# Methods of speed control of DC motor

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• Speed control methods of a DC motor
• Field control method
• Armature control method
• Armature terminal voltage control method
• Ward-Leonard speed control system

## Speed of a DC motor: -

As we know that the speed of a DC motor is given by the equation as given below: -

N = Eb / k Φ

Where,
Eb = V - IaRa

So,
N = V - IaRa / kΦ

Where,
V = supply voltage
Ia = armature current
Ra = armature resistance
Φ = flux per pole

So from above equation, we can conclude that the speed of a DC motor can be changed by changing the supply voltage, armature resistance or field flux.

## Speed control methods of a DC motor: -

There are mainly three methods used to control the speed of the DC motor as given below: -
1. Field control method
2. Armature control method
3. Armature terminal voltage control method

Let's go to discuss about these methods in detail.

## 1. Field control method: -

• As concluded in the above equation that the speed of a DC motor is inversely proportional to the field flux. So by increasing the field flux, the speed will decrease and vice versa.
• To change the field flux, we just add a rheostat in series with the field winding of the motor.
• The rheostat is nothing but just a variable resistance which vary the resistance of the field winding.
• When resistance increases, the current flow through the field winding decreases, which decrease the flux of the field.
• As the speed is inversely proportional to the field flux, therefore the speed will increase in this case. And if the field flux increases the speed will decrease.

## 2. Armature control method: -

• In this method the speed is changed by changing the resistance of the armature winding.
• As mentioned above in the equation, the speed is indirectly proportional to the armature resistance, therefore when the armature resistance increases the speed decreases and vice versa.
• In this method also a rheostat is connected in series with the armature winding to vary its resistance.
• By varying the resistance of the armature winding, we can vary the current flow through it.
• Further the armature field will also change with change in armature current.

## 3. Armature terminal voltage control method: -

There are mainly two methods used to vary the armature terminal voltage: -

### 1. Multiple voltage control: -

• In this method a constant voltage is given to the field winding and variable voltage is given to armature winding.
• By varying the armature terminal voltage, speed can be varied. If terminal voltage increases, then speed will increase and vice versa.
• The suitable switchgear devices are used to change the armature voltage accordingly.

Read: - Three phase induction motor.

### 2. Ward-leonard control system: -

• This system is nothing but just a combination of motor-generator set and motor whose speed requires to change.
• There is a DC generator coupled with an AC motor may be a three phase induction motor or synchronous motor.
• The three phase AC motor works as a prime mover for the generator.
• When the AC motor connected to the supply it will rotate the rotor of the generator. The generator will produce a DC supply.
• The field winding of this DC generator is a variable field winding means the output voltage can be varied according to the requirements.
• This field winding supply the armature of the DC motor. The supply voltage of the armature of the DC motor can be varied by variable field of the generator.
• As supply voltage of the motor armature terminal can be varied from the generator field winding, so speed can be controlled.

## Conclusion: -

• The speed of a DC motor is directly proportional to the supplied voltage and inversely proportional to the field flux and armature resistance.
• There are mainly three methods are used to control the speed of a DC motor-
1. Field control method
2. Armature control method
3. Armature terminal voltage control method

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