According to the speed equation of a d.c. motor we can write,
The factors Z, P, A are constants for a d.c. motor.
But as the value of armature resistance Ra and series field resistance Rse is very small, the drop Ia Ra and (Ra + Rse) is very small compared to applied voltage V. Hence neglecting these voltage drops the speed equation can be modified as,
Thus the factors affecting the speed of a d.c. motor are,
1. The flux ?
2. The voltage across the armature
3. The applied voltage V
depending upon these factors the various methods of speed control are,
1. Changing the flux ? by controlling the current through the field winding called flux control methods.
2. Changing the armature path resistance which in turn changes the voltage applied across the armature called rheostatic control.
3. Changing the applied voltage called voltage control method.