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 AC generator means "Alternating Current generator". That is, an A.C. generator produces alternating current, which reverses its direction continuously. A.C. generator is also known as an alternator. We will now describe the construction and working of an A.C. generator.

Construction of an A.C. Generator          

A simple A.C. generator consists of a rectangular coil ABCD which can be rotated rapidly between the, poles N and S of a strong horseshoe-type permanent magnet M .The coil is made of a large number of turns of insulated copper wire. The two ends A and D of the rectangular coil are connected  to the two circular pieces of copper metal called slip rings R1 and R2. As the slip rings R1 and R2 rotate with this coil, the two fixed pieces of carbon called carbon brushes, B1 and B2 keep contact with them. So, the current produced in the rotating coil can be tapped out through slip rings into the carbon brushes. The outer ends of carbon brushes are connected to a galvanometer to show the flow of current in the external circuit (which is produced by the generator)

Working of an A.C. generator       

Suppose that the generator coil ABCD is initially in the horizontal position (as shown in Figure) Again suppose that the coil ABCD is being rotated in the anticlockwise direction between the poles N and S of a horseshoe-type magnet by rotating its shaft.

(i) As the coil rotates in the anticlockwise direction, the side AB of the coil moves down cutting the magnetic field lines near the N-pole of the magnet, and side CD moves up, cutting the magnetic field line near the S-pole of the magnet (see Figure). Due to this, induced current is produced in the sides AB and CD of the coil. On applying Fleming's right-hand rule to the sides AB and CD of the coil, we find that the currents are in the directions B to A and D to C. Thus, the induced currents in the two sides of the coil are in the same direction, and we get an effective induced current in the direction BADC (see Figure). Thus in the first half revolution (or rotation) of coil, the current in the external circuit flows from brush B1 to B2.

ii)After half revolution, the sides AB and CD of the coil will interchange their positions. The side AB will come on the right hand side and side CD will come on the left side. So, after half a revolution, side AB starts  moving up and side CD starts moving down. As a result of this, the direction of induced current in each side of the coil is reversed after half a revolution giving rise to the net induced current in the direction CDAB (of the reversed coil). The current in the external circuit now flows from brush B2 to B1. 

Since the direction of induced current in the coil is reversed after half revolution so the polarity (positive and negative) of the two ends of the coil also changes after half revolution. The end of coil which was positive in the first half of revolution becomes negative in the second half. And the end which was negative in the  first half revolution becomes positive in the second half of revolution. Thus, in 1 revolution of the coil,the current reverses its direction 2 times. In this way alternating current is produced in this generator.

   If we replace the slip rings of an A.C. generator by a commutator, then it will  become  a  D.C. generator. When the two half rings of commutator  are connected to the two ends of the generator coil, then one carbon brush is at all times in contact with the coil arm moving down in the magnetic field while the other carbon brush always remains in contact with the coil arm moving up in the magnetic field.Due to this , the current in outer circuit always flows in one direction.So,it is direct current.