What are the factors that control the speed of a DC motor ?

Factors that control the speed of a DC motor:

The speed of a motor is given by the equation

S=(V-IaRa)/kɸ

It is obvious that the speed can be controlled by varying

·         Flux/pole(ɸ) (Flux control)

·         Resistance Ra of armature circuit(Rheoststic control)

·         Applied voltage V(Voltage control)

Advantage of field control method:

The advantage of field control method is to increase the speed above the base speed.

Disadvantage of field control method:

The disadvantage of field control method is excessive sparking at the brushes caused by the distortion of the magnetic field resulting from the weakened shunt field.