The types of DC motors with field coils are classified as
- Series wound motor,
- Shunt wound motor,
- Compound motor, and
- Separately excited motor.
Series wound motor
The armature and field coils are in series. The motor exerts highest starting torque and has greatest no load speed. At light loads, the motor will run at high speed. The reversal of polarity of supply has no effect on direction of rotation of motor.
Shunt wound motor
The armature and field coils are in parallel. The motor has low starting torque and good speed regulation. Shunt motors are widely used because of good speed regulation and the direction of rotation is reversed by reversing the armature or field supplies.
The compound motor has two field windings one in series and another in parallel with armature. This types of DC motors combines the feature of series and shunt motor and is used to get high starting torque and good speed regulation.
Separately excited motor
This motor is considered to be a special case of shunt motor and has separate control over armature and field currents.
The graph shows the torque speed characteristics of the types of motors. Here, the speed can be changed by changing the armature current.
The speed control can be obtained by controlling the voltage applied to the armature. Fixed voltage supplies are often used. A variable voltage is obtained by an electronic circuit.
The commutator and the brushes suffer from wear and produce sparks often when they contact. Hence brushless permanent magnet D.C motor is designed.
In conventional D.C motor, the magnet is fixed and the current carrying conductors are made to move. In the brushless permanent magnet D.C motor, the current carrying conductors are fixed and the magnet moves. The rotor is a ferrite or ceramic permanent magnet.
Feedback is used in this circuit to modify the motor speed even if the conditions are changed.