Transformer Construction:-
There are two
general types of transformers, the core type and the shall type. These two
types differ from each other by the manner in which the windings are wound
around the magnectic core.
The magnetic core is a stack of thin silicon-steel
laminations about 0.35mm thick for 50Hz transformers. In order to
reduce the eddy current losses, these laminations are insulated from one
another by thin layers of varnish. For reducing the core losses, nearly all
transformers have their magnetic core made from cold-rolled grain-oriented sheel-steel
(C.R.G.O). This material, when magnetized in the rolling direction, has
core loss and high permeability.
In the core-type, the winding surround
a considerable part of steel core as shown in fig (a). In the shell type the steel core surround a
major part of the windings as shown in fig (b).
(a)
(b)
For a given output and voltage rating, core-type
transformer requires less iron but more conductor material as compared to a
shell-type transformer. The vertical portions of the core are usually called
limbs or legs and the top and bottom portions are called the yoke. This means
that for single phase transformers, core-type has two legged core whereas shell
type has three legged core. In iron-core transformers, most of the flux is
confined to high permeability core. Leaks flux through the core legs and non-magnetic material
surrounding the core. This flux called leakage flux
links one winding and not the other. A reduction in this leakage flux
is desirable as it improves the transformer performance considerably.
Consequently, an effort is always made to reduce it. In the core-type
transformer, this is achieved by placing half of the low voltage winding over
leg and other half over the second leg or limb. For the high voltage winding
also, half of the winding is over one leg and the other half over the second
leg as shown in fig (c)
(c)
low voltage winding is placed adjacent to the steel core and H.V. winding
outside, in order to minimise the amount of insulation required.
In the shell type transformer, the L.V. and H.V. windings are wound over the
central limb and are interleadved or sandwiched as shown in fig (d).
(d)
Note that the botton and top
L.V. coils are of half the size of other L.V. coils. Shell-type transformers
are preferred for low voltage low-power levels, whereas core type construction
is used for high voltage . In core type transformer, the flux, has a single
path around the legs or yokes. In the
shall type transformer, the flux in the central limb divides equally and
returns though the outer two legs.There are two types of windings employed for
transformers. The connecntic coils are used for core-type transformer. One type
of laminations for the core and shell-type of transformers is illustranted as shown in fig (d)
(d)
The
steel core is assembled in such a manner that the butt joints in adjacent
layers are staggened as illustrated. The staggering of the joints avoids
continuous air gap and therefore, the reluctance of the magnetic circuit is not
increased. At the same time, a continuous air gap would reduce the mechanical
strength of the core and therefore, the staggering of the butt joint is
essential.
During the transformer construction first
the primary and secondary winding are wound, then the laminations are pushed
through the coil openings, layer by layer and the steel core is prepared. The
laminations are then tightened by means of clamps and bolts. Low power
transformers are air-cooled whereas large power transformers are immersed in
oil for better cooling. In oil-cooling
transformers, the oil serves as a coolant and also as an insulating medium.
For power frequency range of 25 Hz to 400
Hz, transformers are construnted with 0.35mm thick silicon-steel laminations.
For audio-frequency range of 20Hz to 20,000Hz, iron core with suitable
refinements is used. For high frequencies employed in communication circuits,
core is made up of powderd ferromagnetic
alloy. In special cases, the magnetic circuit of a transformer may be made of
non-magnetic material and in such a case, the transformer is referred to as an
air-core transformer. The air-core transformer is primarily used in radio
devices and in certain types of measuring and testing instruments. Cores made
of soft ferrites are also used for pulse transformers as well as for high
frequency electronic transformers.
