DC servomotors are further classified in two more categories:
Field Controlled DC servomotor:
The construction of field Controlled DC motor is shown below:
We are getting and signal from servo amplifier and it is applied to the field winding. In this type of motor armature is connected to the constant current source. The field current will be controlled by the voltage from servo amplifier and this will control the flus produced in the motor. The voltage produced in the servo amplifier will be in proportional to the error signal produced in the control system in which the servomotor is being used this amplified error signal obtained from servo amplifier is used for controlling the field current.
The response of DC servomotors to any change in the servo amplifier output is explained below:
1) As the error will change, the voltage from servo amplifier will also change
2) Due to this filed current will also change
3) This will change the flus
4) And then motor will respond to the change in flux Î¦ so as to minimize the error.
Disadvantages of field controlled DC servo motors:
1) The time Constant Lf, Rf is large where Lf is the inductance of the field winding and Rf is its resistance.
2) Hence the field current will also change slower in response to the any change in Vf.
3) And due to the motor will respond slowly to any error produced in the system in which it is being used.
4) This drawback can be overcome by using the armature controlled servo motors.