Basics of Servo motor, it's advantages and disadvantages

What is a Servo motor?

A servo is a small device which has an output shaft which positions on coded signal. It is a rotary or linear actuator that allows for precise control of angular or linear position, velocity and acceleration.

The servo motor is which respond to signal abruptly and accelerate the load quickly are called servo motor.

What is a servo?

Servo motor is constructed out of basic DC motor, by adding:

  • Some gear reduction
  • A position sensor for a motor shaft
  • An electronic circuit that controls the motor operation
  • A variable resistor potentiometer that measures the output shaft at all times so the controller can accurately place.


Servo motor has the same construction of DC motor It has the stator, rotor and controlling parts. It has feedback for controlling the speed & torque. It has two port one for dc supply and other for controlled DC supply.

They have separate DC source for field winding and armature winding. The control is achieved either by controlling field current or armature current. The type of control is defined as the type of application.

##Types of servo motors:

DC servomotors:

DC Servomotors are separately excited dc motor or permanent magnet dc motor. They are controlled by armature voltage. The armature is designed to have large resistance so that the torque-speed characteristics are linear and have a large negative slope as shown below. Therefore, a step change in the armature voltage results in a quick change in the position or speed of the motor.

AC servomotor:

It is a two-phase a.c. induction motor. There are two winding, one fixed or reference winding is supplied with a fixed voltage and frequency from a constant voltage source. Second winding is called control winding, with variable supply voltage of same frequency

The stator has two distributed windings displaced 90 electrical degree apart. One winding is the reference phase and is connected to a constant voltage source. The other winding is the control phase and is supplied with a variable voltage of the same frequency as the reference phase but is phase-displaced by 90 electrical degree.


  • High output power relative to motor size and power

  • Encoder determines accuracy and resolution.

  • Resonance and vibration free operation

  • High efficiency

  • There is no out-of-step condition, as heavy load placed on the motor the driver will increase the current to the motor

  • High speed operation is possible


  • Require tuning to stabilize feedback loop

  • Complex, require encoder

  • Poor motor cooling

  • Motor can be damaged by sustained overload.