current in the primary coil produces
Distribution transformers are used to
an alternating magnetic field in the
reduce high primary voltages to lower
iron core.
A voltage is induced in
usable secondary voltages.
the secondary coil by the alternating
field.
c. Taps.
Many transformers have
extra taps on the primary winding so
a. Substation
transformers.
that full secondary voltage can be
Substation transformers should be of
maintained even though the primary
the outdoor type and of adequate self-
voltage is reduced.
Figure 9 shows a
cooled rating.
2,400/120-volt transformer (20:1) with
two extra taps on the primary winding.
b. Distribution
transformers.
In the transformer, one tap, the 95
Utilization voltage for most military
percent tap, gives a 19:1 ratio, while
electrical equipment is 120, 208, or
the 90 percent tap gives an 18:1
240 volts.
However, since it is not
ratio.
If the primary voltage is 5
practical
to
transmit
appreciable
percent
below
2,400
volts,
the
quantities
of
electrical
energy
transformer is connected to the 95
farther than 1,000 to 1,500 feet at
percent
tap,
thus
keeping
the
these
low
voltages,
higher
secondary voltage at 120 volts.
distribution
voltages
are
used.
EXERCISES
First requirement.
Each of the
1
a.
2W
120V
following
multiple-choice
exercises
1
b.
3W
120/240V
has four choices with only ONE best
3
c.
2W
120V
answer. Select the choice you believe
3
d.
3W
120/240V
is best.
Then turn to the answer
sheet and mark an X through the letter
3. In
a
single-phase
two-wire
representing that choice. Exercises 1
system what is the relationship
through 8 provide an opportunity for
between the amounts of current
you to show that you understand the
carried in each of the two wires?
basic types of electric power systems.
a. currents are equal
b. relationship depends on the size
1. Direct
current,
under
the
of the load
condition of constant load, flows
c. currents differ by a factor of
in the same direction at all times.
1.73
Which of the following is also a
d. relationship
depends
on
the
characteristic of direct current?
magnitude of the voltage
a. is economical for use in 3 3W
systems
4. What type of system has no
b. has a constant magnitude
phase difference between any of the
c. has a variable frequency
available voltages?
d. is necessary in the operation of
a. single-phase (1)
transformers
b. three-phase (3)
c. three-wire (3W)
2. You are assigned the project
d. four-wire (4W)
of
classifying
the
electrical
systems
of
a
captured
enemy
5. You are attempting to balance
installation.
How
would
you
the loads in a single-phase three-
classify the system in figure 10?
wire
system.
What
would
you
1-15
Previous Page
