Direct torque control of induction machines over a wide speed range

A control scheme for direct torque control (DTC) of a three-phase induction machine operating over a wide spread range is discussed. The DTC is based on deadbeat control of torque and flux utilizing the principles of stator flux field orientation. The stator flux can be calculated from the terminal quantities and the stator resistance. Variations in the stator resistance result in significant errors in the stator flux, especially at low speed. This problem is overcome in the proposed scheme by using the slip relation from indirect rotor flux field orientation to locate the position of the rotor flux. The rotor flux position is then used to locate the position of the stator flux. The magnitude of the stator flux is found using a frequency-dependent combination of the reference value and the flux value calculated from the stator quantities. Experimental results are presented for a 4 kW bipolar transistor inverter induction machine drive and a digital signal processor-based controller