Servoamplifier device

Claim 1

Apparatus for operating an AC servomotor having control and exciting coils, the apparatus being arranged to accept an AC power signal from an AC power source and arranged to accept a DC input signal and to control the AC servomotor therewith through a transformerless control circuit having reduced susceptibility to errors due to noise, comprising: a common potential reference line; an input circuit for comparing the DC input signal with a feedback signal controlled by the servomotor to obtain a difference signal referred to the common potential line and including chopper means balanced about the common potential line for converting the difference signal into two 180* out of phase signals referred to the common potential line and having amplitudes varying with the amplitude of the difference signal; a differential-input voltage amplifier having two differential inputs balanced about the common potential line and receiving the two AC signals on the two differential inputs and combining them into an AC output signal with amplitude and phase varying with the amplitude of the difference signal; a power amplifier for amplifying the output of the voltage amplifier to produce a signal referred to the common potential line to be applied to the control coil of the AC servomotor for driving the AC servomotor; power transformer means for coupling the AC power source to the servomotor exciting coil to isolate the AC power source from the servomotor exciting coil and to apply a signal to the exciting coil in phase with the AC power source signal, and means connected to the AC power source and including a 90* phase shifting circuit for operating the chopper means 90* out of phase with respect to the AC power source signal, whereby AC input noise signals and common mode noise signals are effectively elminated without an isolating transformer between the input circuit and the servomotor, and whereby safe operation of the servomotor is afforded.