The dc motor converts the mechanical power into dc electrical power. One of the most important features of the dc motor is that their speed can easily be control according to the requirement by using simple methods. Such type of control is impossible in an AC motor.
The equation (1) that the speed is dependent upon the supply voltage V, the armature circuit resistance Ra and the field flux ϕ, which is produced by the field current
𝜔 = (𝑉a − 𝐼a 𝑅a)/ 𝐾∅……1
For controlling the speed of DC separately excited Motor, the variation in voltage, armature resistance and field flux is taken into consideration. There are three general methods of speed control of a DC Motor. They are as follows.
1. Adjusting the supply voltage applied to the armature without changing the voltage applied to the field. Hence, the flux is kept constant. This can applied to separately excited motors only. Hence, at a certain load, since the flux is fixed, increasing the armature voltage, increases the motor speed. This method is known as Armature Voltage Control.
2. Adjusting the field resistance 𝐼𝐹 = 𝑉𝑠 𝑅𝐹 (and thus the field flux). This can be applied to separately excite and shunt motors hence, for a constant supply voltage, at a certain load, increasing the flux decreases the motor speed, this method is known as Field Flux Control.
3. Inserting a resistor in series with the armature circuit this method called Armature Resistance or Rheostatic control. This can be applied to separately excited and shunt motors hence, for a constant supply voltage and fixed flux, at a certain load, increasing 𝑅𝐴 decreases the motor speed.