Page 90 - Electric Machinery Fundamentals

66 ELECTRIC MACHINERY FUNDAMENTALS

A transformer is a device that changes ac electric power at one frequency and
voltage level to ac electric power at the same frequency and another voltage level
through the action of a magnetic field. It consists of two or more coils of wire
wrapped around a common ferromagnetic core. These coils are (usually) not di-
rectly connected. The only connection between the coils is the common magnetic
flux present withjn the core.
One of the transformer windings is connected to a source of ac electric
power, and the second (and perhaps third) transformer winding supplies electric
power to loads. The transformer winding connected to the power source is called
the primary winding or input winding, and the winding connected to the loads is
called the secondary winding or output winding. If there is a third winding on the
transformer, it is called the tertiary winding.

2,1 WHY TRANSFORMERS ARE
IMPORTANT TO MODERN LIFE

The first power distribution system in the United States was a 120-V de system in-
vented by Thomas A. Edison to supply power for incandescent light bulbs. Edison's
first central power station went into operation in New York City in September
1882. Unfortunately, his power system generated and transmitted power at such
low voltages that very large currents were necessary to supply significant amounts
of power. These high currents caused huge voltage drops and power losses in the
transmission lines, severely restricting the service area of a generating station. In
the 1880s, central power stations were 10cated every few city blocks to overcome
this problem. The fact that power could not be transmitted far with low-voltage de

FIGURE 2-1
The first practical modern transformer, built by William Stanley in 1885. Note that the core is made
up of individual sheets of metal (laminations). (Courtesy of General Electric Company.)