Short note: AC servo motor

An AC servo motor is a two-phase induction motor designed to give torque proportional to a control voltage, used in feedback control systems for accurate position and speed control.

Construction

• Stator: two windings displaced by $90^\circ$ electrical:
  • Reference (fixed) winding, supplied with a constant AC voltage $V_r$.
  • Control winding, supplied from a servo amplifier with variable voltage $V_c$, $90^\circ$ out of phase with $V_r$.
• Rotor: squirrel cage (or drag-cup) with high resistance and small diameter, long length, giving low inertia and fast response.

   Vr (fixed) --> [Reference wdg]
                       |  90 deg
   Vc (from   --> [Control wdg] --> rotor --> load
   amplifier)

Working

• The two quadrature fluxes produce a rotating field; torque is proportional to $V_c$.
• When $V_c = 0$ the field is only pulsating, and because of the high rotor resistance the motor stops (it cannot keep running as a single-phase motor).
• Reversing the phase of $V_c$ reverses rotation.
• High $R_2/X_2$ gives an almost linear torque-speed curve with negative slope, which gives positive damping and stability.

 T |\
   | \  Vc = rated
   |\ \
   | \ \  Vc = 0.5 rated
   +--\-\---- speed

Applications

Position control in radar and antenna drives, instrument servos, X-Y recorders, robotics and computer peripherals.