Control of an AC brushless permanent-magnet machine using long supply cables

The use of a long cable between an inverter and motor is recognised as problematic and a cause of insulation failure in motors. The cable also introduces a voltage drop and phase shift in the fundamental voltage at the motor terminals, reducing the available motor torque in open-loop control, and affecting the stability of a closed-loop controller. This paper investigates the performance of a brushless, permanent magnet motor in order to determine the maximum operating distance between the motor and its inverter. The study develops simulation models of the motor in open and closed-loop modes, with and without the inclusion of a line filter. The ability to track changes in reference speed and reject torque disturbances is shown to depend on the resistance of the damper circuits in open-loop control, but is not an issue in closed-loop control. The maximum operating distance depends on cable resistance, and a practical operating limit of 2km is given for the motor-cable combination studied.