Speed Regulation of DC Motor

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 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

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 Nno load = no load speed and Nfull 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.