Construction and Working of D.C. Servo Motor: The D.C. motor that is used in servo-mechanism is called a D.C. servo motor. There are three main types of D.C. servo motor.
Field controlled D.C. servo motor.
Armature controlled D.C. servo motor.
Permanent magnet armature controlled D.C. servo motor.
Field Controlled D.C. Servo Motor:
In this method the armature is supplied by the constant current source.
The error voltage represents the difference between the measured signal and the desirable signal.
The torque produced by the motor is zero when no field excitation is supplied by the d. c. error amplifier.
Since the armature current is always constant, the torque is directly proportional to the field flux and also it is proportional to the field current up to saturation.
If the polarity is reversed the motor direction is reversed
Armature Controlled D.C. Servo Motor:
In this method the armature is controlled by the electronic amplifier. The field winding is supplied by the constant current source. The error voltage represents the difference between the measured signal and the desirable signal. A sudden large or small change in armature voltage produces an error signal to cause change in torque. If the error signal and the polarity of the armature voltage is reversed, the motor reverses its direction.
Permanent Magnet Armature Controlled D.C. Servo Motor:
It uses permanent magnets for constant field excitation instead of the constant current source. These types of motors use either alnico or ceramic magnets to produce magnetic field. Permanent magnets have several advantages over wound field servo motors including increased efficiency, reduced frame size and high accelerating torque. The speed of a PMDC servo motor is varied by changing the voltage applied to the armature. Conventional wound field motors are often equipped with interpoles in order to improve commutation and minimize armature reaction. The high coercive force of permanent magnet materials used in PM motors eliminates the need for interpoles due to their excellent commutation characteristics.