D.C. Servomotor

D.C. Servomotor

Basically d.c. servomotor is more or less same as normal d.c.motor. There are some minor differences between the two. All d.c. servomotors are essentially separately excited type. This ensures linear torque-speed characteristics.

The control of d.c. servomotor can be from field side or from armature side. Depending upon this, these are classified as field controlled d.c. servomotor and armature controlled d.c. servomotor. 

1. Field Controlled D.C. Servomotor

In this motor, the controlled signal obtained from the servoamplifier is applied to the field winding. With the help of constant current source, the armature current is maintained constant. The arrangement is shown in the Fig. 9.9.1.

This type of motor has large L_f/R_f ratio where L_f is reactance and R_f is resistance of field winding. Due to this the time constant of the motor is high. This means it can not give rapid response to the quick changing control signals hence this is uncommon in practice.

a. Features of Field Controlled D.C. Servomotor

It has following features :

i) Preferred for small rated motors.

ii) It has large time constant.

iii) It is open loop system. This means any change in output has no effect on the input.

iv) Control circuit is simple to design.

2. Armature Controlled D.C. Servomotor

In this type of motor, the input voltage 'V_a' iS applied to the armature with a resistance of Ra and inductance La. The field winding is supplied with constant current I_f. Thus armature input voltage controls the motor shaft output. The arrangement is shown in the Fig. 9.9.2.

The constant field can be supplied with the help of parmanent magnets. In such case no field coils are necessary.

a. Features of Armature Controlled D.C. Servomotor

It has following features :

i) Suitable for large rated motors.

ii) It has small time constant hence its response is fast to the control signal.

iii) It is closed loop system.

iv) The back e.m.f. provides internal damping which makes motor operation more stable.

v) The efficiency and overall performance is better than field controlled motor.

As the armature controlled d.c. servomotor is closed loop system, in comparison with open loop field controlled system, generally armature controlled motors are used.

3. Characteristics of D.C. Servomotors

The torque-speed characteristics of a d.c. motor basically describe the static torque producing capability of the motor with respect to the applied voltage and motor speed.

The effect of armature inductance is negligible in the steady state and hence the torque equation for a d.c. servomotor can be written as

From the above equation (9.9.1), it can be seen that the graph of Tm against rom is a straight line which is dropping in nature, for constant value of Va- This line represents torque-speed characteristics of a d.c. servomotor, for constant Va.

The Fig. 9.9.3 shows a typical set of torque-speed characteristics for various values of applied voltage.

The slope of the characteristics can be obtained from the equation (9.9.1), which is expressed as, 

The slope of such characteristics is negative.

The torque-speed characteristics of d.c. servomotors are determined experiementally in practice, by conducting various tests on the servomotor.

a. Other Performance Characteristics

The performance characteristics other than the torque-speed characteristics of a d.c. servomotor are shown in the Fig. 9.9.4.

As torque developed increases, the current and power input increases while the efficiency increases upto certain limit and then starts decreasing.

4. Applications of D.C. Servomotor

These are widely used in air craft control systems, electromechanical actuators, process controllers, robotics, machine tools etc.

Review Question

1. Write a note on d.c. servomotors.