A.C. Motor control circuit

An a.c. motor control circuit comprising:

• an a.c. motor having a plurality of stator windings and a rotor;
• a tachometer coupled to said rotor and operable to produce an output signal proportional to the speed of said rotor;
• means for generating a velocity command signal having an amplitude proportional to a desired speed for said motor;
• means for comparing the output signal of said tachometer to said velocity command signal and for generating an error signal proportional to the difference between said tachometer output signal and said velocity command signal;
• a d.c. power source;
• a first switching circuit coupled to the output of said d.c. power source;
• modulator means coupled to said first switching circuit and responsive to said error signal for periodically opening and closing said first switching circuit to cause said first switching circuit to produce a train of d.c. pulses having a substantially fixed frequency and having a variable pulse width which is proportional at any time to the amplitude of said error signal;
• a filter coupled to the output of said first switching circuit for filtering said train of d.c. pulses to produce a d.c. voltage which is substantially proportional to the amplitude of said error signal;
• an inverter coupled between the output of said filter and the stator windings of said motor, said inverter being operative to change said d.c. voltage into an a.c. voltage of variable frequency to drive said motor, said inverter including a variable frequency oscillator responsive to said velocity command signal for controlling the frequency of said a.c. voltage in accordance with said velocity signal.