DC Servo-motor:
The motors which are utilized as DC servo motors, generally have separate DC source for field winding and armature winding.
The control can be achieved either by controlling the field current or armature voltage. Field control has some specific advantages over armature control and on the other hand the armature control also has some specific advantages over field control. The type of control to be applied to the DC servo motor, is being decided depending upon its specific application.
The direction of rotation can be changed by changing polarity of the field. The direction of rotation can also be altered by using split field DC motor, where the field winding is divided into two parts, one half of the winding is wound for clockwise direction and other half is wound for anticlockwise direction. The amplified error signal is fed to the junction point of these two halves of the field.
The magnetic field of both halves of the field winding opposes each other. During operation of the motor, magnetic field strength of one half dominates other depending upon the value of amplified error signal fed between these halves. Due to this, the DC servo motor rotates in a particular direction according to the amplified error signal voltage.
The main disadvantage of field controlled DC servo motor, is that the dynamic response to the error is slower because of longer time constant of inductive field circuit. The field being an electromagnet, it is basically a highly inductive circuit.
Due to sudden change in error signal voltage, the current through the field cannot change suddenly, but reach to its steady state value after certain period depending upon the time constant of the field circuit.
That is why field controlled DC servo motor arrangement is mainly used in small servo motor applications.
The figure below illustrates the schematic diagram for a field controlled DC servo motor. In this arrangement the field of DC motor is excited by the amplified error signal and armature winding is energized by a constant current source.
The figure below shows the schematic diagram for an armature controlled DC servo motor. Here the armature is energized by amplified error signal and field is excited by a constant current source.
0 Comments