An
understanding of how an AC generator develops an AC output will help us analyze
the AC power generation process.
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. The induced voltages add in series,
making slip ring X (Figure 1) positive (+) and slip ring Y (Figure 1) negative
(-). The potential across resistor R will cause a current to flow from Y to X
through the resistor. This current will increase until it reaches a maximum
value when the coil is horizontal to the magnetic lines of force (Figure 2,
90o). The horizontal coil is moving perpendicular to the field and is cutting
the greatest number of magnetic lines of force. As the coil continues to turn,
the voltage and current induced decrease until they reach zero, where the coil
is again in the vertical position (Figure 2, 180o). In the other half
revolution, an equal voltage is produced except that the polarity is reversed
(Figure 2, 270o, 360o). The current flow through R is now from X to Y (Figure
1).
Figure
2 Developing a Sine-Wave Voltage
The
periodic reversal of polarity results in the generation of a voltage, as shown
in Figure 2. The rotation of the coil through 360° results in an AC sine wave
output.
Summary
• A
simple generator consists of a conductor loop turning in a magnetic field,
cutting across the magnetic lines of force.
•
The sine wave output is the result of one side of the generator loop cutting
linesof force. In the first half turn of rotation this produces a positive
current and in the second half of rotation produces a negative current. This
completes one cycle of AC generation.
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