How Transformers Work
There are many sizes, shapes and
configurations of transformers from tiny to gigantic like those used in power
transmission. Some come with stubbed out wires, others with screw or
spade terminals, some made for mounting in PC boards, others for being screwed
or bolted down.
Transformers are composed of a laminated
iron core with one or more windings of wire. They are called
transformers because they transform voltage and current from one level to
another. An alternating current flowing through one coil of wire, the
primary, induces a voltage in one or more other coils of wire, the secondary
coils. It is the changing voltage of AC current that induces voltage in
the other coils through the changing magnetic field. DC voltage such as from a
battery or DC power supply will not work in a transformer. Only AC makes
a transformer work. The magnetic field flows through the iron core.
The faster the voltage changes, the higher the frequency.
The lower the frequency, the more iron
is required in the core for the efficient transfer of power. In the USA,
the line frequency is 60 Hertz with a nominal voltage of 110 volts.
Other countries use 50 Hertz, 220 volts. Transformers made for 50 Hertz
must be a little heavier than ones made for 60 Hertz because they must have
more iron in the core. Line voltage can vary a little and usually runs
between 110 volts and 120 volts or between 220 and 240 volts depending on
country or power connections. A house in the USA has 220 volts coming in
but is split to two legs of 110V by grounding the center tap (see
configuration section below)
The ratio of input voltage to output
voltage is equal to the ratio of turns of wire around the core on the
input side to the output side. A coil of wire on the input side is
called the primary and on the output side is called the secondary.
There can be multiple primary and secondary coils. The current ratio
is opposite the voltage ratio. When the output voltage is lower than
the input voltage, the output current will be higher than the input
current. If there are 10 times the number of turns of wire on the
primary than the secondary and you put 120 volts on the primary, you will
get 12 volts out on the secondary. If you pull 2 amps out from the
secondary, you will only be using 0.2 amps or 200 milliamps going into the
primary.
Transformers can be built so they
have the same number of windings on primary and secondary or different
numbers of windings on each. If they are the same, the input and
output voltage are the same and the transformer is just used for isolation
so there is no direct electrical connection (they are only linked through
the common magnetic field). If there are more windings on the
primary side than the secondary side, then it is a step down transformer.
If there are more windings on the primary side, then it is a step up
transformer.
A transformer can actually be used in
reverse and will work fine. For example, if you have a step up
transformer built for transforming 120 volts to 240 volts, you can also
use it for a step down transformer by putting 240 volts into the secondary
side and you will get 120 volts on the primary side. Effectively,
the secondary becomes the primary and vice versa.
Transformer Power Ratings
Voltage is measured in volts, current
is measured in amps, and the unit of measure for power is watts.
Watts is equal to the volts times the amps. There is a little loss
of power in a transformer due to the combination of resistance and
reactance. Reactance is similar to resistance except it is the
resistance to an AC current or more technically, the resistance to change
in a change in current due to the change in the field created. This
heat is what limits the amount of current or power a transformer can
handle. The higher the current, the more heat is produced.
When the wires get too hot, the insulation breaks down and shorts with
adjacent wires which causes more heat which eventually melts wires and
ruins the transformer.
A basic transformer has no additional
components and so nothing to protect it from overloading. If you
were to connect the two output wires directly together, that will
constitute a short circuit and cause far too much current to flow in both
the primary and secondary and you will burn out the transformer. In
the same way, if you use the transformer to power a hot wire foam cutter
and you are using a wire with too little resistance for your foam cutter,
you will burn out your transformer if you don't have it protected by a
proper value fuse or breaker. You have to make sure that the wire
resistance, in other words, the gage or diameter, and the length is
correct to limit the amount of current to under the rating of the
transformer.
The higher the current, the larger
the wires need to be that carry that current. When the wires are
larger, there is less resistance and so less heat. The power that is
changed to heat and lost can be calculated as P=I2R. That
means that if you double the current, the power lost to heat increases by
four times. If the transformer is a step down transformer, then
there will be more current on the output and so the wire in the secondary
windings will be heavier than the primary. The reverse is true for a
step up transformer.
A transformer may be rated in Amps,
Volt-Amps (VA), or Watts (W). For small transformers, VA and Watts
are the same thing for all practical purposes. In large industrial
transformers, power factors get involved and the two can be different.
If the transformer is rated in amps, it usually says X amps at X volts and
is rated on the output or secondary side. A 120V transformer with
24V out rated at 2 amps means that you can only safely pull 2 amps from
the secondary side. You can find the power rating of the transformer
by multiplying the rated amps times the output voltage so 2 X 24 = 48
watts.
If the transformer is rated in VA or
watts, you can calculate the maximum allowable output current by dividing
the VA or watts by the output voltage. So if the transformer is
rated at 48 VA with 24 volts output, the allowable output current is 48 /
24 = 2 amps.
Transformer Configurations
A 120 volt transformer with two wires
in and two wires out is very simple. You hook up the two wires on
the primary side, the 120V side, to a wall outlet and your output voltage
is on the two wires coming from the secondary side.
When a transformer is shown in an
electronic circuit, it is shown as a diagram like shown here. The
parallel lines represent the laminated iron core, the curved lines
represent the primary and secondary windings, the circles represent the
terminations whether terminals or short wires.
Center Tap
A common configuration is a center
tap or CT. The secondary side has three wires out. The middle
wire on the output side is attached to the secondary coil, usually at the
middle. If the winding ratio is 5 to 1, then with 120V input, you
get 24 volts output on the two outside wires but if you connect an
outside wire and the center wire, you get 12 volts because you are using
only half the secondary winding making the connection a 10 to 1 ratio.
If the transformer is rated at 2 amps, you still can only use 2 amps
output whether you use 12 volts or 24 volts. Often the center tap is
grounded so you then have two 12 volt sources that can be used to make +
and - 12V DC after running through a converter (rectifier and filter).
Dual Output
The
dual output configuration is similar to the center tap except that instead
of connecting a wire to the center of the coil, the coil is separated into
two separate coils with wires with terminals or wires coming out from both
ends of both coils so four wires come out of the secondary side instead of
three.
If the transformer is a 110V input
with two 12V outputs, you can connect the two secondary coils in series to
get 24 volts out, or you can connect them in parallel to get 12V out.
You have to be careful to connect the right ends of the two secondary
coils in both the series and in the parallel connections. If you
reverse the connections, you will get 0 volts out because the two voltages
will cancel each other out.
If the transformer is rated at 48VA,
then you can use up to 2 amps out for the 24 volt connection which is no
different than the center tap or single 24V output configuration.
However, when connected in parallel, you get 12 volts out but double the
output current available so you can get 4 amps out. You get the full
48VA output where with the center tap 12V output, you can only get half
the rated output or 24VA. This is an advantage in hot wire foam
cutters because you have a wider range of wire diameters and lengths
depending on whether you connect the outputs in parallel or series.
The series and parallel connections are shown below.
Dual Input
The dual input transformer is often
used to make the transformer able to be used in both countries with 120V
line voltage and 240V line voltage. The primary is separated into
two separate windings with terminals at each end of both windings so there
are four wires or terminals on the primary side.
To use it with 110 volts input, the
two primary windings are connected in parallel as in the left diagram
below. Care must be taken to connect the correct ends together.
If they are reversed, the fields cancel each other out because the fields
generated by each section of the primary are opposite. Normally,
terminals are labeled with numbers or letters and a diagram is provided on
the transformer or in an accompanying data sheet showing how the
connections must be made for 110V and 220V.
If the transformer is to be connected
to a 220V supply, then the two coils are connected in series and again,
care must be taken to connect the correct terminations together.
Parallel connections for 110V and series connections for 220V is shown
below.
Dual Input and Output
And of course, you can have both a
dual input and a dual output so you have four wires in and four wires out
which gives even more flexibility to the use of the transformer.
Some specialized transformers may
have several secondary taps or several secondary windings to provide
different voltages and they need not be even numbers. A transformer
could have a 3V, 5V, 12V, and 24 volt output for example.
Autotransformers (Variac)
An autotransformer is often referred to
as a Variac which is actually one company's trade name for their
autotransformer. It has a continuous output voltage from zero to a
little over the input value. It works similar to a potentiometer or
rheostat except the change in the voltage is due to the field change rather
than resistance. Another difference is that a potentiometer or rheostat
is very inefficient because it converts the current flowing through it to heat
(Watts = Amps X Volts). As in all transformers, the resistance is low so
the amount of heat generated is much less and so much more efficient at
transforming voltage
An autotransformer has only one winding
which serves as both the primary and the secondary winding. Because
there is only one winding, there is no electrical isolation between the input
and the output but if isolation is not required, then it provides an
alternative to multiple winding transformers in some situations.
This transformer has the input wires
connected to one end of the winding and the other a little ways from the other
end. The secondary is connected the same point as the input side that is
on the end. The other secondary connection is to a wiper that rides on
the top of the windings where the insulation has been removed so the wiper can
make contact with the windings at any point on one surface. The wiper is
connected to a knob on the top of the autotransformer so a person can turn the
knob to get the voltage they want. Because one primary wire is connected
a ways from the end of the winding, the wiper can go past that point and so
provide a voltage higher than the input, typically a 110V output can go up to
around 130V on the secondary side.
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Because the autotransformer has only one
winding, there is only one wire size and so the maximum input current is also
the maximum output current. If a 110 Volt autotransformer is rated at 10
amps, then the maximum output current is 10 amps regardless of the voltage.
If it is rated in Watts or VA, then the amps is calculated by dividing the
Watts or VA by the rated input voltage.
The autotransformer is a good
alternative to a step down transformer when the range of desired voltages is
on the high end or the whole range of voltages is needed but becomes more
expensive if the range is on the low end because you have a lot of unused
windings. A step down transformer is more economical.
For hot wire foam cutting, an
autotransformer is much more expensive than step down transformers in most
applications. If the voltage required is more than 24 volts, then an
autotransformer might be considered.
