Specifications

8
Transformers Once the electrical energy gets near the end user, the utility
steps down the voltage to the level needed by the user. The
device that utilities use to step up the voltage at the generator
end and step down the voltage at the user end is called a
transformer.
The transformer transfers energy from a primary coil to a
secondary coil by mutual induction. The AC generator provides
electrical power to the primary coil. The magnetic field produced
by the primary coil induces a voltage into the secondary coil,
which supplies power to the connected load. The load in this
case is the entire electrical distribution network including all
residential, commercial, and industrial customers.
A step-up transformer is used when it is desirable to step
voltage up from one level to another. The following simplified
example shows a 1:2 step-up transformer could be used to
step 120 volts up to 240 volts. A 2:1 step-down transformer
could be used to step 240 volts down to 120 volts. Keep in
mind that most transformers step up or down the voltage more
significantly than this simple example displays.
1:2
120 VAC
I = 10 amps
Primary Coil
900 Turns
Secondary Coil
1800 Turns
V
load
= 240 VAC
I
load
= 5 amps
Load
2:1
240 VAC
I = 5 amps
Primary Coil
1800 Turns
Secondary Coil
900 Turns
V
load
= 120 VAC
I
load
= 10 amps
Step-Up Transformer Step-Down Transformer
AC
Source
Three-Phase Voltage For simplicity, the generator and transformers shown so far have
been single-phase devices. While single-phase power is needed
for many applications, utilities generate and transmit three-
phase power. In a three-phase system, the generator produces
three voltages. Each voltage phase rises and falls at the same
frequency (60 Hz in the U.S., 50 Hz in many other countries);
however, the phases are offset from each other by 120°.
Phase A Phase B Phase C
+
0
-