Duty cycle based volt/hertz inverter control for induction motor drives

A simple scheme for speed control of induction motors has been presented in this paper. The scheme employs a timer designed to have variable OFF time keeping the ON time of the pulses constant. The timer is digitally locked with a three-phase pulse generator and the inverter circuit in such a way that any change in the OFF time changes the frequency and the duty cycle of the inverter in the same proportion. Hence the V/f ratio is maintained constant over a wide range of frequencies. A new inverter circuit has also been designed which does not require any pulse transformers or opto-couplers to isolate the bases of the transistors. Thus, the complexity of the circuit has been greatly reduced. The circuit was built and tested in the laboratory and the performance was found to be highly satisfactory