurate motor parameters are needed to successfully estimate motor performance. Plenty of research has been done on this issue. However, standard procedures are not established yet. This paper concentrates on the measurement of the most critical parameter for permanent magnet motors, namely motor inductances of such motors. Frequency dependence, current dependence and the effect of the power supply waveform on measurements are investigated. Tests are done both at standstill and under running conditions and thus the results of various measurement methods could be compared. Also the effect of the d-axis stator current on the inductance is investigated. Tests are performed on a surface mount PM motor, for which little work appears to be reported as far as measurements are concerned. It is discovered that the inductance measurements with sinusoidal and PWM supply give very close results. It is recognized that during standstill tests measurements are advisable also by reversing the direction of the stator MMF. It is found that direct axis inductance measurements from no-load running test, with stator current direct-axis component in the magnetization direction, gives very close results to standstill tests. Inductance measurements are done also under load. It is discovered that for accurate results this test must be done at a low supply frequency and with a large phase current.