Generators & Motors — A/L Physics

How do I practise Generators & Motors?

1 question with a worked solution

1. A direct-current (DC) electric motor with an armature resistance of 1.0 Ω is operated from a 200 V DC supply. When running at full operating speed, the armature draws a current of 10 A. What back electromotive force (back EMF, E_b) is developed in the spinning armature?
   1. A) 200 V
   2. B) 100 V
   3. C) 210 V
   4. D) 190 V
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   Correct answer: D) 190 V

   In a running DC motor, the applied voltage V overcomes the back EMF E_b and supplies the ohmic drop across armature resistance: V = E_b + I_a R_a. Rearranging: E_b = V - I_a R_a = 200 V - (10 A × 1.0 Ω) = 200 V - 10 V = 190 V.

   Show worked solution

   Step 1: Motor voltage equation: V = E_b + I_a · R_a. Step 2: E_b = V - I_a · R_a = 200 - (10 × 1.0) = 190 V. Answer: 190 V.
   Modelled on: the A/L Physics examination pattern

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Where does Generators & Motors fit in the syllabus?

Unit 8.7 · Year 2 · Term 1