What is alternating current (AC)?
Most students of electricity begin their
study with what is known as direct current (DC),
which is electricity flowing in a constant direction, and/or
possessing a voltage with constant polarity. DC is the kind
of electricity made by a battery (with definite positive and
negative terminals), or the kind of charge generated by
rubbing certain types of materials against each other.
As useful and as easy to understand as DC
is, it is not the only "kind" of electricity in use. Certain
sources of electricity (most notably, rotary
electro-mechanical generators) naturally produce voltages
alternating in polarity, reversing positive and negative
over time. Either as a voltage switching polarity or as a
current switching direction back and forth, this "kind" of
electricity is known as Alternating Current (AC):
Whereas the familiar battery symbol is used
as a generic symbol for any DC voltage source, the circle
with the wavy line inside is the generic symbol for any AC
voltage source.
One might wonder why anyone would bother
with such a thing as AC. It is true that in some cases AC
holds no practical advantage over DC. In applications where
electricity is used to dissipate energy in the form of heat,
the polarity or direction of current is irrelevant, so long
as there is enough voltage and current to the load to
produce the desired heat (power dissipation). However, with
AC it is possible to build electric generators, motors and
power distribution systems that are far more efficient than
DC, and so we find AC used predominately across the world in
high power applications. To explain the details of why this
is so, a bit of background knowledge about AC is necessary.
If a machine is constructed to rotate a
magnetic field around a set of stationary wire coils with
the turning of a shaft, AC voltage will be produced across
the wire coils as that shaft is rotated, in accordance with
Faraday's Law of electromagnetic induction. This is the
basic operating principle of an AC generator, also known as
an alternator:
Notice how the polarity of the voltage
across the wire coils reverses as the opposite poles of the
rotating magnet pass by. Connected to a load, this reversing
voltage polarity will create a reversing current direction
in the circuit. The faster the alternator's shaft is turned,
the faster the magnet will spin, resulting in an alternating
voltage and current that switches directions more often in a
given amount of time.
While DC generators work on the same general
principle of electromagnetic induction, their construction
is not as simple as their AC counterparts. With a DC
generator, the coil of wire is mounted in the shaft where
the magnet is on the AC alternator, and electrical
connections are made to this spinning coil via stationary
carbon "brushes" contacting copper strips on the rotating
shaft. All this is necessary to switch the coil's changing
output polarity to the external circuit so the external
circuit sees a constant polarity:
The generator shown above will produce two
pulses of voltage per revolution of the shaft, both pulses
in the same direction (polarity). In order for a DC
generator to produce constant voltage, rather than
brief pulses of voltage once every 1/2 revolution, there are
multiple sets of coils making intermittent contact with the
brushes. The diagram shown above is a bit more simplified
than what you would see in real life.
The problems involved with making and
breaking electrical contact with a moving coil should be
obvious (sparking and heat), especially if the shaft of the
generator is revolving at high speed. If the atmosphere
surrounding the machine contains flammable or explosive
vapors, the practical problems of spark-producing brush
contacts are even greater. An AC generator (alternator) does
not require brushes and commutators to work, and so is
immune to these problems experienced by DC generators.
The benefits of AC over DC with regard to
generator design is also reflected in electric motors. While
DC motors require the use of brushes to make electrical
contact with moving coils of wire, AC motors do not. In
fact, AC and DC motor designs are very similar to their
generator counterparts (identical for the sake of this
tutorial), the AC motor being dependent upon the reversing
magnetic field produced by alternating current through its
stationary coils of wire to rotate the rotating magnet
around on its shaft, and the DC motor being dependent on the
brush contacts making and breaking connections to reverse
current through the rotating coil every 1/2 rotation (180
degrees).
So we know that AC generators and AC motors
tend to be simpler than DC generators and DC motors. This
relative simplicity translates into greater reliability and
lower cost of manufacture. But what else is AC good for?
Surely there must be more to it than design details of
generators and motors! Indeed there is. There is an effect
of electromagnetism known as mutual induction,
whereby two or more coils of wire placed so that the
changing magnetic field created by one induces a voltage in
the other. If we have two mutually inductive coils and we
energize one coil with AC, we will create an AC voltage in
the other coil. When used as such, this device is known as a
transformer:
The fundamental significance of a
transformer is its ability to step voltage up or down from
the powered coil to the unpowered coil. The AC voltage
induced in the unpowered ("secondary") coil is equal to the
AC voltage across the powered ("primary") coil multiplied by
the ratio of secondary coil turns to primary coil turns. If
the secondary coil is powering a load, the current through
the secondary coil is just the opposite: primary coil
current multiplied by the ratio of primary to secondary
turns. This relationship has a very close mechanical
analogy, using torque and speed to represent voltage and
current, respectively:
If the winding ratio is reversed so that the
primary coil has less turns than the secondary coil, the
transformer "steps up" the voltage from the source level to
a higher level at the load:
The transformer's ability to step AC voltage
up or down with ease gives AC an advantage unmatched by DC
in the realm of power distribution. When transmitting
electrical power over long distances, it is far more
efficient to do so with stepped-up voltages and stepped-down
currents (smaller-diameter wire with less resistive power
losses), then step the voltage back down and the current
back up for industry, business, or consumer use use.
Transformer technology has made long-range
electric power distribution practical. Without the ability
to efficiently step voltage up and down, it would be
cost-prohibitive to construct power systems for anything but
close-range (within a few miles at most) use.
As useful as transformers are, they only
work with AC, not DC. Because the phenomenon of mutual
inductance relies on changing magnetic fields, and
direct current (DC) can only produce steady magnetic fields,
transformers simply will not work with direct current. Of
course, direct current may be interrupted (pulsed) through
the primary winding of a transformer to create a changing
magnetic field (as is done in automotive ignition systems to
produce high-voltage spark plug power from a low-voltage DC
battery), but pulsed DC is not that different from AC.
Perhaps more than any other reason, this is why AC finds
such widespread application in power systems.
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REVIEW:
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DC stands for "Direct Current," meaning
voltage or current that maintains constant polarity or
direction, respectively, over time.
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AC stands for "Alternating Current,"
meaning voltage or current that changes polarity or
direction, respectively, over time.
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AC electromechanical generators, known as
alternators, are of simpler construction than DC
electromechanical generators.
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AC and DC motor design follows respective
generator design principles very closely.
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A transformer is a pair of
mutually-inductive coils used to convey AC power from one
coil to the other. Often, the number of turns in each coil
is set to create a voltage increase or decrease from the
powered (primary) coil to the unpowered (secondary) coil.
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Secondary voltage = Primary voltage
(secondary turns / primary turns)
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Secondary current = Primary current
(primary turns / secondary turns)
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