DC Shunt Motor Speed Control

In the shunt motor, the motor winding is in parallel with the armature.

From the figure above, the supply current I = I_a  + I_f . Unlike a series motor, if the load is removed from the motor, only the armature current will decrease, the field remaining at the same strength. Therefore the motor will not continue to speed up to destruction. The graphs below shows the load characteristics of a shunt motor.

Speed Control of a DC shunt motor

Speed control in a DC shunt motor is achieved by controlling the field strength, by placing available resistance in series with the shunt winding.

Starting a Shunt Motor

Starting large dc shunt wound motors is usually carried out using a dc face-plate; the face-plate starter comprises the following items:

• A series of resistances connected to brass studs
• A spring-loaded handle which makes contact with two brass strips and also the brass studs
• A no-volt release
• An overload release

The operation of the DC face-plate starter

When the handle is located on the first stud, the field is supplied via the overload release, the top brass strip via the handle and the no-volt release (The field is continuously supplied this way). The armature is supplied via the resistances.

As the handle is moved around, the resistances in the armature circuit is gradually cut out. On the final stud the handle is held in place by the no-volt release electromagnet. Should a failure in supply occur, the no-volt release will de-energize and the handle will spring back to the off position, If a serious overload occurs, the overload release will energize sufficiently to attract its soft iron armature which will short out the no-volt release coil; and the handle will return to the off position. Please note that, the handle should be moved slowly from the stud to stud.

Applications of Shunt motors

Since the speed of a shunt motor is almost constant over a wide range of loads, it is most suitable for small machine tools.

Related articles:

• Speed Control of a DC Series Motor
• Speed Control of a DC Compound Motor
• Pulse Width Modulation (PWM) Speed Control of a DC motor
• Power MOSFET Motor Control
• DC Motor Power Op-amp Speed Controller