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If you have read the
page on ELECTROMAGNETISM
then you will know that
when a current is passed
through a coil, the coil
becomes surrounded by a
magnetic field.
This field is made up
from lines of force and
has the same shape as a
bar magnet.
If the current is
increased, the lines of
force move outwards from
the coil.
If the current is
reduced, the lines of
force move inwards.
If another coil is
placed adjacent to the
first coil then, as the
field moves out or in,
the moving lines of
force will "cut" the
turns of the second
coil.
As it does this, a
voltage is induced in
the second coil.
With the 50 Hz AC mains
supply , this will
happen 50 times a
second.
This is called MUTUAL
INDUCTION and forms the
basis of the
transformer.
The input coil is called
the PRIMARY WINDING, the
output coil is the
SECONDARY WINDING.
The voltage induced in
the secondary is
determined by the TURNS
RATIO.
Primary
voltage
Number of primary turns
-------------------------------- =
------------------------------------------------
Secondary
voltage
Number of secondary
turns
For example, if the
secondary has half the
primary turns, the
secondary will have half
the primary voltage.
Another example is if
the primary has 5000
turns and the secondary
has 500 turns, then the
turns ratio is 10:1.
If the primary voltage
is 240 volts then the
secondary voltage will
be x 10 smaller = 24
volts.
Assuming a perfect
transformer, the power
provided by the primary
must equal the power
taken by a load on the
secondary.
If a 24 watt lamp is
connected across a 24
volt secondary, then the
primary must supply 24
watts.
If it is a 240 volt
primary then the current
in it must be 0.1 amp.
(Watts = volts x amps).
To aid magnetic coupling
between primary and
secondary, the coils are
wound on a metal CORE.
Since the primary would
induce power, called
EDDY CURRENTS, into this
core, the core is
LAMINATED.
This means that it is
made up from metal
sheets insulated from
each other.
Transformers to work at
higher frequencies have
an iron dust core, or no
core at all.
Note that the
transformer only works
on AC which has a
constantly changing
current and moving
field. DC has a steady
current and therefore a
steady field and there
would be no induction.
Some transformers have
an electrostatic screen
between primary and
secondary.
This is to prevent some
types of interference
being fed from the
equipment down into the
mains supply, or in the
other direction.
Transformers are
sometimes used for
IMPEDANCE MATCHING.
There is a page on this
subject.
There is also a page on
transformer types. |
