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Transformer
Chapter-1
Core
The magnetic core is a stack of thin silicon-steel laminations about 0.35 mm thick for 50 Hz
transformer. It provides magnetic coupling between two (or more) isolated electrical circuits.
To minimize core losses, core should be made of a highly permeable material. Hence, core is
made from cold-rolled grain-oriented sheet-steel (C.R.G.O.) magnetized in the rolling direction.
The core is laminated to reduce the eddy current losses.
In the core-type, the windings surround a considerable part of the steel core. In the shell-type,
the steel core surrounds a major part of the windings. For single-phase transformers, core-type
has two legged core while shell-type has three legged core.
Both the high voltage winding and the low voltage winding are divided on both the limbs.
When half of winding is placed on either limb, the leakage flux would correspond to only half
magnetic vector. Thus, the windings are divided to reduce leakage flux (flux that links
one winding and not the other). The low voltage (L.V.) winding is placed near to the steel core
in order to minimize 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 interleaved or sandwiched as shown in Figure. The bottom 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, high-power transformers.
In core-type transformer, the flux has a single path around the legs or yokes. In the shell-type
transformer, the flux in the central limb divides equally and returns through the outer two
legs.
There are two types of windings employed for transformers. The concentric coils are used
for core-type transformers and interleaved (or sandwiched) coils for shell-type
transformers.
During the assembly of the steel core, the butt joints are staggered to reduce the reluctance
and it provides mechanical strength to the core. Low power transformers are air-cooled where
as large power- transformers are immersed in oil for better cooling. In oil-cooled transformers,
oil acts as coolant and as insulating medium.
Pulse transformers have cores made of soft ferrites. In communication circuits (high
frequency circuits), cores are made of powdered ferromagnetic alloys. Air-core (core of nonmagnetic
material) transformers are used in radio devices.
The cores are stepped in transformers to obtain optimum use of the copper material used for
winding and ferroelectric material for core.
Principle of Transformer Action
A transformer works on the principle of electromagnetic induction according to which emf is
induced in a coil if it links a changing flux. When the primary winding is supplied with an ac
source, an alternating mutual flux linking both the windings is set up, which induces
voltage in the primary and secondary windings.
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