Improving the open-loop torque step response of induction motors

Driving functions which require changes of torque and speed of induction motors produce DC offsets or transients in the torque-producing flux and current waves in the machine. There are two time constants controlling the respective rates at which they disappear and if these are short compared with the rate at which torque and speed are required to change, they can be neglected. Robotic systems place critical constraints on the time taken by the motor to execute its instruction. The flux wave transient has a much longer time constant than the current wave and seriously limits the application of the motor in a control system. It is shown how it can be avoided by prior injection of a direct current and subsequent control of the driving functions to keep the flux constant. The motor becomes comparable in its response to DC servomotors when operated in this manner. The application of a single-phase high rate of rise current pulse to the stator at the appropriate space orientation, can improve the response so that it is better than would normally be allowed by the leakage reactance of the motor.