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 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 for DC cumulative compound the speed regulation 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.


Therefore, the speed of a DC motor is directly proportional to the electromotive force (emf) and inversely proportional to the flux per pole (φ).

Speed Regulation of a DC Motor

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,
Therefore,
A motor that maintains nearly constant speed at all loads below full rated load has good speed regulation.