Transformer is a
stationary device used to transform electrical power from one circuit to
another without changing its frequency. The applied alternating voltage is
either increased or decreased with corresponding decrease or increase of
current in the circuit.
If the transformer
converts an alternating current with low voltage into an alternating current
with high voltage, it is called step-up transformer. On the contrary, if the
transformer converts alternating current with high voltage into an alternating
current with low voltage, then it is called step-down transformer.
Construction and working of transformer
The principle of
transformer is the mutual induction between two coils. That is, when an
electric current passing through a coil changes with time, an emf is induced in
the neighbouring coil.
In the simple
construction of transformers, there are two coils of high mutual inductance
wound over the same transformer core. The core is generally laminated and is
made up of a good magnetic material like silicon steel. Coils are electrically
insulated but magnetically linked via transformer core (Figure 4.37).
The coil across which
alternating voltage is applied is called primary coil P and the coil
from which output power is drawn out is called secondary coil S. The
assembled core and coils are kept in a container which is filled with suitable
medium for better insulation and cooling purpose.
If the primary coil is
connected to a source of alternating voltage, an alternating magnetic flux is
set up in the laminated core. If there is no magnetic flux leakage, then whole
of magnetic flux linked with primary coil is also linked with secondary coil.
This means that rate at which magnetic flux changes through each turn is same
for both primary and secondary coils.
As a result of flux
change, emf is induced in both primary and secondary coils. The emf induced in
the primary coil εp is almost equal and opposite to the applied
voltage Ï…p and is given by
The frequency of
alternating magnetic flux in the core is same as the frequency of the applied
voltage. Therefore, induced emf in secondary will also have same frequency as
that of applied voltage. The emf induced in the secondary coil εs is
given by
where N p
and Ns are the number of turns in the primary and secondary
coil respectively. If the secondary circuit is open, then εs = υs
where Ï…s is the voltage across secondary coil.
This constant K is known
as voltage transformation ratio. For an ideal transformer,
Input power Ï…p
ip = Output power Ï…s is
where ip
and is are the currents in the primary and secondary coil
respectively.
Therefore,
Equation 4.33 is written
in terms of amplitude of corresponding quantities,
i) If N s
> N p
( or K >1), ∴
Vs >Vp and I s < Ip .
This is the case of
step-up transformer in which voltage is increased and the corresponding current
is decreased.
ii) If N s
< N p
( or K <1), ∴
Vs <Vp and I s > Ip .
This is step-down
transformer where voltage is decreased and the current is increased.
The efficiency η of a
transformer is defined as the ratio of the useful output power to the input
power. Thus
Transformers are highly
efficient devices having their efficiency in the range of 96 – 99%. Various
energy losses in a transformer will not allow them to be 100% efficient.
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