94904

THREE-PHASE SYSTEM PRODUCTION OF THREE-PHASE CURRENTS

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Коммуникация, связь, радиоэлектроника и цифровые приборы

In an ordinary a. c. .circuit the current goes through all its phases in succession, but at any particular instant the current has only one phase. In ttie three-phase system there are three circuits, and the currents in these have three different phases at the same instant of time. The phase difference between any two of theseююю

Английский

2015-09-18

27.5 KB

1 чел.

THREE-PHASE SYSTEM

PRODUCTION OF THREE-PHASE CURRENTS

In an ordinary a. c. .circuit the current goes through all its phases in succession,' but at any particular instant the current has only one

phase. In ttie three-phase system there are three circuits, and the currents in these have three different phases at the same instant of time. The phase difference between any two of these three phases is 120°.

Imagine an armature core to be rotated in a counter-clockwise direction between the two poles of a magnetic field excited by D. C., as shown in Fig, 5. The two conductors A and A' are connected in series to form a turn, the front end of the conductor A being considered as the front end of the turn, and the front end of the conductor A' ' being considered as the rear end. As the armature core is rotated,' a sinusoidal e. m. f. is induced in the turn AA'. Next consider the turn BB', where B is regarded as the front end and B' the rear end. A sinusoidal e. in. f. will also be induced in this turn, but it does not reach its maximum value in the positive direction until the core has been rotated through 120°. In other words, this e. m. f. although having the same maximum and r. m. s. value as that induced in the turn AA, is behind it in phase by 120°. Finally, consider the turn CC in the same way. The induced e» m. f. again has the same maximum and r. m. s. value, but it is a further 120° behind in phase. * Following on this, yet another 120° behind in phase, the first turn AA' induces an e. m. f. which is 3 X 120° == 360° behind the original e. m. f. induced in AA. Putting this another way, the turn AA is now beginning to induce the second cycle of e. m. f. The three e. m.f.'s induced in the three turns are represented graphically in Fig. 6, where it is seen that there is a phase difference of 120°, or one third, of a cycle, between the e. m. f.'s of. each pair. If each of these turns is connected to the ends of a resistance,. three currents will be obtained, also having a mutual phase difference of 120°, these currents being called three-phase currents.

In practice it is usual to arrange the armature conductors on the stationary element of the machine, now called the stator, the d. c. excited field forming the rotating element, or rotor. Each winding also is made to consist of many turns. It does not matter, however, whether the conductors cut the magnetic flux, or the magnetic flux cuts the conductors; the action is the same.

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 C' must alt 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.fs 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.

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 fourth (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 to earth the system at one point, this being done conveniently at the generator common junction as shown in Fig. 8.

Three-Phase Four-Wire System.— This is a system of connections which permits the employment of a three-phase load and thres single-phase loads simultaneously, as shown in Fig. 9. It re-introduces the fourth conductor to act as a common return for the three singlephase loads, this conductor being called the neutral. The other three conductors are called the line conductors. The system is earthed as before, by connecting the neutral conductor to an earthed plate or other earthing connection.

Colour Scheme.—For purposes of standardization it is now the

general practice to mark each particular phase by a distinctive colour. The three phases are coloured red, white (or yellow), and blue, respectively, while the neutral conductor is coloured green.

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