Техническое чтение для энергетиков. Бухарова Г.П. - 34 стр.

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* but it is a further 120° behind in phase но она (эдс) еще на 120° отстает по фазе

TRANSMISSION OF THREE-PHASE POWER

The three windings discussed above can be made to supply three individual circuits,

when all six ends must be used. It is possible, however, to link the three circuits

electrically with the result that the number of conductors necessary for the

transmission of the power is reduced.

In the first instance it is possible to effect an economy by using a common return, this

being permissible since it does not disturb the electrical arrangement. This implies

that the three rear ends of the turns, A', B', and С' must all be joined, together with the

three rear ends of the three resistances used as loads. This arrangement is illustrated

in Fig. 7 which shows the three generator windings connected together at one end, the

other ends being connected to three conductors for the purpose of transmitting the

power. The return conductor carries the vector sum of these three currents back to the

common junction of the three generator windings.

If the three e. m. f. s are all equal and the three load resistances are also all equal, the

three currents will also be all equal and will have a phase difference of 120° from one

another. In these circumstances the system is said to be balanced.

Fig. 7. Transmission of Three-Phase

Power by Four Conductors

Fig. 8. Transmission of Three- Phase

Power by Three Conductors

Three-Wire Transmission. – In a balanced three-phase system the three currents are

equal and can be represented by the graphs shown in Fig. 6, substituting current for e.

m. f. The resultant current in the tourth (return) conductor is, at any instant, the

algebraic sum of the three line currents and, on examination of the graphs, it is found

that this algebraic sum is zero at every instant. The fourth (return) conductor thus

carries no current and it can be omitted. The connections now take the form shown in

Fig. 8, three conductors only being employed.

Each conductor now acts in turn as the return for the other two. This can be checked

from Fig. 6, where it is seen that the reverse current in one phase is always equal to

the forward current in the other two. (At certain instants, two conductors act as the

return for the forward current in the remaining conductor). It is also general practice