Servo Motor

Servo motors are also called Control motors. They are used in feedback control systems as output actuators and does not use for continuous energy conversion. The principle of the Servomotor is similar to that of the other electromagnetic motor, but the construction and the operation are different. Their power rating varies from a fraction of a watt to a few hundred watts.

The rotor inertia of the motors is low and has a high speed of response. The rotor of the motor has a long length and smaller diameter. They operate at very low speed and sometimes even at zero speed. The servo motor is widely used in radar and computers, robots, machine tools, tracking and guidance systems, processing controlling, etc.

Applications of the Servo Motor

The power rating of the servo motor may vary from the fraction of watts to a few hundred watts. The rotor of the servo motor has low inertia strength, and therefore they have a high speed of inertia. The Applications of the Servomotor are as follows:

  • They are used in Radar systems and process controllers.
  • Servomotors are used in computers and robotics.
  • They are also used in machine tools.
  • Tracking and guidance systems.

Classification of Servo Motor

They are classified as AC and DC Servo Motor. The AC servomotor is further divided into two types:

servo-motor-figDC Servo Motor

DC Servo Motors are separately excited DC motors or permanent magnet DC motors. Figure (a) shows the connection of Separately Excited DC Servo motor and figure (b) shows the armature MMF and the excitation field MMF in quadrature in a DC machine.


This provides a fast torque response because torque and flux are decoupled. Therefore, a small change in the armature voltage or current brings a significant shift in the position or speed of the rotor. Most of the high-power servo motors are mainly DC.

The Torque-Speed Characteristics of the motor are shown below:

Servo Motor fig 2

As from the above characteristics, it is seen that the slope is negative. Thus, a negative slope provides viscous damping for the servo drive system.

AC Servo Motor

The AC Servo Motors are divided into two types 2 and 3 Phase AC servomotor. Most of the AC servomotors are of the two-phase squirrel cage induction motor type. They are used for low-power applications. The three-phase squirrel cage induction motor is now utilized for applications where a high power system is required.

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