Specifications

7
If you track the rotation of the AC generator through a complete
revolution of 360°, you would see that, during the first quarter
of a revolution, voltage increases until it reaches a maximum
positive value at 90°. Voltage decreases during the second
quarter of a revolution until it reaches zero at 180°. During the
third quarter of a revolution, voltage increases in the opposite
direction until it reaches a maximum negative value at 270°.
During the last quarter of a revolution, voltage decreases until it
reaches zero at 360°.
This is one complete cycle of operation. If the armature of this
simple AC generator rotates 3600 times per minute (3600 RPM),
it produces 60 cycles of voltage per second, or 60 hertz.
Energy Transfer The role of the generator just described is to change mechanical
energy into electrical energy. In order for this energy to be
useful, however, it must be transmitted to the utility’s customers
via transmission lines. The most efficient way to do this is
to increase the voltage while at the same time reducing the
current. This is necessary to minimize the energy lost in heat
on the transmission lines. These losses are referred to as I
2
R (I-
squared-R) losses because they are equal to the square of the
current times the resistance of the power lines.