# How to control Speed of DC series Motor?

### DC Series Motor Speed Control methods

In this post, let us discuss about various DC Series Motor Speed Control methods.

(a) The speed of a d.c. series-wound motor is given by:
$N=k\left(\frac{V-IR}{\phi }\right) \$
where
N= Speed of DC series Motor
k = Constant
V = Terminal voltage
R = Combined resistance of the armature and series field and is the flux

From the above equation it is clear that the speed of DC series motor can be varied

• by varying applied voltage (V),
• by varying armature resistance(R) and
• by varying flux(Φ).

In general, The speed of series-wound can be controlled by two methods:

1. Flux or Field Control Method
2. Voltage or Armature Control Method

#### [1] Field Control Method

From the speed equation, we know that, reducing the flux will increase the speed. There are three ways to control the flux of the DC series motor.

• By field diverter
• By motor diverter method
• By armature diverter

#### [1.a] Field Control by Field Diverter Method:

• In this method, a low value resistance (Diverter) is connected across the field as shown in the below figure. [A variable resistor connected in parallel with the series-wound field to control speed is called a diverter]
• By adjusting the diverter resistance, any desired amount of current can be passed through the diverter.
• So the flux can be decreased, consequently the speed of the motor is increased.
• In this method, the speed can be controlled only above normal speed.

#### [1.b] Field Control by Motor Diverter Method:

In this method, a line resistance(R) and diverter are connected as shown in the below figure.
Using this method, speed less than normal speed can be obtained.

#### [1.c] Field Control by Armature Diverter Method:

• As shown in the below figure, a high variable resistance is connected across the armature.
• This method is better than the motor diverter method.
• The current taken by the armature diverter also flows through the series field.
• So the field flux is increased and so the speed is reduced below the normal speed.

#### [2] Armature Control (Voltage) Method:

In this method as shown in the below figure, a variable resistance is directly connected in series with the supply and the armature circuit.

This will increase the resistance R value in the equation
$N=k\left(\frac{V-IR}{\phi }\right) \$
which results in reduction in speed.

Since the additional resistor reduces the voltage available across the armature and thus the speed gets reduced. By varying the rheostat(diverter), any speed below the normal speed can be achieved.