Construction & Working of Servo Motor


The servo motor is also called as control motor. The torque capacity of the servo motor is very high. The application of the servo motor is for higher torque, accurate speed and position control. The main characteristic of servo motor is

  • It generates torque at all speed except zero speed.
  • It does not overheat at low speed or at standstill condition.
  • It reverses in the reverse direction very fast due to low inertia.
  • It accelerates very fast and speed decreases also very fast.
  • Large length & small rotor diameter
  • It operates at very low speed. When it operates at low speed, blower or cooling fan is used for cooling.
  • Generally, speed sensor device or tachometer is connected to servo motor shaft.
  • Its rating is usually less than 100 W.

How servo motor is operated?

  • In the servo motor, control circuit and potentiometer are connected at the outer shaft. The control circuit closely monitors the angle of servo motor shaft.
  • The servo motor shut down, if the shaft is at correct angle.
  • The servo motor continues run until the shaft of the motor does not set at correct angle.
  • Normally, the servo runs for 0 degree to 180 degree. It does not run greater than 180 degree due to mechanically stop in the output gear. 
  • The input power in the motor depends upon shaft rotation angle. 
  • The servo motor rotates at low speed if the shaft rotates at small angle.
  • In the three wire servo system, control signal is given to one wire, power signal is given to other wire and third wire is grounded. 
  • The control wire controls the shaft angle. The angle depends upon pulse given to the control wire. It is called as – Pulse Coded Modulation.

Rotation of shaft

  • The servo motor monitors pulse signal at every 0.02 second.  
  • The rotation of shaft depends upon width of pulse. 
  • The pulse of 1.5 msec is called as neutral position. 
  • The motor shaft rotates at 90 degree due to this pulse. 
  • If the pulse width is less than 1.5 msec, the motor shaft rotates at 0 degree similarly the motor shaft rotates at 180 degree for pulse width greater than 1.5 msec

Pulse time

Rotation shaft angle

1.5 msec

90 degree

< 1.5 msec

0 degree

> 15 msec

180 degree



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