Speed controlling of DC motors

There exist three methods to control the speed of DC motors. They are flux control method (or field control method), rheostat control method (or armature voltage control method) and applied voltage control method. In flux control method, flux is decreased to enhance the speed of the DC motor. A field rheostat is used to vary the field and hence the flux. A shunt field rheostat is used in shunt motor for this purpose. In a series motor, to achieve field variations, we use field diverters, paralleling field coils, armature diverter and tapped field controls. A field diverter is a variable resistance. Field control method is economical and efficient. In armature voltage control method, a variable rheostat called controller resistance is inserted in series with the armature circuit. A speed below normal speed can be achieved by this method but against the huge power wastage. In multiple voltage control method, the shunt field of the motor is permanently connected to a fixed exciting voltage.