DC Motors

DC Motors

DC motors consist of two magnets: a stator and an armature.

The stator is an electromagnet or a permanent magnet. The stator coil windings are called field windings. The stator is fixed in place so that it does not move.

The armature winding is supported by bearings. It is free to rotate. The armature is also known as the rotor.

Fig. 17 Cutaway of DC motor

The stator creates a magnetic field. When the armature is energized it creates a new magnetic field. These two fields interact and try to line up. For the armature field to line up with the stator field, the armature must rotate.

Fig. 18 The rotor magnetic field tries to line up with the stator magnetic field, and so the rotor rotates.

Once the armature and stator fields are aligned there is no more tendency for the armature to move. If the voltage energizing the armature is reversed, the armature will turn in order to line up the magnetic fields again.

For the motor to rotate continuously, the polarity of the armature voltage must be reversed every time the stator and rotor fields line up. This is achieved by using a commutator. Carbon brushes make contact with the commutator on the armature and reverse the voltage on the coil just before the magnetic fields line up. A commutator is also called a reversing switch.

Fig. 19 Reversing of voltage polarity

Types of DC motor

DC motors have either a permanent magnet stator or a wound stator.

Permanent magnet motor - A permanent magnet is used to provide the constant stator field. This motor has a high starting torque, is suitable for rapid positioning applications, and is physically smaller than other motors.

Fig. 20 Permanent Magnet motor characteristics

Shunt motor - the field winding (shunt winding) and armature are connected in parallel. This configuration gives a relatively constant speed-torque relationship and good speed regulation over wide load ranges. A drawback is the shunt motors relatively lower starting torque.

Fig. 21 Shunt motor characteristics

Series motor - the field winding (series winding) and armature are connected in series. This configuration gives high starting torque, but should be avoided in applications where they are likely to lose load. Under no-load conditions this motor can accelerate

uncontrollably. e.g. Power tools

Fig. 22 Series motor characteristics

Compound motor - this is a combination of a shunt motor and series motor. A series winding is connected in series with the armature, and a shunt winding is connected in parallel with the series winding and the armature. The ratio of shunt to field windings gives various torque-speed characteristics which are chosen to suit the application.

Fig. 23 Compound motor characteristics