Transformer
Basic Working of Transformer
E.M.F Equation of transformer
Voltage Transformation ratio (K)
Introduction
A transformer is a static piece of apparatus by means of which electric power in one
circuit is
transformed into electric power of the same frequency in another circuit.
It can
raise or
lower the voltage in a circuit but with a corresponding
decrease or increase in current .
It has an input side (primary) and an output side (secondary). Electrical energy applied to the
primary is converted to a
magnetic field which in turn, induces a current in the secondary which
carries energy to the load connected to the secondary. The energy applied to the primary must
be in the form of a
changing voltage which creates a constantly
changing current in the primary,
since only a
changing magnetic field will produce a current in the secondary.
In brief, a transformer is a device that
(a) transfers electric power from one circiut to another.
(b) it does so without a change of frequency.
(c) it accomplishes this by electromagnetic induction and
(d) where the two circuit are in mutual inductive influence of each
other.
In its most basic form a transformer consists of :
(1)A
primary coil or winding.
(2)A
secondary coil or winding.
(3)A
core that supports the coils or windings.
It consist of
two inductive coils which are electrically separated but magnetically linked through a
path of low reluctance. If one coil (primary) is connected to source of
alternating voltage ,
an
alternating flux is set up in the laminated core, most of which is linked with the other
coil in which it produces mutually-induced e.m.f. (according to
Faraday's Laws of
Electromagnetic Induction) . If the second coil (secondary) circuit is closed, a current
flows in it and so electric energy is transferred (entirely magnetically) from the first
coil to the second coil.
Let
N1 = No. of turns in primary
N2 = No. of turns in secondary
Øm = Maximum flux in core in webers = Bm x A
f = Frequency of a.c. input in Hz.
The
flux increases from it's
zero value to maximum value
Øm in one quarter
of the cycle i.e. in
1/4 f second.
Therefore, r.m.s value of e.m.f./turn = 4.44 Øm volts
Now, r.m.s value of induced e.m.f in the whole primary winding
= ( induced e.m.f. / turn ) x No. of primary winding
E1 =
4.44 f N1Øm -------------------
(i)
Similarly,
r.m.s. value of e.m.f. induced in secondary is,
E2 =
4.44 f N2Øm -------------------
(ii)
From the above equations (i) and (ii), we get
(i) If
K > 1 , then the transformer is called
step-up transformer.
(ii) If
K < 1 , then the transformer is called
step-down transformer.