What Is Alternating Current (AC) ?

Most students of electricity begin their study with what is known as direct current (DC), which is
electricity °owing in a constant direction, and/or possessing a voltage with constant polarity. DC
is the kind of electricity made by a battery (with de¯nite 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): Figure 1.1

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

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: Figure 1.3

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 bene¯ts of AC over DC with regard to generator design is also re°ected 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
¯eld 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 e®ect of electromagnetism known as mutual induction, whereby
two or more coils of wire placed so that the changing magnetic ¯eld 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:
Figure 1.4

As useful as transformers are, they only work with AC, not DC. Because the phenomenon of
mutual inductance relies on changing magnetic ¯elds, and direct current (DC) can only produce
steady magnetic ¯elds, 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 ¯eld (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 di®erent from AC. Perhaps
more than any other reason, this is why AC ¯nds 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 tothe 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)