Methods of speed control of DC motor

Hello guys welcome to my blog. Hope you are all good and well. 

Today we will discuss about,

  • 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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