In this article you’ll learn DC motor speed control by varying current through field winding.
A very simple method for controlling the speed and torque characteristics of a wound-field (nonpermanent magnet) DC motor is to control the amount of current through the field winding.
Decreasing the field control resistor’s resistance allows more current through the field winding, strengthening its magnetic field. This will have two effects on the motor’s operation: first, the motor will generate more torque than it did before (for the same amount of armature current) because there is now a stronger magnetic field for the armature to react against; second, the motor’s speed will decrease because more counter-EMF will be generated by the spinning armature for the same rotational speed, and this counter-EMF naturally attempts to equalize with the applied DC source voltage. Conversely, we may increase a DC motor’s speed (and reduce its torque output) by increasing the field control resistor’s resistance, weakening the stationary magnetic field through which the armature spins.
Regulating field current may alter the balance between speed and torque, but it does little to control total motor power. In order to control the power output of a DC motor, we must also regulate armature voltage and current. Variable resistors may also be used for this task, but this is generally frowned upon in modern times because of the wasted power.