Speed control of A D. C. Machine/Motor

Written On Tuesday, December 1st, 2009 At 09:17:17 am By Sunil Saharan
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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=PΦNZ/60 A
P = Number of poles in the machine
Φ = 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
Now we have to control N by changing values of other variables we have
N=(V-IaRa)(60A)/PZΦ 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)/PZΦ The following are constants i.e. Can't be changed
  • 60 A
  • P
  • Z
  • Φ
Note : Φ 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)/PZΦ The following are constants i.e. Can't be changed
  • 60 A
  • P
  • Z
Here Φ 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 Φ
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=PNZΦ/60A=V-IaR
We have N=(V-IaRa)*(60A/PZΦ)
If Flux kept constant and P, A, Ia, Ra and Z are already constant so
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 then Speed of the motor will be reduced and vice versa.

Field Flux Control

Increasing flux in a DC motor will reduce speed of the machine and vice versa.

Tags :DC Motor Speed Control, Flux Control Technique, Armature Resistance Speed Control Method, DC Motor Control
Article Was Last Updated on Sunday, March 11th, 2018 At 10:26:50 am

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I'm Sunil Saharan, An Electrical Engineer by chance and a Web developer by choice. I love HTML, CSS, JS, Bootstrap and PHP off course. This site is my effort to share my little knowlege with the world.

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