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 d.c. servomotor can be form 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. Filed 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.1.
This type of motor has large Lf/Rf ratio where Lf is reactance and Rf is resistance of field winding. Due to this the time constant of the motor is high. This means it cannot give rapid response to the quick changing control signals hence this is uncommon in practice.
1.1 Feature 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 lop 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 'Va' is applied to the armature with a resistance of Ra and inductance La. The field winding is supplied with constant current Lf. Thus armature input voltage controls the motor shaft output. The arrangement is shown in the Fig. 2.
The constant field can be supplied with the help of permanent magnets. In such case no field coils are necessary.
2.1 Feature 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.
1.3 Characteristics of D.C. Servomotors
The characteristics of d.c. servomotors are mainly similar to the torque-speed characteristics of a.c. servomotor. The characteristics are shown in the Fig.3.
1.4 Application of D.C. Servomotor
These are widely used in air craft control systems, electromechanical actuators, process controllers, robotics, machine tools etc.