An electric motor rotates at various speeds and the current through the armature changes accordingly. Which pa

Question

62.) An electric motor rotates at various speeds and the current through the armature changes accordingly. Which pair of conditions occurs when the motor generates the greatest back emf?

A.) Speed: slowest; Current: largest

B.) Speed: fastest; Current: largest

C.) Speed: slowest; Current: smallest

D.) Speed: fastest; Current: smallest

Answer: D please explain!! thanks!

Answer 1

When the coils of an electric motor pass through the electric field of the motor's magnets, an emf is produced. This is much like the emf that an electric generator produces. The difference is the motion in a generator is produced by an external mechanical force; the motion in a motor is produced by the current through it's coils.

The emf produced by a rotating motor is in the opposite direction of the externally applied current. That's why it is called "back emf."

With a generator a greater emf is produced with a greater speed. Similarly with a motor a greater back emf is produced with a greater speed.

Answer B and D have speed as fastest.

A greater speed produces a greater back emf. This back emf acts against the applied current, thus reducing the current. The current becomes smaller at higher speeds. The answer is D.