Input Admittance Characteristics of Permanent Magnet Brushless AC Motor Drive Systems

This paper analyzes the input admittance/impedance characteristics of a permanent-magnet brushless AC motor drive system, taking account of the influence of both current and speed control loops as well as the motor and inverter losses. It is shown that, if voltage feedforward compensation is not employed, the input admittance of the drive system differs significantly from that of an ideal constant power load, and is influenced by the speed and current control loop bandwidths. It is also shown that, when the drive is connected to a DC supply via an LRC filter, the magnitude of the input admittance at the filter resonant frequency has a crucial effect on system stability margin.