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Servo motor operating principle

Short answer

A servo motor operates in a closed-loop system, where an error signal between reference and feedback drives the motor. In a two-phase AC servo, a control winding voltage 90° out of phase produces torque proportional to the error. High rotor resistance gives a linear torque-speed curve with negative slope for stable positioning.

Explain the operating principle of servo motor.

A servo motor is a motor used in a closed-loop control system to give precise control of position, speed or torque, in response to a control signal. It has a low-inertia rotor, a fast response and can start, stop and reverse quickly. Types are d.c. servo motors and a.c. (two-phase) servo motors.

Operating principle (closed loop)

 Reference ->(+)-> Error -> Amplifier -> Servo ->  Load
 input       ^ (-)  signal              motor     |
             |                                    |
             +------ Feedback (tacho / encoder) -+
  1. A reference signal (desired position or speed) is compared with the feedback signal from the output (a potentiometer, tachogenerator or encoder).
  2. The difference, the error signal, is amplified and applied to the control winding (a.c. servo) or armature/field (d.c. servo).
  3. The motor develops torque proportional to the error signal and turns the load in the direction that reduces the error.
  4. When the output reaches the desired value, the error becomes zero and the torque falls to zero, so the motor stops. The torque reverses if the output overshoots.

Principle of the two-phase a.c. servo motor

  • The stator has two windings 90° apart. The reference (fixed) winding is excited from a constant a.c. supply, often with a capacitor. The control winding is excited by the amplified error signal, 90° out of phase with the reference voltage.
  • The rotor is a squirrel cage or drag cup of high resistance (R/X ratio high), to give a torque which falls linearly with speed (negative slope) and to avoid single-phase running.
  • The torque is proportional to the control voltage, and the direction of rotation depends on the phase (lead or lag) of the control voltage with respect to the reference.
 Torque
   |\   Vc = high
   | \\  Vc = medium
   |  \\\ Vc = low
   +-----\----> speed (nearly linear, negative slope)

Features: low inertia (long, thin rotor), linear torque-speed characteristic, quick response, no hunting, stable.

Uses: robotics, radar and antenna positioning, machine tools, process control and instruments.

✓ Answer checked by Claude Opus 5.5 (high effort)

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