Speed Control of DC Series Motor

Speed Control of DC Series Motor

Various quantities can control the speed of a DC series motor, and according to that, the speed control methods of a DC series motor is classified as the below table.

Speed Control of DC Series Motor
Speed Control of DC Series Motor

As shown in the above chart, the speed of a DC series motor depends on the flux per pole, armature voltage, and applied voltage.

In the DC series motor, the field winding and armature winding are connected in series. Hence, the current flowing through both winding is the same.

The flux produced in any winding is expressed as below equation.

    \[ \phi = \frac{MMF}{Reluctance} = \frac{NI}{S} \]

Where,

N = Number of turns
I = Current
S = Reluctance

In this article, we will discuss each method in brief.

Flux Control for DC Series Motor

The flux of a DC series motor can be controlled by using the below methods;

  • Field diverter method
  • Armature diverter method
  • Tapped field method
  • Series-parallel connection of field

Field Diverter Method

In this method, a variable resistor R is connected across the field winding. This resister is known as a field diverter.

The connection diagram of a field diverter or flux diverter method is as shown in the figure below.

Field Diverter Method
Field Diverter Method

When the field diverter is placed at point A, the resistance offered by the diverter is zero (R=0). Hence, all current diverted away from the field winding.

Therefore, the flux per pole is minimum. And it results in a maximum speed of the motor. So, if the field diverter is set at point A, the motor offers maximum speed.

When the diverter is set at point B, the entire resistance of the diverter is connected and the field current increases.

As the field current increases, the flux increases, resulting in the minimum speed of the motor. So, if the field diverter is set at point B, the motor offers minimum speed.

In this method, the speed is controlled only above the rated speed without changing the armature current.

Armature Diverter Method

In this method, a variable resistor is connected across the armature winding. The connection diagram of this method is shown in the figure below.

Armature Diverter Method
Armature Diverter Method

With the help of an armature diverter, we can control the armature current Ia. For constant load torque, if the armature current is reduced, the flux increase.

    \[ T_a \propto \phi I_a \]

This will result in an increase in current taken from the supply, and it increases the flux. Hence, the speed is decreased.

    \[ N \propto \frac{1}{\phi} \]

In this method, the speed of the motor is controlled below the rated speed.

Tapped Field Method

In this method, the series field winding is provided with taping. Hence, we can change the number of turns of the series field winding.

The connection diagram of this method is shown in the figure below.

Tapped Field Method
Tapped Field Method

By changing the number of turns of the series field winding, we can change the flux and hence, it results in a change in the speed of a motor.

If the number of turns of field winding is reduced, the flux will reduce, resulting in increased speed.

This method of speed control is used to control speed above the rated speed. Generally, this method is used in applications like traction.

Series-Parallel Connection of Field Winding

In this method, the field winding of a DC series motor is divided into a number of parts. And these parts are connected either in series or parallel.

The connection diagram of this method is shown in the figure below.

Series Parallel Connection of Field Winding
Series-Parallel Connection of Field Winding

The MMF (flux) produced by the field winding depends on the series-parallel connection of winding. The speed of the motor decides by the flux.

Hence, by connecting winding differently, we can control the speed of the motor. But in this method, the speed of the motor is not changing smoothly. It varies in step.

Generally, this type of speed control method is used in fan motor speed control.

So, these are the methods to control the flux of DC motors, and we can control the speed of a motor by controlling produced flux.

Rheostatic Control

In this method, the flux produced by the field winding is constant, and we control the voltage across the armature. Hence, this method is also known as the armature voltage control method.

The connection diagram of this method is shown in the figure below.

Rheostatic Control
Rheostatic Control

As shown in the connection diagram, a variable resistor is connected in series with the armature winding.

By controlling the resistance of variable resistance, we can control the voltage across the armature. Because the armature current will flow through the variable resistance, it will create a voltage drop Ia2R across the variable resistance R.

The speed of a motor is directly proportional to the armature voltage, so, as the voltage decreases, the speed decreases and vice versa.

Applied Voltage Control

The speed of a DC series motor is directly proportional to the applied voltage. So, by controlling the applied voltage, we can control the speed.

The connection diagram of this method is shown in the figure below.

Applied Voltage Control
Applied Voltage Control

Before using this method, keep in mind that you cannot give voltage more than the rated voltage. This may lead to damage to the motor.

Hence, the maximum speed of this motor is at rated speed, and we can control the speed of a motor below rated speed.

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