Construction & Working of Servo Motor

Introduction of Servo Motor

  • In this article, characteristics, operation and rotation of shaft of servo motor is given. 
  • 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. 

Characteristics of Servo Motor

  • 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.

Servo Motor Working

  • 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 of Servo Motor

  • 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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