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Figure 3-7.—Connections of a two-phase, three-wire alternator output. | Up
Neets Module 05-Introduction to Generators and Motors | Next
Three-Phase Connections |
3-10
Now, look at the smaller schematic diagram in figure 3-7. Only three connections have been brought
out from the stator. Electrically, this is the same as the large diagram above it. Instead of being connected
at the output terminals, the B1-A2 connection was made internally when the stator was wired. A two-
phase alternator connected in this manner is called a two-phase, three-wire alternator.
The three-wire connection makes possible three different load connections: A and B (across each
phase), and C (across both phases). The output at C is always 1.414 times the voltage of either phase.
These multiple outputs are additional advantages of the two-phase alternator over the single-phase type.
Now, you can understand why single-phase power doesn’t always come from single-phase
alternators. It can be generated by two-phase alternators as well as other multiphase (polyphase)
alternators, as you will soon see.
The two-phase alternator discussed in the preceding paragraphs is seldom seen in actual use.
However, the operation of polyphase alternators is more easily explained using two phases than three
phases. The three-phase alternator, which will be covered next, is by far the most common of all
alternators in use today, both in military and civilian applications.
Q11. What determines the phase relationship between the voltages in a two-phase ac generator?
Q12. How many voltage outputs are available from a two-phase three-wire alternator?
Q13. What is the relationship of the voltage at C in figure 3-7 to the voltages at A and B?
THREE-PHASE ALTERNATOR
The three-phase alternator, as the name implies, has three single-phase windings spaced such that the
voltage induced in any one phase is displaced by 120º from the other two. A schematic diagram of a
three-phase stator showing all the coils becomes complex, and it is difficult to see what is actually
happening. The simplified schematic of figure 3-8, view A, shows all the windings of each phase lumped
together as one winding. The rotor is omitted for simplicity. The voltage waveforms generated across
each phase are drawn on a graph, phase-displaced 120º from each other. The three-phase alternator as
shown in this schematic is made up of three single-phase alternators whose generated voltages are out of
phase by 120º . The three phases are independent of each other.
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