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what is ac generator?

A.C. Generator is a device used to convert mechanical energy into electrical energy. It is based upon the principle of electromagnetic induction in which e.m.f. Is induced in a coil rotating in a magnetic field due to change in the number of field lines associated with the coil. The direction of current induced in the coil is given by Fleming's right hand rule.

Construction : Figure shows the essential parts of an a.c. Dynamo.

Armature : A rectangular coil ABCD of many turns of insulated copper wire wound on a soft iron core. The coil and its core as a whole is called as armature. The function of the coil is to increase strength of the magnetic field.

Field magnets : The armature rotates around a spindle between pole pieces N and S of a strong magnet. Axis of rotation is perpendicular to the magnetic field lines.

Slip rings : The ends of the armature are connected to two circular rings R1 and R2 which are insulated from each other. The rings rotates along with the armature.

Brushes : B1 and B2 are two flexible metal plates or carbon rods fixed to the base of the generator and are kept pressed against rings R1 and R2.

Load : Brushes are connected to a load resistance R in the external circuit. Here load means the device through which the generated current is to be passed.

Working : let the armature ABCD is initially in the horizontal position in the magnetic field. Now let it be rotated in the clockwise direction, the side AB of the coil moves in the upward direction and side CD moves down. According to Fleming's right hand rule, the current induced flows from A to B in the arm AB and C to D in the arm CD as shown in the fig (a). thus, the induced current flows in the external circuit through brush B1 to B2. After half the rotation of the armature, the arm AB moves down and the arm CD moves up. Now the direction of induced current in the armature is DCBA while in the external circuit, current flows from B2 and B1 as shown in fig (b).

Thus, in one complete rotation of the armature, in half the rotation the current induced flows in the direction ABCD and in the external circuit from B1 to B2; in next half rotation of the armature, the induced current flows in the direction DCBA and in the external circuit from B2 to B1. Thus in one rotation of the coil the current changes its direction twice alternately. Hence, the current induced is alternating in nature. In India, the alternating current is produced which has a frequency of 50 Hz. i.e., the armature rotates in the magnetic field at the rate of 50 rotations per second. Since in one rotation of the armature current changes its direction twice, therefore, in 50 rotations of the armature, the current will change its direction 100 times. In other words, the alternating current changes its direction every 1/100 second.