There are two kinds of servo-motor:

  1. A.C. servo-motors.
  2. D.C. servo-motors.

8.2.1. A.C. servo-motors


These motors are utilized for low power applications. A Precision servo-motor is used as:

  1. Instrument servos
  2. Inertial guidance systems
  3. Computers,etc.
  • The output power of any A.C. servo-motor ranges from 2 watts upto a few hundred watts.
  • The primary difference between a standard split-phase motor and an A.C. servomotor is that the second one has higher resistance because of has thin conducting bars in the squirrel cage motor in comparison to the first one. Some specially designed features, an A.C. servomotor is typically had two-phase induction motor. Graph follows:

A.C. servo-motors:

  1. Drag-cup rotor servo-motor. Refer Fig. 31.
  • Drag-cup rotor servo motor is used in low inertia purposes.
  • These motors contain a squirrel cage type motor where only a light cup rotates while the rotor core is stationary.
  • The voltages that act on the windings remains at right angles.The servo-motors comprises of two windings. Namely, main winding also called fixed, or reference winding and the controlled winding.  The excited winding is controlled by the fixed voltage while the other one is controlled by the control voltage.
  • While in operation, the output torque of the motor is almost considered proportional to the applied control voltage, and the direction of torque is calculated by the polarity of the control voltage.
  1. Shaded-pole type servo-motor:
  • These motor works as a phase-sensitive relay to make those contacts come in function which helps in the production of a short-circuit of the shaded-pole winding to create the rotation in the desired direction.
  • The only drawback of this motor is that it returns only when the amplifier error signal shows some proper magnitude to cause the relay to come in action.


8.2.2. D.C. servo-motors

These are the most choice motors because of its very high power systems,and also they operate very efficiently in contrast with A.C. servo-motors. These are the following types of D.C. servo motors:

  1. Series motors;
  2. Permanent magnet (fixed excitation) shunt motor.
  3. Split series motors;
  4. Shunt control motors;
  1. Series motors:
  • This motor has an excellent initial torque,
  • The speed regulation is thus weak.
  • It lures a large number of currents.
  • By reversing the field voltage polarity with the split series field winding the system can be reversed.
  1. Split series motor:
  • The D,C. series motor with the split field can be functioned as a distinctly excited field-controlled motor (Fig. 29).

The armature is provided with a constant current source.

A characteristic torque curve consists the following:

  • High stall torque
  • The quick decrease in torque with the escalation in speed

iii.            Shunt control motor:

  • This motor has two separate windings: Armature winding on the rotor and Field winding on the stator. Both the windings are linked up to a D,C.Supply source.
  • In a conventional D.C. shunt motor, the two windings are connected up to a parallel D.C. supply source, but in a servo application, the windings move by separate D.C. stores.
  1. Permanent magnet shunt motor:
  • It is a shunt motor where the field is created by a permanent magnet.
  • The performance of it is similar to that of fixed field motor produced by the armature.


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