Field Controlled D.C. Servomotor

Servomotor Part 3-2

       In a field controlled d.c. servomotor, the electrical signal is externally applied to the field winding. Hence current through the field winding is controlled, inturn controlling the flux. This motor is also known as separately excited motor or variable magnetic flux motor.

       In a control system, the error signal is generated by a controller by comparing the actual output with the reference input. Such an error signal is not enough to drive the d.c. motor. Hence it is amplified by an amplifier called servoamplifier. Such a signal obtained from a servoamplifier is applied to the field winding. With the help of constant current source, the armature current is maintained constant.

        The circuit is shown in the Fig. 1.

Fig. 1 Field controlled d.c. servomotor

       When there is change in the voltage applied to the field winding V_f, the current through field winding changes. This changes the flux produced by the field winding responsible for the change in the motor characteristics. The motor has large L_f/R_f ratio where L_f is the field inductance and R_f  is and is the field resistance. Due to this the time constant of the motor is high. Control signals hence field controlled d.c. servomotor is uncommon in practice.

1.1 Features of Field Controlled D.C. Servomotor

       It has following features :

1. It is preferred for small rated motors

2. It has large field inductance to resistance ratio.

The ratio of inductance to resistances determines the time constant and hence the time constant of field controlled d.c. servomotor is large.

3. The overall operation is an open loop system.

4. The control circuit is simple to design.