Motor control system for variable speed induction motors

Abstract

A control system for a multiphase induction motor includes microcontroller control means providing a variable frequency signal and deriving high side/low side phase signals from the variable frequency signal. Power is applied from an AC/DC converter to the motor by an output drive bridge with three pairs of high side/low side semiconductor switches, one pair for each phase. The high side and low side phase signals are applied through high side/low side driver means to control the switches and effectively apply a quasi-square waveform to the motor windings. The motor and the control system electronics are protected from excessive current levels by current sensing means. When the current exceeds a predetermined level, the microcontroller limits the power drawn by the motor by ramping down the variable frequency to lower the speed of the motor. In case the motor or control electronics overheat, a temperature sensor is monitored by the microcontroller to shutdown the motor. When the motor is started, current inrush is avoided by modulating the phase signals during a start up period. The duty cycle of the modulation is gradually increased in steps until a duty cycle of 100% is reached at the end of the starting period. The voltage level applied to the motor may be reduced. In one embodiment, a voltage controller generates a pulse of fixed width which controls the duty cycle of the converter to lower the voltage level to a particular voltage level when enabled by an enabling signal from the microcontroller. In a second embodiment, a binary signal from the microcontroller to the voltage controller selects a particular pulse width to select the voltage level applied to the motor when enabled by the enabling signal.