Speed control of D. C. Machines

On Dec 1, 2009 by Sunil Saharan At 9:17 AM | | 0 comments »
This article is about speed control of a D. C. Motor. It describe how speed of a D. C. Motor can be controlled.

Hello Friends,
In our current article we will discuss how speed of a D. C. Motor can be controlled i.e. Which factors are controlled to control the speed of the motor ?
From the back e.m.f. Equation of a D. C. Motor we have :
Eb=P¤NZ/60 A
Where
P = Number of poles in the machine
¤ = Flux enclosed to a pole
N = Speed of Machine in r. p. m.
Z = Number of Armature Conductors
A = Number of Parallel Paths in D. C. Motor
Also from equivalent circuit of a D. C. Motor we have
V = Eb+IaRa
Here V is applied voltage to the D. C. Motor
Eb is induced back e.m.f. in motor Armature.
Ia is Armature current and
Ra is Armature resistance.
Normally Armature resistance of a D. C. Motor is of the order of 0.5 ohm.
From above two equations we have
Eb=V-IaR=NP¤Z/60A
Now we have to control N so by changing position of o other variables we have
N=(V-IaRa)(60A)/PZ¤
This equation decide what should be changed to change the speed of a D. C. Motor. We know that for shunt motor flux is constant i.e. Independent of Armature current while in series motor the flux induced in D. C. Motor depends upon Armature current, so speed control method of both motors will be different.

Speed Control of D. C. Shunt Motor


We know that in equation
N=(V-IaRa)(60A)/PZ¤
The following are constants i.e. Can't be changed
  • 60 A
  • P
  • Z
  • ¤

Note : ¤ can be changed in D. C. Shunt motor by adding external variable resistance in field winding.

Speed Control of D. C. Series Motor


Similarly in D. C. Series Motor, We have the same equation
N=(V-IaRa)(60A)/PZ¤
The following are constants i.e. Can't be changed
  • 60 A
  • P
  • Z

Here ¤ can be changed by simply increasing or decreasing Armature resistance.

Speed Control of D. C. Motor ( Series and Shunt Both )


From above discussion it is clear that following can be changed in both series and shunt motors

  • Applied Voltage V
  • Armature Current Ia
  • Armature Resistance Ra
  • Flux ¤

Based on the quantity to be controlled the method of speed controlled are named as

  • Voltage Control Method
  • Armature Resistance Control.
  • Field Flux Control

Lets discuss one by one in detail.

Voltage Control Method



From the back e.m.f. Equation E=PNZ¤/60A=V-IaR
We have N=(V-IaRa)*(60A/PZ¤)
If Flux kept constant and P, A, Ia, Ra and Z are already constant so
N=(V-K)*K1
here K and K1 are constant.
Now N is directly proportional to applied voltage. So if applied voltage is increased then the speed of the dc machine will increased and vice versa.

Armature Resistance Control.


If we increase armature resistance it will Speed will be reduced and vice versa.

Field Flux Control


Incresing flux in a DC motor will reduce speed of the maachine and vice versa.

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