INDUCTION MOTOR SPEED CONTROL HAVING IMPROVED SENSING OF MOTOR OPERATIVE CONDITIONS

摘要

An apparatus and method controls the speed of an AC induction motor by varying the frequency and current to the motor from an inverter power supply. A Counter EMF regulator receives a reference signal corresponding to the desired speed, and a signal proportional to CEMF for feedback. The output of the CEMF regulator is provided to the power supply to control the current to the motor. The CEMF feedback signal is also utilized to vary the output frequency of the power supply in accordance with the magnitude of the CEMF. This maintains the magnetizing component of stator winding current at a constant value for all operating frequencies of the motor. A signal corresponding to the load current component of the stator winding adds to the speed reference signal to the CEMF regulator to increase CEMF and frequency with increasing load, thereby to compensate for increased slip and maintain constant speed. The phase displacement between CEMF and stator winding current is determined. If the phase displacement is less than 90 DEG , the motor is in the motoring condition and the frequency of the power supply is determined solely by the CEMF. If the phase displacement is greater than 90 DEG , the motor is regenerating, and the frequency of the power supply is increased to prevent regeneration, and to avoid the need for the power supply to absorb regenerative energy.