DC Motor Speed Control

There are three main ways to achieve speed regulation in series DC motors–flux control, voltage control, and armature resistance control. 

1. Flux Control Method

In the flux control method, a rheostat (a type of variable resistor) is connected in series with the field windings. The purpose of this component is to increase the series resistance in the windings which will reduce the flux, consequently increasing the motor’s speed. 

2. Voltage Regulation Method

The variable regulation method is typically used in shunt dc motors. There are, again, two ways to achieve voltage regulation control:

• Connecting the shunt field to a fixed exciting voltage while supplying the armature with different voltages (aka multiple voltage control)

• Varying the voltage supplied to the armature (aka the Ward Leonard method)

3. Armature Resistance Control Method

The armature resistance control is based on the principle that the speed of the motor is directly proportional to the back EMF. So, if the supply voltage and the armature resistance are kept at a constant value, the speed of the motor will be directly proportional to the armature current.