delta-connected system has only a single voltage
rating (220 V in fig. 9-4). However, in a
Y-connected system, the voltage developed in
different combinations of wires is different. In
figure 9-5, you can see that lines 1 and 2 take
power from two stator coils (A and C). The same
applies to lines 1 and 3 (power from coils C and
B) and lines 2 and 3 (power from coils A and B).
However, the neutral (N) and line 2 take power
from coil A only; neutral (N) and line 1, from
coil C only; and neutral (N) and line 3, from
coil B only.
It follows from this that a Y-connected
alternator can produce two different voltages: a
higher voltage in any pair of hot wires, or in all
three hot wires, and a lower voltage in any hot
wire paired with the neutral wire.
Output taken from a pair of wires is SINGLE-
PHASE voltage; output from three wires is
THREE-PHASE voltage. The practical signifi-
cance of this lies in the fact that some
electrical equipment is designed to operate only
on single-phase voltage, while other equipment
is designed to operate only on three-phase voltage.
This equipment includes the alternators them-
selves, and a system with a three-phase alternator
is called a three-phase system. However, even in
such a system, single-phase voltage can be
obtained by tapping only two of the wires.
Figure 9-6 shows a four-wire system serving
the same facilities. Here there is a Y-connected
alternator rated at 110/220 V. You can see that
to get 110 V single phase for the secondary mains,
no transformers are necessary. These mains are
simply tapped into pairs of wires, one of each pair
being a hot wire and the other, the neutral wire.
The 220-V, three phase motor is tapped into the
three hot wires that develop 220 V, three-phase.
You can see that the neutral wire in a four-wire
system exists to make it possible for a lower
voltage to be used in the system.
Figure 9-7 shows a wiring diagram for the
system shown in figure 9-6.
Now, let’s discuss the device called a
DISTRIBUTION TRANSFORMER. A trans-
former is simply a device for increasing or
reducing the voltage in an electrical circuit. It
ranges in size from one that is portable (those used
for appliances inside the building) to heavy ones
that are mounted permanently on platforms or
Figure 9-7.-Wiring diagram of the four-wire system in
figure 9-6.
hung with crossarm brackets attached to an
electric pole. Ask one of the CES to show you a
transformer. It is very probable that one is nearby.
Now, for long-distance power transmission,
a voltage higher than that normally generated is
required. A transformer is used to step the voltage
up to that required for transmission. Then at the
service distribution end, the voltage must be
reduced to that required for lights and equipment.
Again a transformer is used; but this time it is to
step down the voltage.
The reason for stepping up the voltage in a
line lies in the fact that the greater the distance,
the more resistance there will be to the current
flow; and a much greater force will be required
to push the current through the conductor.
Perhaps you can best understand this reasoning
if you examine Ohm’s Law.
(Refer to chapter 1 of this book.)
You can see from the formula above that the
CURRENT (I) varies inversely to the RESIST-
ANCE (R). To maintain the required current flow
9-5
