The electricity that comes from the
power grid is generated by
alternating-current generators, also known as alternators. The
alternator on a car is a small-scale version of those used by power
companies. An alternator works on the same principle as a DC generator
(review DC motors and generators
in DC Circuits).
A coil of wire, called the armature, rotates in a magnetic field. Each
end of the armature coil connects to a slip ring and each slip ring
connects to a contact called a brush. As the armature rotates, the
coils pass through the magnetic field in one direction then the
other direction. This causes the induced
electrical current to flow one direction and then the other. The
current is taken from brushes to pass through the circuit. (See the
YouTube video at the end of this page for an animation of a DC
generator.)
This is the classic
view of an alternator compared to a generator.
However, most alternators actually use a rotating magnetic field and
the electricity is produced in stationary coils.
Practical AC generators
Most alternators, including the ones used in cars are three-phase
alternators. A three phase alternator is essentially three alternators
in one housing. A simplified model has three coils of wire placed 120
degrees apart in a circular housing.
A
simplified model of a three phase alternator
The armature consists of a magnet
that rotates inside the space
between the three coils. Alternating current is produced as the
magnetic field of the armature
passes the coils. In a real world alternator DC is sent
through the brushes to the slip rings and through the coil of the
rotating armature (called the field coil). This produces a magnetic
field around the armature coil. Since this magnetic field is produced
by DC it is a steady unchanging field like a permanent magnet. The
armature rotates, rotating the
magnetic field with it. This rotating magnetic field is moved past the
stationary stator coils in the alternator housing (the stator). As the
alternate
north and south poles of the armature move past the stator coils,
alternating current is produced in each stator coil.
Regulating the Alternator Output
The output of a generator or alternator is regulated by controlling the
amount of direct current in the field coil. If the output voltage rises
too high, the current in the field coil is reduced. If the output
voltage drops too low, the current in the field coil is increased.
AC on the Power Grid
The three phase power, produced by power company alternators, is
optimized for industrial motors. Industrial areas have three phase
power distributed by three wires from the power plants. The power that
goes to homes is taken from a single coil in the alternator (actually
from two of the three wires that carry three phase power to the
industrial areas). Therefore, the AC power most of us encounter is a
single-phase sine wave, as described below.
Other Methods of Generating AC
Alternating current is also generated by oscillator circuits. These
are circuits that have a level of instability. This instability causes them to have
a constantly changing voltage that repeats the same pattern over and
over. We will see some of these patterns in AC Waveforms later.
Generating Alternating Current
Olde Tyme Video
Here is a U.S. Army training film on AC motors and generators from 1961.
AC Motors and Generators (US Army Training Film)
Here is a short animation of an AC generator from the above video.
