The back emf in an electric motor
- A. adds to the supply voltage
- B. opposes the supply voltage and limits the current drawn
- C. has no effect on the current
- D. appears only when the motor is stationary
Explanation
As the armature spins it acts as a generator, inducing an emf that by Lenz's law opposes the supply, so the effective driving voltage and therefore the current fall as the motor speeds up. At the instant of starting there is no back emf, which is why the starting current is very large and why motors need starters. Overloading a motor slows it, reduces the back emf and can burn out the windings.
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