Components
Transformers
Transformers
How does a transformer work?
A transformer is based on a simple fact about electricity: when a
fluctuating electric current flows through a wire, it generates a
magnetic field (an invisible pattern of magnetism) or "magnetic
flux" all around it. The strength of the magnetism (which has the
rather technical name of magnetic flux density) is directly related
to the size of the electric current. The larger amount of current,
the stronger the magnetic field. When a magnetic field fluctuates
around a piece of wire, it generates an electric current in the
wire. So if we put a second coil of wire next to the first one, and
send a fluctuating electric current into the first coil, we will
create an electric current in the second wire.
Transformers
The current in the first coil is usually called the primary
current and the current in the second wire is the
secondary current. What we've done here is pass an
electric current through empty space from one coil of
wire to another. This is called electromagnetic induction
because the current in the first coil causes (or "induces")
a current in the second coil. We can make electrical
energy pass more efficiently from one coil to the other
by wrapping them around a soft iron bar (called a core).
Transformers
Transformers
To make a coil of wire, we simply curl the wire round into loops
or "turns". If the second coil has the same number of turns as the
first coil, the current in the second coil will be virtually the same
size as the one in the first coil. But (and here's the clever part) if
we have more or fewer turns in the second coil, we can make the
secondary current and voltage bigger or smaller than the primary
current and voltage. One important thing to note is that this trick
works only if the current is fluctuating in some way. In other
words, you have to use a type of constantly reversing electricity
called alternating current (AC) with a transformer. Transformers
do not work with direct current (DC), where a steady current
constantly flows in the same direction.
Transformers
Step-down transformers
If the first coil has more turns that the second coil, the secondary voltage is
smaller than the primary voltage:
This is called a step-down transformer. If the second coil has half as many
turns as the first coil, the secondary voltage will be half the size of the
primary voltage; if the second coil has one tenth as many turns it has one
tenth the voltage.
Transformers
Transformers
So a step-down transformer with 100 coils in the
primary and 10 coils in the secondary will reduce the
voltage by a factor of 10 but multiply the current by a
factor of 10 at the same time. The power is equal to the
current times the voltage (watts = volts x amps is one
way to remember this), so you can see the power in the
secondary coil is theoretically the same as the power in
the primary coil. (In reality, there is some loss of power
between the primary and the secondary because some of
the "magnetic flux" leaks out of the core, some energy is
lost because the core heats up, and so on.)
Transformers
Step-up transformers
Reversing the situation, we can make a step-up transformer that
boosts a low voltage into a high one:
Transformers
Transformers
Transformers in the home
There are huge transformers in towns and cities where the highvoltage electricity from incoming power lines is converted into
lower-voltages. There are transformers in your home as well.
Electric appliances such as washing machines and dishwashers
use voltages of 110-240 volts, but electronic devices (laptop
computers and chargers for MP3 players and mobile cellphones)
use lower voltages: a laptop needs about 15 volts, an iPod charger
needs 12 volts, and a cellphone typically needs less than 6 volts
when you charge up its battery. Thus, electronic appliances like
these have small transformers built into them (often mounted at
the end of the power lead) to convert the 110-240 volt domestic
supply into a smaller voltage they can use.
Transformers
Photo: Typical home transformers. Anticlockwise from top left: A
modern transformer, the white transformer in an iPod charger,
and a cellphone charger.
Transformers
Induction chargers
What happens with an electric toothbrush, which has no power lead? It
charges up with a slightly different type of transformer, which has one of its
coils in the base of the brush and the other in the charger that the brush stands
on. You can read more about how it works in our article about induction
chargers.
Photo: An electric toothbrush standing on
its induction charger. The battery in the
brush charges up by induction: there is no
direct electrical contact between the plastic
brush and the plastic charger unit in the
base.
Transformers
Transformers on the Electrical Grid
Let's look at the electricity that comes to your home. When electricity moves
from a power plant it is put into a very high voltage to be able to travel long
distances. The high voltage lines can be as high 155,000 to 765,000 volts to
travel many hundreds of miles.
In order for your home or a store to use the electricity, it has to be at a lower
voltage than on the long-distance lines. So, the electricity is "stepped-down to
a lower level using a transformer. This lower voltage electricity is put into the
local electric wires at a substation. The substation breaks the larger amount of
power down into smaller pieces at lower voltage. It then is stepped down again
and again.
Once smaller transformers take that voltage down to usually 7,200, the power
leaves this substation.
Transformers
In your neighborhood, a transformer on top of a utility pole, or one connected
to underground wires, transforms the 7,200 volts into 220-240 volts. This is
then sent into your home over three wires. The three wires go through the
electric meter, which measures how much electricity you use. One of the three
wires is the ground, and the other two are the positives.
Transformers
Some of the electrical appliances in your home use the 220-240 volts. These
are things like a water heater, stove and oven, or air conditioner. They have
very special connections and plugs. Other devices, like your TV or computer
only use one-half of the electricity - 110-120 volts.
In a toy train set, the voltage is reduced even more from 110-120 and is
changed from alternating current into direct current.