Text 11. Production of Three-Phase Currents

In an ordinary a.c. circuit the current through all its phases in succession, but at any particular instant the current has only one phase. In the 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 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. m. 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 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, wet another 120 behind in phase,the first turn AA” induces an e. m. f. which is 3X120=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, there 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.

Дата добавления: 2015-08-20