DC Servo Motor

Field Controlled DC Servo Motors

Its speed is controlled by varying field current and its armature current is kept constant by using a constant current source as shown in Figure 3.

Figure 3.

Field controlled DC servo motor

Its torque is given by T = K φ I_a

If the polarity of the field is reversed, its speed is reversed. The control of field current by this motor is used for small motors. Its dynamic response is slower than armature controlled motors.

Armature Controlled DC Servo Motors

In this motor, a fixed DC current is supplied from a constant current source. A small change or sudden change in armature voltage produced by error signal will cause immediate response in torque because the armature circuit is resistive compared to highly inductive field current. The schematic diagram of this type of servo motor is shown in Figure 4.

Figure 4.

Armature controlled DC servo motor

This motor is usually operated will beyond the knee point on saturation curve to keep the torque less sensitive for slight changes of voltage from the constant armature source.

Its torque is given by T = K φ I_a. DC servo motor up to 1000 hp are armature controlled.

Permanent Magnet Armature Controlled DC Servo Motor

This type of motor uses permanent magnets either ceramics or alnico for constant field excitation. It is usually manufactured in 6V and 28 Volt ratings in fractional hp size and in 150V rating is integral hp sizes up to 20 hp. It is shown in Figure 5.

Figure 5.

Permanent magnet DC servo motor

Series Split Field DC Servo Motor

These are built in for fractional hp size. These can be used as separately excited field controlled motors. Its schematic is shown in Figure 6(a) and 6(b).

Figure 6.

(a) Separately excited DC servo motor (b) Directly excited DC servo motor

Its field is split into two windings. One is called main winding and the other is called auxiliary winding. Both windings has same MMF and are wound on field poles so as to produce reverse flux with respect to each other.

The advantages of split field are:

• 1.

  The dynamic response is improved because the field windings are always energized, so there is no delay due to inductive time constant.

• 2.

  The finer degree of control is obtained due to the smaller difference of current in main and auxiliary winding.

Large series motors are operated in directly excited made because separate large constant current source is not available. Obviously from the Figure 6(b), the armature current is sum of main and auxiliary windings but when the series field currents are equal and opposite, no torque produced. A small change of auxiliary winding current will produce an immediate torque and rotation in either direction.