Control system for regulating the torque and speed of an electric motor

Abstract

A control system for regulating the torque and speed of an alternating current wound rotor motor used in the traverse drive of an overhead traveling crane comprises an SCR bank controlled by a firing circuit responsive to actuation of an operator-controlled induction master control. The control system further comprises means for deriving a torque signal indicative of instantaneous rotor torque, means for deriving a speed signal indicative of instantaneous rotor speed, and summing means for combining the torque signal, the speed signal, and a reference signal from the induction master control to provide an error signal which is used to operate the firing circuit to thereby control the power delivered by the SCR bank to the motor and insure that motor speed is constant. The means for deriving the torque signal comprises means for deriving a voltage signal indicative of the instantaneous line-to-neutral voltage across one of the motor stator windings; means for deriving a current signal indicative of the instantaneous current in a phase line to the motor stator winding; and multiplying means for multiplying the voltage and current signals to provide a wattage or torque signal which is proportional to the instantaneous torque being developed by the motor. The wattage supplied to the motor minus the wattage attributable to fixed motor loses is directly related to the torque delivered by the motor shaft. The means for deriving the speed signal measures the instantaneous frequency of the instantaneous current flow in the rotor (secondary).