Speed Regulation of DC Motor

Speed Regulation of DC Motor Points : Speed Regulation of DC Motor The speed regulation of a dc motor is defined as the change in speed when the load on the motor is reduced from fill load to zero and is expressed in percentage of rated full load speed.

Percentage speed regulation = N₀ _ N_f/ N_f x 100

where N₀ and N_f are no-load and full-load speeds respectively. The motor capable of maintaining a nearly constant speed for varying loads is said to have a good speed regulation.

The speed regulation of permanent magnet motors is quite good, about 10 to 15%. This is due to the fact that the voltage drop in armature is quite low. The speed regulation of dc shunt motors is usually less than that for permanent magnet types, somewhat less than 10%. This is due to slight reduction in flux per pole on account of flow of current through armature. The speed regulation of a dc series motor is normally very poor. The no-load speed is usually several times of the rated full-load speed, high enough for centrifugal force to damage the armature of any but the very smallest machines. The reason for this high no-load speed is that, with the rise in speed, the field flux is weakened causing further increase in speed. The speed regulation for cumulative compound wound dc motors varies from 20% to 50% whereas for differential compound wound dc motors it is about 5%.

Both dc shunt and compound motors have definite no-load speed, while the dc series motor is quite unstable and tends to operate at an extremely high, often runaway, speed when the mechanical load is removed.