Servomotor Part 3-3
In a armature controlled d.c. servomotor, the control signal available from the servoamplifier is applied to the armature of the motor. The field winding is supplied with constant current hence the flux remains constant. So these motors are also called as constant magnetic flux motors. It is easy to keep field current constant rather than armature current. This is because presence of back e.m.f. in the armature.
The circuit of armature controlled d.c. motor is shown in the Fig. 2.
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| Fig. 2 Armature controlled d.c. motor |
In some case constant field is obtained with the help of permanent magnets. A control signal Va (t) available from the servoamplifier, is applied to the armature. This signal is based on the feedback information, supplied to the controller. Due to this, armature current Ia changes which inturn changes the torque produced. This changes the output of the motor as per the requirement. As flux is kept constant, the torque developed is proportional to the armature current.
The Ra and La are the resistance and inductive values of the armature, respectively. The inductance is usually small and can be neglected.
1.1 Features of Armature Controlled D.C. Motor
1. It is suitable for large rated motors.
2. The value of armature inductance is small. Hence its time constant is small. Hence motor can give quick response to the changes in the control signal.
3. The overall operation is a closed loop system.
4. The back e.m.f. provides internal damping which makes the motor operation more stable.
5. The efficiency and overall performance is better than field controlled motor.