On application of load the speed of a dc motor decreases gradually. This is not at all desirable. So the difference between no load and full load speed should be very less. The motor capable of maintaining a nearly constant speed for varying load is said to have good speed regulation i.e the difference between no load and full load speed is quite less. The speed regulation of a permanent magnet DC motor is good ranging from 10 - 15% whereas for dc shunt motor it is somewhat less than 10 %. DC series motor has poor value of regulation. In case of compound DC motor the speed regulation for dc cumulative compound is around 25 % while differential compound has its excellent value of 5 %.

## Speed of a DC Motor

The emf equation of DC motor is given by

Here N = speed of rotation in rpm.

P = number of poles.

A = number of parallel paths.

Z = total no. conductors in armature.

Hence, speed of a DC motor is directly proportional to emf of rotation (E) and inversely proportional to flux per pole (φ).

## Speed Regulation of a DC Motor

The speed regulation is defined as the change in speed from no load to full load, expressed as a fraction or percentage of full load speed.

Therefore, as per definition per unit (p.u) **speed regulation of DC motor** is given as,

Similarly, percentage (%) **speed regulation** is given as,

Where N_{no load} = no load speed and N_{full lod} = full load speed of DC motor.

Therefore, Percent speed regulation = Per unit (p.u) speed regulation X 100 %.

A motor which has nearly constant speed at all load below full rated load, have good speed regulation.