ALTERNATOR (A.C. GENERATOR)
In DC Generator. the conductors are rotated in a stationary magnetic field and current flows in the same direction. But in Alternator or A.C. generator, the magnetic field is rotating and the conductors remain stationary. The current flows first in one direction and then in other alternately.
As shown in Figure, a rotating bar magnet furnishes the moving field which cut the U-shaped conductor. As at when the north pole passes the upper leg of the conductor, the current flows in one direction. As at (B), when the south pole passes the upper leg of the conductor, the direction of the magnetic lines of force is reversed and the current flows in reverse direction in the conductor. Thus, the magnet is revolving continuously, the direction of lines of force is also changing continuously, the current alternate in direction-alternating current flows. The strength of the current depends upon the following factors:
- Strength of the magnetic field.
- Speed of the magnetic field with which it revolves.
- No. of conductors.
Instead of a simple bar magnet, two or more pole pieces, having field winding, are assembled on a shaft. When the current flows in the winding, a strong magnetic field is created and the pointed ends of the pole pieces become alternately north and south poles. The two ends of the conductors are connected to the two slip rings fixed on the shaft. The brushes make continuous sliding contact with the slip rings, by which the current is taken out to an external circuit for power transmission and distribution. The alternator type generator different from the commutator type generator in the voltage and current control and also in design. It uses silicon diodes to change the alternating current to direct current for the vehicle’s electrical system. Also, it does not require any cutout to prevent the battery from discharging through it, because the diode rectifiers allow the current to flow in one direction only.
CONSTRUCTION OF A.C. GENERATOR
The figure shows the rotor of an A.C. generator. The rotor consists of a circular field coil encased by two end pole pieces, each having six protruding fingers spaced 60 degrees apart. These fingers are alternately spread over to provide twelve poles. The end of the field coils is connected to the slip rings. The carbon brushes make sliding contact with the slip rings by which the current is taken out to an external circuit. The rotor is united on a shaft supported in the front by ball bearings and in the rear by needle bearings. A pulley is keyed at the front end of the shaft to turn by means of an engine-driven belt. The stator of the A.C. generator consists of an internally slotted, laminated iron core around which three sets of windings are made. One of each winding is connected to a common Y-junction. The other end is connected to one positive diode rectifier and one negative rectifier.
The figure shows the charging circuit of the A.C. generator. When the ignition switch is turned on, the rotor gains current from the battery through the voltage regulator. This current produces the magnetic field of the rotor, As the rotor, is turned by the pulley, the current is induced in the stator winding. This AC or alternating current is changed into DC or direct current by rectifiers. The voltage depends upon the rotor speed. A voltage regulator is used to limit the AC generator voltage to a correct value.
Watch the animation video below to know how an Alternator Works?
Applications Or Uses of AC Generator
- Used in automobiles.
- Power generation system of windmills.
- Power generation system of Hydro-electric power plants.
- Construction projects.
- Oil and gas mining areas.
- Rail and marine transportation.
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