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The basic principle of the DC motor is a device which converts DC energy into mechanical energy. When the current carrying armature  is connected to the supply end though commentator segment, brushes are placed within the North South Poles of permanent or electromagnets. By using these electromagnets operating principle is depends on the Fleming’s left hand rule to determine the direction of the force acting on the armature conductors of the DC motor.

Speed Control Methods of a DC Motor

Speed of a DC motor can be varied by varying flux, armature resistance or applied voltage. Different speed control methods for different DC shunt and series methods are there.

Speed Control of Shunt Motors

• Flux control method
• Armature and Rheostatic control method
• Voltage control method

1. Multiple voltage control
2. Ward Leonard system

Speed Control of Series Motors

• Flux control method

1. Field diverter
2. Armature diverter
3. Trapped field control
4. Paralleling field coils

• Variable Resistance in series with motor
• Series -parallel control method

Flux Control Method

In this flux control method, speed of the motor is inversely proportional to the flux. Thus, by decreasing flux and speed can be increased vice versa. To control the flux , he rheostat is added in series with the field winding will increase the speed (N), because of  this flux  will decrease. So, the field current is relatively small and hence I2R loss is  decreased. This method is quite efficient.

So in this method, the speed can be increased by reducing flux, it puts a method to reducing flux with this method, it puts a method to maximum speed as weakening of flux beyond the limits will adversely affect the commutator.

Armature Control Method

In the armature control method, the speed of the DC motor is directly proportional to the back emf (Eb) and Eb = V- IaRa. When supply voltage (V) and armature resistance Ra are kept constant, the Speed is directly proportional to armature current (Ia). If we add resistance in series with the armature, the armature current (Ia) decreases and hence speed decreases.

This armature control method is based on the fact that by varying  the voltage across the required voltage. The motor back EMF (Eb) and Speed of the motor can be changed. This method is done by inserting the variable resistance (Rc) in series with the armature.