Speed Control of DC Shunt Motor

Before discussing the different methods of speed control of DC shunt motor, we have to find the factors affecting the speed of a DC motor.
 
The speed equation of a DC motor shows that,
 
N α Eb / φ
 
or N α  (V – IaRa)/ φ
 
But the resistance of armature winding is small. Therefore the voltage drops IaRa will be negligible as compared to the external supply voltage V. Therefore, the expression for the speed can be approximated as follows:
 
N α  V/φ    (because V>> IaRa)
 
From this expression we can obtain the methods of speed control of DC shunt motor as follows:

  • The speed is inversely proportional to flux φ.
  • It is directly proportional to armature voltage drop (IaRa).
  • It is directly proportional to applied Voltage V.

So by varying one of these parameters, it is possible to change the speed of DC shunt motor. Depending on the parameter being controlled, methods of speed control of shunt DC Motor are classified as follows:
 

Speed Control of DC Shunt Motor by Flux Control Method

 

speed control method of dc shunt motor

 
The set up for speed control of DC shunt motor using flux control technique is shown in the figure. In order to change the speed, we have to change flux. This can be achieved by changing the field current. The field current can be changed by changing the rheostat R connected in series with the field.
 
At the time of starting the motor, we need to run the motor slowly, therefore, the flux should be maximum, because,
 
N α  1 / φ 
 
To obtain maximum flux at the start, the field current should be maximum at the time of starting. To obtain this, the value of rheostat (R) should be minimum.  
 
The speed of DC shunt motor can be varied by varying the field current. As we increase the resistance R of the rheostat, the field current Ish decreases.  So the flux φ decreases. This results in increasing the speed of the motor. As the R is increased, the speed increases.
 
We can use this technique to control motor speed above its rated value. It is the most commercial method. There is a limit to the maximum obtainable speed by this method due to poor commutation at week fluxes.  Most common maximum to minimum speed ratio is 6:1.
 

Speed Control of DC Shunt Motor by Armature Control Method

 

speed control method of dc shunt motor image

 
The set up for speed control of DC shunt motor by armature voltage control method is shown in the figure. A rheostat is connected in series with armature winding. By varying the value of R we can vary the voltage across the armature.
 
Because speed N is directly proportional to armature voltage, it is possible to change the speed by changing the value of rheostat R.
 
We can use this technique to control motor speed below its rated value. But it is neither efficient nor economical method because, in this method, speed is reduced at the cost of power loss in rheostat (Ia2R).
 

Speed Control of DC Shunt Motor by Applied Voltage

 

speed control method of dc shunt motor www.yourelectricalguide.com

 
In this method, the field winding of the motor is connected to a constant DC voltage. But armature is supplied with different voltages with the help of suitable switch gear as shown in the figure.
 
Advantages

  • The biggest advantage of this method is that the direction can be reversed.
  • Smooth increase and decrease in speed are possible.
  • The speed control can be obtained over the wide speed range.

Disadvantages
 
The arrangement for changing the applied voltage is expensive.
 

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