The supply of current for electrical devices may come from
a direct current (DC) source or analternating current (AC)
source. In a direct current circuit, electrons flow continuously
in one direction from the source of power through a conductor
to a load and back to the source of power. Voltage polarity for
a direct current source remains constant. DC power sources
include batteries and DC generators.
By contrast, an AC generator makes electrons flow first in one
direction then in another. In fact, an AC generator reverses its
terminal polarities many times a second, causing current to
change direction with each reversal.
AC Sine WaveAlternating voltage and current vary continuously. The graphic
representation for AC is a sine wave. A sine wave can represent
current or voltage. There are two axes. The vertical axis
represents the direction and magnitude of current or voltage.
The horizontal axis represents time.
When the waveform is above the time axis, current is flowing in
one direction. This is referred to as the positive direction. When
the waveform is below the time axis, current is flowing in the
opposite direction. This is referred to as the negative direction.
A sine wave moves through a complete rotation of 360
degrees, which is referred to as one cycle. Alternating current
goes through many of these cycles each second.
Basic AC GeneratorA basic generator consists of a magnetic field, an armature,
slip rings, brushes and a resistive load. In a commercial
generator, the magnetic field is created by an electromagnet,
but, for this simple generator, permanent magnets are used.
Anarmature is any number of conductive wires wound in loops
which rotates through the magnetic field. For simplicity, one
loop is shown. When a conductor is moved through a magnetic
field, a voltage is induced in the conductor. As the armature
rotates through the magnetic field, a voltage is generated in the
armature which causes current to flow. Slip rings are attached
to the armature and rotate with it. Carbon brushes ride against
the slip rings to conduct current from the armature to a resistive
An armature rotates through the magnetic field. At an initial
position of zero degrees, the armature conductors are moving
parallel to the magnetic field and not cutting through any
magnetic lines of flux. No voltage is induced.
Generator Operation fromAs the armature rotates from zero to 90 degrees, the
Zero to 90 Degreesconductors cut through more and more lines of flux, building up
to a maximum induced voltage in the positive direction.
Generator Operation fromThe armature continues to rotate from 90 to 180 degrees,
90 to 180 Degreescutting fewer lines of flux. The induced voltage decreases from
a maximum positive value to zero.
Generator Operation fromAs the armature continues to rotate from 180 degrees to 270
180 to 270 Degreesdegrees, the conductors cut more lines of flux, but in the
opposite direction, and voltage is induced in the negative
direction, building up to a maximum at 270 degrees.
Generator Operation fromAs the armature continues to rotate from 270 to 360 degrees,
270 to 360 Degreesinduced voltage decreases from a maximum negative value
to zero. This completes one cycle. The armature continues to
rotate at a constant speed causing the cycle to repeat as long
as the armature rotates.
Four-Pole AC GeneratorAn AC generator produces one cycle per revolution for each
pair of poles. An increase in the number of poles, causes an
increase in the number of cycles completed in a revolution. A
two-pole generator completes one cycle per revolution and a
four-pole generator completes two cycles per revolution.
FrequencyThe number of cycles per second of voltage induced in the
armature is thefrequency of the generator. If a two-pole
generator armature rotates at a speed of 60 revolutions
per second, the generated voltage have a frequency of 60
cycles per second. The recognized unit for frequency ishertz,
abbreviatedHz. 1 Hz is equal to 1 cycle per second.
Power companies generate and distribute electricity at very low
frequencies. The standard power line frequency in the United
States and many other countries is 60 Hz. 50 Hz is also a
common power line frequency used throughout the world. The
following illustration shows 15 cycles in 1/4 second which is
equivalent to 60 Hz.
AmplitudeAs previously discussed, voltage and current in an AC circuit
rise and fall over time in a pattern referred to as a sine wave.
In addition to frequency, which is the rate of variation, an AC
sine wave also hasamplitude, which is the range of variation.
Amplitude can be specified in three ways: peak value, peak-topeak
value, and effective value.
Thepeak value of a sine wave is the maximum value for each
half of the sine wave. Thepeak-to-peak value is the range
from the positive peak to the negative peak. This is twice the
peak value. Theeffective value of AC is defined in terms of an
equivalent heating effect when compared to DC. Instruments
designed to measure AC voltage and current usually display the
effective value. The effective value of an AC voltage or current is
approximately equal to 0.707 times the peak value.
The effective value is also referred to as theRMS value. This
name is derived from the root-mean-square mathematical
process used to calculate the effective value of a waveform.
Instantaneous ValueThe instantaneous value is the value at any one point on the
sine wave. The voltage waveform produced as the armature of
a basic two-pole AC generator rotates through 360 degrees is
called a sine wave because the instantaneous voltage or current
is related to the sine trigonometric function.
As shown in the following illustration, the instantaneous
voltage (e)and current (i) at any point on the sine wave are
equal to the peak value times the sine of the angle. The sine
values shown in the illustration are obtained from trigonometric
tables. Keep in mind that each point has an instantaneous
value, but this illustration only shows the sine of the angle
at 30 degree intervals. The sine of an angle is represented
symbolically as sinq, where the Greek letter theta (q)
represents the angle.
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