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Armature resistance Control Of DC Motor | Rheostatic Speed Control of Dc Motor

What is the Armature Control Method of DC Motor?

Armature resistance Control Of DC Motor-In this method of speed control of DC motor, an external variable resistor is connected in series with the armature circuit. On increasing the value of the external resistor, the voltage drop across the armature circuit increases which reduces the speed of the motor. 

armature control method of dc motor

In the case of shunt motor, the field circuit is directly connected across the applied voltage, hence there is no effect on magnetic flux due to variation of armature resistance. hence magnetic flux is constant.

Working of Armature Control of Speed of DC Motor

To explain the working of speed control of DC Motor by armature control method. I shall use the relation between terminal voltage and Back EMF along with armature resistance.

From the above-given circuit diagram of the DC Motor, On applying Kirchoff Voltage Law

V = Eb + IaRa

Eb = V - IaRa ----------(1)

As we Know that EMF Induced in Armature which also called back EMF is equal to Eb

Eb = (NPФZ)/60A

Where N =Speed of Rotor in RPM, P =Number of Pole, Ф = Flux per Pole In Weber ,Z = Number of Conductor Per Pole, A = Parallel Path

For a given Machine of Particular Rating, P, Ф and Z are Constant hence the above equation can be written as

Eb =(PФZ/60A)N

Now from this equation (PФZ/60A) can be replaced by new Constant k (Say) then above equation can be written as

Eb = kN

Now put this Value in equation (1)

kN = V - IaRa

N =( V - IaRa)/k

N ∝ (V - IaRa) --------(2)

From equation (2) it is clear that Speed of Motor depends on Armature Current and Armature Resistance. If we increase the armature resistance then the Voltage drop across the armature resistance will increase and due to this (V - IaRa) will decrease and the speed of the motor will also decrease.

The drawback of the Resistance Control of DC Motor

  • A large amount of electrical energy is wasted in external resistance.
  • For a given value of external resistance the speed reduction is not constant, it varies with the external load.
  • Control is limited to give speeds below normal and an increase of speed can not be obtained by this method.
  • This method is suitable for small motors.
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