Page 44 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

Source: ELECTRICAL EQUIPMENT HANDBOOK

CHAPTER 3
TRANSFORMERS

A transformer is a device that uses the action of a magnetic field to change ac electric energy
at one voltage level to ac electric energy at another voltage level. It consists of a ferromag-
netic core with two or more coils wrapped around it. The common magnetic flux within the
core is the only connection between the coils. The source of ac electric power is connected
to one of the transformer windings. The second winding supplies power to loads. The wind-
ing connected to the power source is called the primary winding or input winding. The
winding connected to the loads is called the secondary winding or output winding.

IMPORTANCE OF TRANSFORMERS

When a transformer steps up the voltage level of a circuit, it decreases the current because
the power remains constant. Therefore, ac power can be generated at one central station. The
voltage is stepped up for transmission over long distances at very low losses. The voltage
is stepped down again for final use. Since the transmission losses are proportional to the
square of the current, raising the voltage by a factor of 10 will reduce the transmission
losses by a factor of 100. Also, when the voltage is increased by a factor of 10, the cur-
rent is decreased by a factor of 10. This allows the use of much thinner conductors to
transmit power.
In modern power stations, power is generated at 12 to 25 kV. Transformers step up the
voltage to 110 to 1000 kV for transmission over long distances at very low losses.
Transformers then step it down to 12 to 34.5 kV for local distribution and then permit
power to be used in homes and industry at 120 V.

TYPES AND CONSTRUCTION OF
TRANSFORMERS

The function of a transformer is to convert ac power from a voltage level to another voltage
level at the same frequency. The core of a transformer is constructed from thin laminations
electrically isolated from each other to reduce eddy current losses (Fig. 3.1).
The primary and secondary windings are wrapped one on top of the other around the
core with the low-voltage winding innermost. This arrangement serves two purposes:

1. The problem of insulating the high-voltage winding from the core is simplified.
2. It reduces the leakage flux compared to if the windings were separated by a distance on
the core.

3.1
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