SOLUTION

Field controlled D.C. servomotor is an open loop control system.

• In this motor the control signal obtained by the servo amplifier is applied to the field winding and with the constant current source, the armature current is maintained constant.
• This type of motor has large reactance to resistance ratio (L/R ratio). Hence the time constant of the motor is high. Therefore it can’t give a rapid response to the quick changing control signals.
• It is an open loop control system because any change in the output has no effect on the input i.e speed. The speed will not be constant for load variations on the motor since there is not any feedback to the controller to indicate the change in speed due to load.