Basic AC Theory
An understanding of how an AC generator develops an AC output will help the
student analyze the AC power generation process.
DESCRIBE the construction and operation of a simple
EXPLAIN the development of a sine-wave output in an
The elementary AC generator (Figure 1) consists of a conductor, or loop of wire in a magnetic
field that is produced by an electromagnet. The two ends of the loop are connected to slip rings,
and they are in contact with two brushes. When the loop rotates it cuts magnetic lines of force,
first in one direction and then the other.
Development of a Sine-Wave Output
Figure 1 Simple AC Generator
At the instant the loop is in the vertical position (Figure 2, 0o), the coil sides are moving parallel
to the field and do not cut magnetic lines of force. In this instant, there is no voltage induced
in the loop. As the coil rotates in a counter-clockwise direction, the coil sides will cut the
magnetic lines of force in opposite directions. The direction of the induced voltages depends on
the direction of movement of the coil.