In a DC series motor, what is the relationship between speed and armature current? 

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SSC JE Electrical 4 Dec 2023 Official Paper II
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  1. Speed is independent of armature current. 
  2. Speed is directly proportional to armature current. 
  3. Speed is directly proportional to the square of armature current. 
  4. Speed is inversely proportional to armature current. 

Answer (Detailed Solution Below)

Option 4 : Speed is inversely proportional to armature current. 
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Detailed Solution

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Concept:

DC series motor:

F1 Savita  Engineering 08-4-22 D1

  • In DC series motor, armature, and field windings are connected in series.
  • Induced EMF in DC series motor is given by:

E = V - IaRa

where E = induced emf

V= terminal voltage

Ia = armature current

Ra = armature resistancence

Explanation:

E = V - IaRa

kϕN = V - IaRa

N = \({V \over kϕ } - {{I_a}{R_a} \over kϕ }\)

In DC series motor, ϕ is proportional to Ia

N = \({V \over kI_a} - {{I_a}{R_a} \over kI_a }\) 

N α \( {1\over I_a}\)

So, the speed armature current characteristics will be a rectangular hyperbola.

F1 Savita  Engineering 08-4-22 D2

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