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 circuit 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.
- REVIEW:
- DC stands for "Direct Current,"
meaning voltage or current that maintains constant polarity or
direction, respectively, over time.
- AC stands for "Alternating Current,"
meaning voltage or current that changes polarity or direction,
respectively, over time.
- AC electromechanical generators, known
as alternators, are of simpler construction than DC
electromechanical generators.
- AC and DC motor design follows
respective generator design principles very closely.
- 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.
- Secondary voltage = Primary voltage
(secondary turns / primary turns)
- Secondary current = Primary current
(primary turns / secondary turns)
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