Shunt DC Motors
Shunt motors are used in applications where a good speed regulation is required. In the shunt-wound DC motor, as shown in Figure 4.10, the field winding is connected in parallel with the armature winding.
In this type of motor, the field winding has many turns of small diameter wire to keep the resistance high.
The strength of the field is not affected appreciably due to changes in the load. A more or less constant speed is obtainable.
Shunt-wound DC motors are used where a more or less constant speed, a low staring torque, and a light overload on the motor are required.
The shunt-wound motor can also work as an adjustable-speed motor by means of the armature control or field control.
If a variable resistance is placed in the field circuit, the amount of current in the field windings can be controlled and the speed of the motor can be controlled.
As the motor speed increases, the torque decreases proportionately, resulting in an approximately constant horsepower.
If a variable resistance is placed in the armature circuit, the voltage applied to the armature can be reduced, and hence the motor speed can be reduced. With an armature control, speed regulation becomes poorer as the speed is decreased. Since the current in the field remains unchanged, the torque remains constant.
Adjustable-speed shunt-wound motors are used on large machines for boring mills, lathes, planners, etc. These are particularly adapted to spindle drive because of the constant horse-power characteristic that permits heavy cuts at low speeds and light cuts at high speeds.