Control and stabilizing system for damperless synchronous motor

Abstract

Stabilized operation of a damperless synchronous motor, which is driven by a voltage source inverter energized by a d-c bus voltage, is obtained under transient conditions, such as during sudden variations of load torque, in order to maintain the torque angle in the motor relatively stable at all times and within the stability limit. This is achieved by employing transient changes in either the d-c bus voltage, the d-c bus current or the motor voltage, which changes reflect transient variations of the torque angle, to rapidly adjust the inverter frequency as necessary to hold the torque angle reasonably constant. For example, if there is an abrupt load increase on the motor, the bus voltage tends to drop and the motor tends to slow down, the torque angle thereby tending to increase. The extent to which the angle tends to increase will, however, be rapidly reduced by dynamically decreasing the inverter frequency so that the rotating magnetic flux or mmf, produced by the stator windings of the motor, slows down to preserve the desired torque angle. When there is a normal variation in load demand, necessitating a different steady state motor speed, a set point voltage may be varied to change the amplitude and frequency of the inverter output voltage to the steady state levels needed to drive the motor at the new required speed.