Abstract: The voltage and frequency of an A.C. excitation signal applied to the stator windings of an A.C. induction motor are varied in proportion to the amplitude of current flowing in the stator windings. The stator current flow is sensed by a feedback resistor in series with the inverter which generates the A.C. excitation signal for the stator windings. The voltage drop aross the feedback resistor is summed with a VELOCITY COMMAND voltage which indicates the desired speed of the motor and the summed voltage controls the frequency and the amplitude of the A.C. excitation voltage in a constant volts per hertz manner. The stator current feedback holds the motor speed constant in spite of variations in the load on the motor.

Abstract: A tachometer is coupled to the rotor of an A.C. motor and generates an output voltage proportional to the rotor's speed. A VELOCITY COMMAND voltage is generated having an amplitude which specifies a desired rotor speed. The tachometer voltage is subtracted from the VELOCITY COMMAND voltage to produce a VELOCITY ERROR voltage. The output of a D.C. power source is chopped at a fixed frequency and variable pulse width, the pulse width being proportional at any time to the amplitude of the VELOCITY ERROR voltage. The chopped D.C. voltage is filtered to provide a variable D.C. voltage having an amplitude which is proportional to the amplitude of the VELOCITY ERROR voltage. The variable D.C. voltage is applied to an inverter which changes it into an A.C. voltage. The A.C. voltage is applied to the stator windings of the motor to drive the rotor thereof. The frequency of the A.C. voltage is variable and is proportional, at rotor speeds above 150 RPM, to the amplitude of the velocity command voltage.

Abstract: Two transistors each have their emitter-collector circuits coupled in series between one end of a motor field winding and the opposite terminals of a D.C. voltage source to switch current flow in either direction through the winding. Two current sensors are provided, one coupled in series with each emitter-collector circuit. The voltage developed across each current sensor is applied through a driver circuit to the base of the other transistor so as to cut one transistor off when the other is conducting and vice versa, thereby eliminating the possibility of having both transistors conduct simultaneously which would short out the voltage source and burn out one or both transistors.