G azərbaycan respublikasi təHSİl naziRLİYİ azərbaycan texniKİ universiteti M. M. QƏNDİLova e. Q. İsmayilova

Alternating Current
In alternating current (A.C) circuits, the voltage and current in each phase build up from zero to maximum in one direction, return to zero, build up maximum in the opposite direction, again diminish to zero, and continue in this manner for as long as the current flows. A sine wave chart shows this pattern in an alternating current circuit; it also shows voltage building up and diminishing in the same way. In a sine wave chart, the zero line or base line represent both time and electrical degrees. When the current has reached the maximum in one direction, returned to zero, reached the maximum in the opposite directions, and again retuned to the zero, one cycle, or 360 electrical degrees, has been completed. A two-pole generator produces one cycle in making one complete turn. A four-pole generator produces two cycles, or 720 electrical degrees, in making one complete turn. (In a four-pole generator, electrical degrees have twice the value of degrees of revolution.)

For the following discussion of single-phase and polyphase alternating current, produces a single alternating voltage that reaches a peak or maximum in each direction for every complete revolution of the rotor. This is a single-phase voltage, and the current that flows because of this voltage is a single-phase alternating current.

Polyphase currents, are produced by generators having two or more sets of windings, in which two or more voltage are generated at the same number of hertz, or cycles per second. These cycles reach the same values and have the same sine wave form, but they have a time lapse of 90 electrical degrees or more between the sine waves.

A two-phase generator is actually a combination of two single-phase generators wound on the same shaft and inducing e.m.f. in each of two circuits 90 electrical degrees apart.

A polyphase circuit made up of three alternating voltages of equal value spaced 120 electrical degrees apart is called a three-phase circuit; voltage in such a circuit reaches a peak in each direction three times in each cycle. In the three-phase generator, the spacing of the stator windings causes the voltage in each phase to alternate exactly a third of a cycle after the phase ahead of it.

The advantages of using three-phase alternating current may be compared with those of using an engine with many cylinders: with a single cylinder, power from the engine comes in spurts; but with the addition of more cylinders, the spurts of power overlap, producing a steadier level of power as long as the engine continues to run. In the same way, the overlapping phases of the three-phase circuit maintain a higher level of power. Power is generally produced and transmitted in three-phase systems, because they are best adapted to motors, and involve the least expensive power lines. In addition, three-phase motors are smaller, simpler in structure, and have better operating characteristics than single-phase motors of same rating.