A new speed-control method for induction motors
Presently, speed control of electrical drives is preferably done by cascaded control loops. In an inner control loop, the torque of the machine or some equivalent quantity ( e.g. torque generating current) and the flux of the machine is controlled, whereas in the next control loop the speed of the drive is controlled. To achieve fast control action, direct control methods should be used.
Two types of direct control methods have been developed in the past: Direct control of stator currents and direct control of torque and flux . In all cases, speed is controlled by a superimposed control loop.
In contrast to this, the presented method has no superimposed speed control loop. The switching events in the inverter are calculated directly in order to control the speed of the drive, without any subordinated control loop or any puls width modulator. The switching events in the inverter are put into effect in such a way, that in steady state the speed of the drive is kept in an (extremely small-) hysteresis band and a disturbed system will return to steady state as fast as possible, i.e. time-optimal.
As many drives e. g. in machine tools need a superimposed position control loop, the inner speed control loop is desired to be as fast as possible. This is achieved by the proposed time optimal speed control method.