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ALTERNATORS AND A.C. GENERATORSThe national electricity supply system of every country is an alternating current supply; in the United Kingdom and in Europe the polarity of the supply changes every V50 s or every 20 ms, and every l/60s or 16.67 ms in the United States of America.The basis of a simple alternator is the following one. It comprises a rotating permanent magnet (which is the rotating part or rotor) and a single-loop coil which is on the fixed part or the stator of the machine. You will see that at this instant of time, current flows into the terminal (A) and out of terminal b (that is, terminal B is positive with respect to (A) so far as the external circuit is concerned).When the magnet has rotated through 180A, the S-pole of the magnet passes across A conductor and the N-pole passes across a conductor (B). The net result at this time is that the induced current in the conductors is reversed when compared with the previous case. That is, terminal B is negative with respect to (A).In this way, alternating current is induced in each turn of wire on the stator of the alternator. In practice a single turn of wire can neither have enough voltage induced in it nor carry enough current to supply even one electric light bulb with electricity.A practical alternator has a stator winding with many turns of wire on it, allowing it to deal with high voltage and current. The winding in such a machine is usually distributed around the stator in many slots in the iron circuit. The designer arranges the coil design so that the alternator generates a voltage which follows a sinewave, that is, the voltage waveform is sinusoidal.
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