Principle. Alternators are based on the principle that an induced current is produced in a conductor whenever there is change in magnetic lines of force linked with it. The direction on induced current is given by Fleming.s right hand rule stated below.
Fleming.s right hand rule. If the thumb, forefinger and central finger of right hand are stretched out forefinger points along the direction of magnetic field and thumb along the direction of motion of the conductor, then the central finger indicates the direction of the current produced in the coil.
Construction. A simple A.C. generator consists of the following important parts :
(i) Armature : It consists of a coil ABCD having a large number of insulated copper wires wound over a soft iron core. The coil is called armature. The armature is mounted on an axle which can be rotated by force exerted by falling water, winf or steam.
(ii) Field Magnets. The coil is held between the pole pieces of a strong magnet called the field magnet. In small dynamos, the magnetic field is provided by permanent magnet. In large dynamos, the magnetic field is provided by eletromagnets.
(iii) Split Rings. These consist of two hollow metallic ring `R_(1)` and `R_(2)` mounted on the axle of the coil. The ends of the limbs AB and CD of the coil are connected to the rings `R_(1)` and `R_(2)` respectively. These rings rotate along with the rotation of the armature.
(iv) Current from `B_(1) : B_(2)` is taken out to load connected across two brushes.
Working. Let the coil initially be in horizontal position as shown in Fig. Let the coil be rotated in anticlockwise direction between the pole pieces N and S of a horse-shoe magnet.
At an instant, let Ab move down and CD move up as shown in the figure. The coil AB cut the magnetic lines of force near N-pole of the magnet, side CD cut the lines near S pole of the magnet. Due to the change in the magnetic field cutting the coil, an induced current is produced in AB and DC. Using Fleming.s right hand rule to sides AB and DC of the coil, we observe that the current flows in the direction B to A and D to C. Effective current flows along DCBA.
After half rotation, sides AB and DC of the coil will interchange their positions. AB will come towards right and DC towards left. Now AB starts moving up and DC downwards. The direction of induced current in each side is reversed after half a revolution. Due to the change in the direction of induced current in the coil is reversed after half revolution, hene positive or negative polarity of the two ends is also changed.
Function of brushes. Brushes help conveying the current to pass through load while the coil is rotating.
