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Types of Servo-motors

The servo-motors are of the following two types:

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

8.2.1. D.C. servo-motors

These motors are preferred for very high power systems since they operate more efficiently (as compared to A.C. servo-motors).

These motors may be of the following types:

–        Series motors;

–        Split series motors;

–        Shunt control motors;

–        Permanent magnet (fixed excitation) shunt motor.

i.            Series motors:

  • This motor has a high starting torque,
  • It draws large current.
  • The speed regulation is poor.
  • Reversal can be obtained by reversing field voltage polarity with split series field winding.

ii.            Split series motor:

  • The D,C. series motor with split field (small fractional kW) may be operated as a separately excited field-controlled motor (Fig. 29).


The armature may be supplied from a constant current source.

  • A typical torque curve shows the following:

–        High stall torque;

–        Rapid reduction in torque with increase in speed,

iii.            Shunt control motor:

  • This type of motor has two separate windings: Field winding placed on the stator and the armature winding placed on the rotor of the machine. Both the windings are connected to a D,C. supply source.
  • Whereas in a conventional D.C. shunt motor, the two windings are connected in parallel across the D.C. supply mains, but in a servo-application the windings are driven by separate D.C. supplies.

iv.            Permanent magnet shunt motor:

  • It is-a fixed excitation shunt motor where the field is actually supplied by a permanent magnet.
  • Its performance is similar to that of armature controlled fixed field motor.

8.2.2. A.C. servo-motors


  • These motors are best suited for low power applications.
  • Precision servo-motors are used in :

–        Instrument servos;

–        Computers;

–        Inertial guidance systems etc.

  • The mechanical output power of A.C. servo-motor varies from 2 watts to a few hundred watts.
  • An A.C. servo-motor is basically a two-phase induction motor except for certain special design features. The main important difference between a standard split-phase motor and an A.C. servo-motor is that the latter has thinner conducting bars in the squirrel cage motor, so that the motor resistance is higher. The torque-speed characteristics should be linear as shown by the curve II in Fig. 30.


Description of A.C. servo-motors:

  1. Drag-cup rotor servo-motor. Refer Fig. 31.
  • Drag-cup construction is used for very low inertia applications.
  • In this type of motor the rotor construction is usually of squirrel cage or drag-cup type ; here only a light cup rotates while the rotor core is stationary (thus inertia is quite small).
  • The servo-motors contains two windings namely, main winding (sometimes called      fixed or reference winding) and control winding. The voltages applied to-the windings are at right angles to one another. Usually one winding is excited with a fixed voltage while the other one is excited by the control voltage {which is the output from servo-amplifier}.
  • While in operation, the output torque of the motor is roughly proportional to the applied control voltage, and the direction of torque is determined by the polarity of the control voltage.


  1. Shaded-pole type servo-motor:
  • This type of motor employs a phase-sensitive relay to actuate those contacts which produce a short-circuit of the shaded-pole winding to produce rotation in the desired direction.
  • The main short-coming of this motor is that it responds only when the amplifier error signal is of adequate magnitude to cause the relay to operate.