Transformer
A transformer is a device which consists of two inductive
coils which are electrically separated but magnetically linked through a path
of low reluctance.
The two
coils possess high mutual inductance. If one is connected to a 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
The first
coil in which electric energy is fed from the a.c supply mains is called
PRIMARY WINDING.
And the
other from which energy is drawn out is called SECONDARY WINDING
A transformer
consists of two basic elements,
1.
two coils having mutual
inductance
2.
laminated core
Iron-core
transformer
It is a
transformer in which laminations of iron, make up part or all of the path for
magnetic lines of force that link the transformers windings.
The core is
constructed of these laminations in order to provide continuous magnetic path
for the lines to flow and minimizing the air-gap in between the laminated
sheets.
Now, consider the N-turn iron core
coil. When a current ‘I’ is passed through the primary winding a magnetic flux
’fy’ is produced. Let this flux vary sinusoidally with the time ‘t’ as:
Fy = fy(m) sin 2 (pie) ft
Where fy(m) being the maximum value
in webers and f the frequency. Therefore, the induced voltage in accordance
with faradays law is
Vp= N d(fy)/dt = 2(pie)fN{fy(m)} cos
2 (pie)ft
and the effective value will be
Vp= 2(pie)/1.414 fN {fy(m)}
Vp=4.44f Np{Fy(m)}
Similarly for secondary winding,
Vs=4.44fNs{fy(m)}
Where Vp and Vs represents voltage
across the primary and secondary windng respectively
f is the frequency
Np and Ns are the no. of turns of the
primary and secondary winding respectively
And {fy(m)} is the flux which links
the two windings.
By dividing Vp with Vs we get;
Vp/Vs = Np/Ns
Which is equal to ‘alpha’ also known as the transformation
ratio defined as
Through a common magnetic core when I
current starts to flow magnetic flux is created which links both windings
together hence a voltage is established across the secondary winding. The
magnitude of this voltage is proportional to the ratio of no. turns on primary
winding to the no. of turns of secondary winding.
Also since ampere turns of primary
winding is equal to the ampere turns of secondary winding.
Which could be written as
Np/Ns= Ip/Is
Where, Np/Ns = Vp/Vs
So,
Np/Ns =Vp/Vs=Ip/Is = α
ASSOCIATED TERMS
IN A TRANSFORMER
Generator
A device that
converts mechanical energy into electrical energy by magnetic induction.
AC
An electrical flow of continuous changing
polarity which rises to maximum voltage in one direction , decreases to zero
and then sinks to maximum voltage n other direction before changing polarity
once again. This pattern is referred to as sinusoidal waves and the number of
cycles /sec is equal to frequency which is measured in hertz
Core
The ferrous part of the transformer or inductor
used to increase the length of the magnetic field. It carries the flux and
forms a magnetic coupling between primary and secondary.
Eddy current
It is induced into metal when magnetic lines of
force moves across it.
Efficiency
Rate of the power output to its power input
Frequency
It means the number of lines which an AC
voltage will change from positive to negative and vice versa with in a precise time, usually expressed in
cycles/sec and identified as hertz or as in 60 hertz.
Flux
The lines of forces of a magnetic field.
Horse power
One horse power is 33000 lb ft/min or 746 volts
Hertz
Cycles/ second
Laminations
The sheets of steel making up the core of the
transformer.
Multiple
winding
a winding which consists of two or more
sections that can be paralleled to specific mode of operation
Ohm’s law
the relationship between voltage and current
and resistance. The current in an electrical circuit is directly proportional
to the voltage and inversely proportional to the resistance.
V=IR
I=V/R
Primary
winding
The coil winding that is directly connected to
the input supply
Phase
Electrical circuits can be single, three and
poly phase, depending on the number of source conductors. Single- phase
transformers can be used on a three phase source when two wires of the three phase
system are connected to the primary of the single phase transformer conversely
three phase sources can be combined to provide three phase power.
Secondary
winding
The coil winding supplying the output voltage
to the load.
Sinusoidal
waveform
A waveform that can be expressed mathematically
using sine function.
Short circuit
A low resistance connection using accidental
across part of a circuit resulting in excessive current flow.
Nameplate
Reading of Transformer
Transformer
name plate shows the rating of the transformer...namely, operating frequency,
rated current n voltage, output power in KVA, type of insulation used, and
transformation turns ratio...
Additional Info:
In addition to that, you would also find the
percent impedance, size of the tank; tap configuration, and vector group. Here
is photo of a typical power transformer nameplate:
Types
of cooling: ONAN & ONAF
ONAN:
uses natural air flow through the radiator to dissipate heat.
ONAF:
uses fan for cooling the radiator.
Dyn11
Delta connected HV winding, star connected LV winding with neutral brought out, LV is leading HV with 30°
Delta connected HV winding, star connected LV winding with neutral brought out, LV is leading HV with 30°
HV-High-voltage (primary)
LV-Low-voltage (secondary)
