CONTROLLER FOR INDUCTION MOTORS

摘要

Many induction motor controllers employ a reference angle relative to the supply voltage which is a target for the phase lag of current behind voltage but the choice of the optimum reference angle for energy saving varies from motor to motor. In the present invention firing signals for thyristors connected between a supply and a motor to be controlled are generated by firing circuits under the control of a microprocessor 18. The firing angle for the thyristors is continuously adjusted according to the phase lag as indicated by supply voltage zero crossings and current sensing circuits, load on the motor, and a signal indicating the tendency of the motor to stall obtained from the back e.m.f. of the motor. Using the said signal can be regarded as equivalent to adjusting the reference angle for the optimum value mentioned above. A procedure for starting a motor and setting the initial value of the reference angle is also described.