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A dc motor run by a source of emf 200 V draws a current of 10 A . If the efficiency of the motor is 40 % , then the resistance of the armature coil is

Options

  1. A5   Ω
  2. B12   Ω
  3. C30 Ω
  4. D25 Ω

Correct answer

B. 12   Ω

Step-by-step solution

Let us assume that the source voltage is E , the back emf developed in the armature coil is e , the armature resistance is R and the armature current is i . The efficiency of the motor is η = i e i E = e 200 = 0 . 4 ⇒ e = 80   V The armature current is i = E - e R ∴   10 = 200 - 80 R ∴   R = 12   Ω

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