Tranformer Action:

        The arrangement of two fixed coils wound on a core is 
known as a Transformer. A transformer is an electric device,
without mechanically moving parts, for transferring electric energy
from one or more circuits to one or more other circuits by 
electromagnetic induction.

        A very important point to remember is that, a transformer
will not operate with a steady current in the primary. For electro
magnetic induction to occur, there must be a changing magnetic field
which is produced when the current rises and falls to maximum then
the least, then rises in the opposite polarity to maximum and then
the least. Induction is caused by the changing magnetic field.

        For a transformer to operate continuously, the primary winding
must be connected to an intermittent source of current or to some
source of alternating current. An alternating current continuously
changes in value and periodically reverses direction. Therefore,
around a primary that is carrying an alternating current, the magnetic
field is constantly building up and dying down, first in one direction
then the other. These changes of the field mean that the magnetic
lines continually cut through the secondary and induce voltage it it.

        Frequently, transformer cores are formed into a square or
rectangular loop to provide a complete, closed path or circuit for
the lines of magnetic flux. A solid block of steel with a square
hole in the center has the primary coil wound around one leg and the
secondary wound around the other leg. Produce by the current in the
primary, the flux is carried almost entirely by the core and passes
through the secondary winding to induce the desired emf (voltage).

Transformer Losses:

        A properly designed transformer is a very efficient device;
there are certain factors that prevent the transformation of the 
input voltage and current to the desired output voltage and current
with 100% efficiency. These factors are transformer losses, and they
are of three general:
 Copper losses, from the materials used
in the transformer windings.
Core losses, which come principally
from the material, size and shape of the transformer core.
Stray
losses of various kinds.

        Copper losses occur in the form of heat which is produced by
the currents in the conductors of the transformer windings. Called
I squared R losses, they relate to the amount of current in the windings
and the resistance to the conductors. These losses are minimized by
employing large diameter conductors to reduce the resistance per unit
length of the wires. Copper losses are generally about twice as great
as core losses in most tranformers.

        Core losses, also called "iron losses", mainly affect transformers
with cores of magnetic material and are of two kinds: Hysteresis losses
and eddy current losses. Hysteresis losses are the main type of
core loss, comprising about three fourths of the total. "Hysteresis"
describes the tendency of the core material to oppose a change in 
magnetism.

        Each time the magnetizing force produced by the primary of
a transformer changes because of the applied ac power, the atoms
realign themselves in the direction of the force. The energy to 
accomplish this realignment of the magnetic atoms comes from the
input power and is not transferred to the secondary winding; it is
therefore a loss. Because various types of core materials have
different magnetizing abilities, the selection of core material
is an important factor in reducing core losses.

        Another way of considering this is that, whenever the flux
density is being increased, the magnetizing force must be a little
greater than it would be if hysteresis were not a factor. Also, 
after the magnetizing force has reduced to zero at the end of an
alternation, a certain magnetization remains in the core which can
be decreased to zero only by the application of magnetizing force 
in the opposite direction. During each cycle, a certain percentage
of the total energy supplied is used to overcome the hysteresis
effect, and this energy is lost as far as the transformer is concerned.

        Earlier, we mentioned currents in the solid metal core caused
by induction from the magnetic field. This current is actually 
Eddy Current loss. In iron core transformers, the iron must also
be considered as a conductor because it is affected by the magnetic
field just as the secondary winding is. Small voltages are induced 
in the core by the changing magnetic field; these currents cause 
I squared R losses in the core.

        By using laminated core, (thin sheets of metal instead of a
solid iron core) the path of the eddy current is broken up without
increasing the reluctance of the magnetic circuit. The laminations
lie in the same direction as the flux. Therefore, the insulating
surfaces are directly across the path of the eddy currents. The 
resulting eddy current reduction improves the efficiency of the
transformer.

        All of these losses make the typical transformer hot when it
operates under full load. In fact, the amount of heat the insulation
can take without breaking down helps to determine the power limitations
of the transformer. Although some transformers operate too hot to
hold comfortably, there should be no odor of burning insulation or
varnish, or signs of discoloration or smoke. Any of these conditions
would indicate that the transformer is overloaded.
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