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35
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°.
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 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 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 В is
regarded as the front end and В' 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
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 3×120° = 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.
Fig. 5. Coils arranged to
induce Three-Phase E.M.F.'s
Fig.6. Three-Phase E. M. F.'s or
Currents.
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