Speed Control of DC Motors

We can control a DC motor by following methods,

• Voltage Control Method
• Armature Control Method
• Field Control Method

Voltage Control Method

By changing the supply voltage V, the speed can be changed. As supply voltage can only be reduced, speed of a dc motor N can also be reduced from its rated value by this method.

Armature Control Method

By putting an extra variable resistance like rheostat in the armature circuit, the speed of a dc motor can be reduced to a desired value. It is clear from the above equation, a speed of a dc motor can only be reduced by reducing the values in the numerator.

Field Control Method

By varying the field flux Φ, speed of a dc motor can be changed. Field current produces the flux Φ, and hence this method is called the field control method. This is simply by putting a variable resistance in the field circuit, field current can be reduced, and hence flux produced can be reduced. When flux is reduced, speed is increased. Thus, by the field control method the speed of the dc motor can only be increased.