A typical two-phase AC servo-motor has a squirrel cage rotor and a field consisting of two windings:
- a constant-voltage (AC) main winding.
- a control-voltage (AC) winding in quadrature with the main winding so as to produce a rotating magnetic field. Reversing phase makes the motor reverse.
An AC servo amplifier, a linear power amplifier, feeds the control winding. The electrical resistance of the rotor is made high intentionally so that the speed/torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load. In the World War II Ford Instrument Company naval analog fire-control computers, these motors had identical windings and an associated phase-shift capacitor. AC power was fed through tungsten contacts arranged in a very simple bridge-topology circuit to develop reversible torque.
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