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A major advantage of alternating current
(AC) electricity over direct current (DC) is the fact
that the potential energy can be readily changed from
one voltage to another through the use of a transformer.
An AC transformer changes power line high voltages to
house current. It also is used to change the voltage
from house current to that used by low voltage devices.
Often the AC in these small transformers or adapters is
also changed to DC.
Questions you may have are:
-
How does the transformer work?
-
What are the principles of
electricity and magnetism involved?
-
What are some uses of transformers?
Basic principles
A transformer combines several major
characteristics of electricity and magnetism to change
AC voltages. First of all, you need to know the
principles for creating an electromagnet and creating
electricity.
A wire with DC electric current flowing
through it has a magnetic field around it. Placing a
compass near a wire and observing the needle move when
the DC current is turned on can demonstrate this.
By wrapping the wire around a piece of
iron, the magnetic field is increased many times due to
the realignment of the iron atoms, each which acts as a
tiny magnet. The iron core and wire wrapping is called
an electromagnet.
The greater the current through the wire
(or the higher the voltage) the greater the strength of
the electromagnet. The strength is approximately
proportional to the voltage used. Double the voltage and
you double the strength of the electromagnet.
The greater the number of turns around
the iron core the greater the strength of the
electromagnet. The strength is approximately
proportional to the number of turns. Triple the number
of turns and you triple the strength of the
electromagnet.
(Experiment idea: measure the change of
strength of an electromagnet by changing the voltage
and/or number of turns.)
The direction of the magnetic field is
determined by the direction of the current and the
direction of the turns around the iron core. If you
change the direction of the current, the north and south
poles of the electromagnet will switch.
With DC electricity, you must physically
change the wires to change the direction of the current.
With AC electricity, the direction changes with each
cycle.
Thus, one end of an AC electromagnet is
switching from north to south and back again 60 times
per second in the U.S. or 50 times per second in some
other countries.
Electricity is created either when a
wire is moved through a magnetic field or when a
magnetic field is moved past a wire. Moving the magnetic
field past the wire can be done by physically moving a
magnet past the wire or by somehow changing the amount
of the magnetic field.
Transforming the voltage
To transform or change the voltage of AC
electricity, you use an AC electromagnet and the
principles described above.
An AC electromagnet continually changes
the direction of its magnetic field. This means the
field goes from zero to N to zero to S and so on. If you
would put an AC electromagnet near a wire, then its
changing magnetic field should create a current in the
wire.
Or better yet, why not wrap the wire
around the iron core of the electromagnet? This is how a
transformer works.
A transformer can be a long piece of
iron with wire--with AC current going through
it--wrapped around it near one end and wire in which to
create current wrapped around it at the other end. A
more common configuration is a square or donut shaped
iron core with the wire wrapping on both sides.
Transformer changes voltage
The strength of the magnetic field is
proportion to the input voltage and the number of turns
around the core (called the primary coil). By reversing
the rule, the output voltage is proportional to the
strength of the changing magnetic field and the number
of turns (called the secondary coil).
For example, if you wanted to increase
your house voltage from 110 volts (110V) to 220V in
order to power your electric stove, you could use a
transformer with twice the turns in the secondary coil
as in the primary coil.
The relationship is written as:
input volts / input turns = output volts
/ output turns
110V / 5 turns = 22 = 220V / 10 turns
Using transformers
Transformers are used to lower voltage
to be safer to use in your house. You may also use an
adapter to lower the voltage even more for some devices
you use. DC transformers are now available, but they
won't replace AC transformers.
Normally, the current in the electrical
lines outside your house are around 1100V AC. The reason
it is so high is that the electricity travels more
effectively over long distances at higher voltages. High
voltage lines carry up to 10,000 volts.
The transformer near to top of the
electrical pole changes the voltage to a safer 110V for
your house.
Most people use adapters when they power
devices that also use batteries. An adapter is a
transformer that changes the 110V AC house current to
12V DC or 9V DC that is used by the device. It also
changes the AC to DC, because the device works on
batteries.
Changing AC to DC is done by electronic
circuitry called a rectifier. It essentially chops off
1/2 of the AC current to make it similar to DC. Some of
the lost AC current is turned into heat. That is one
reason your adapters sometimes get warm.
You can see that DC voltages could not
be changed with the configuration of the transformer.
This is because the DC current would not be changing the
magnetic field the way AC current does. And this was the
reason that AC won over DC when electricity started to
be used around the world.
Since then, electrical engineers have
developed DC transformers, primarily using special
circuitry. Since most everyone now uses AC, it is too
late to change the system.
The principles for creating an AC
electromagnet and how a changing magnetic field leads to
how a transformer works. The output voltage of a
transformer is proportional to the ratio of the number
of turns of the coils. Transformers reduce the high
voltage to a safer home voltage, as well as to reduce
110V AC to what can be used in some battery-powered
devices. |
