Servo motor construction and working

A servo motor is a small, high-performance motor used in a closed-loop control system for accurate control of position, speed or torque. It is fast-responding and its shaft position follows a control signal. Two main types are the AC (two-phase) servo and the DC servo motor.

Construction (two-phase AC servo motor)

• Stator: a laminated stator carries two windings placed $90^\circ$ apart in space: the reference (fixed) winding, fed from a constant ac source (often through a capacitor), and the control winding, fed with the variable ac error signal from the servo amplifier.
• Rotor: a squirrel-cage rotor (or drag-cup rotor) of high resistance ($R/X$ large), with small diameter and long length for low inertia and fast response.
• A tacho-generator and a position sensor (potentiometer or encoder) are fitted on the shaft for feedback.

 Ref. -> [Controller] -> error -> [Amplifier]
 input       ^                       |
 (set)       | feedback              v
        [Sensor] <--- shaft <--- [Servo motor] --> load

Working

1. The reference winding gets a fixed voltage $V_r$. The control winding gets the amplified error voltage $V_c$ (difference between desired and actual position), shifted $90^\circ$ in time from $V_r$.
2. The two currents with $90^\circ$ space and time displacement produce a rotating magnetic field in the air gap, which drags the rotor.
3. The torque and speed depend on the magnitude of the control voltage. A larger error gives a larger torque and speed. The direction of rotation depends on the phase of $V_c$ relative to $V_r$ ($\pm90^\circ$).
4. When the shaft reaches the desired position, the error becomes zero, $V_c = 0$, and the motor stops.

The high rotor resistance gives a negative-slope torque-speed characteristic over the full range, which provides damping and stability and prevents single-phase running when the control signal is removed.

Applications

Robotics, machine tools, radar and antenna positioning, aircraft control surfaces, and printers.