Although electricity comes from power stations, power can also be generated by far small means. Nowadays, electricity generators can be small enough to hold in the hand. Portable generators are made up of two main parts; an engine, which powers the equipment, and an alternator, which coverts motion into electricity. The engine shown (fig.1) runs on petrol. It is started by pulling a cord. This creates a spark inside which ignites the fuel mixture. In a typical four stroke engine, when the piston descends, the air inlet valve open and a mixture of air and petrol is sucked in through a carburetors. The valve closes, the piston rises on the compression stroke and a spark within the upper chamber ignites the mixture. This mini- explosion pushes the piston back down, and it rises again, the fumes formed by the ignition are forced out through the exhaust valve. This cycle is repeated many times per second. The moving piston makes the crankshaft rotate at great speed. The crankshaft extends directly to an alternator, which consists of two main sets of windings – coils of insulated copper wire wound closely around an iron core. One set, called stator windings, is in a fixed position and shaped like a broad ring. The other set, the armature windings, is wound on the rotor which is fixed to the rotating crankshaft. The rotor makes about 3,000 revolutions per minute. The rotor is magnetized and as it spins round, electricity is generated in the stator windings through the process of electromagnetic induction. The electric current is fed to the output terminals or sockets. This type of generator can produce 700 watt output, enough to operate lights television, and some domestic appliances. Larger versions provide emergency power to hospitals and factories
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