Question

In an electric motor, the direction of current in the coil changes in every.
(A) Rotation
(B) Two rotations
(C) Four rotations
(D) Half rotation

Answer 1

Hint: For AC motors, the current flowing through the electromagnet and the current flowing through the coil both reverse, exactly in step, so the force on the coil is always in the same direction and the motor always spins either clockwise or counter-clockwise. Direct current converts electrical energy into mechanical energy.

Complete Answer:
An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. Electric motors, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.
Permanent magnet (PM) direct current converts electrical energy into mechanical energy through the interaction of two magnetic fields. One field is produced by a permanent magnet assembly, the other field is produced by an electrical current flowing in the rectangular coil ABCD, through the brushes and split-ring commutators. These two fields result in a torque which tends to rotate the coil. As the coil turns, the current in the winding is commutated to produce a continuous torque output.
The split ring commutator is made from 2 pieces of copper that are held apart in the center from touching each other. They look like a copper ring split in the middle. The function of the commutator rings is to reverse the direction of current flowing through the coil every time the coil just passes the vertical position during a revolution.
Hence, the direction of current in the coil changes in every half rotation.
Hence, the correct answer is Option D.

Note:
In an AC motor power is sent to the outer coils that make up the stator. The coils are energized in pairs, in sequence, producing a magnetic field that rotates around the outside of the motor. The magnetic field is constantly changing (because it's rotating) so, according to the laws of Faraday's law of electromagnetism, the magnetic field induces an electric current inside the rotor. If the conductor is a ring or a wire, the current flows around it in a loop.
If the conductor is simply a solid piece of metal, eddy currents swirl around it instead. Either way, the induced current produces its own magnetic field and, according to another law of electromagnetism (Lenz's law) tries to stop whatever it is that causes it—the rotating magnetic field—by rotating as well. (You can think of the rotor frantically trying to "catch up" with the rotating magnetic field in an effort to eliminate the difference in motion between them.) Electromagnetic induction is the key to why a motor like this spins—and that's why it's called an induction motor.