Introduction
A transformer doesn't change power
levels. If you put 100 Watts into a transformer, 100 Watts come out the other
end. [Actually, there are minor losses in the transformer because nothing in
the real world is 100% perfect. But transformers come pretty darn close;
perhaps 95% efficient.]
A transformer is made from two coils of
wire close to each other (sometimes wrapped around an iron or ferrite
"core"). Power is fed into one coil (the "primary"), which
creates a magnetic field. The magnetic field causes current to flow in the
other coil (the "secondary"). Note that this doesn't work for direct
current (DC): the incoming voltage needs to change over time - alternating
current (AC) or pulsed DC.
The number of times the wires are
wrapped around the core ("turns") is very important and determines
how the transformer changes the voltage.
·
If the primary has fewer turns than the
secondary, you have a step-up transformer that increases the voltage.
·
If the primary has more turns than the
secondary, you have a step-down transformer that reduces the voltage.
·
If the primary has the same number of turns as
the secondary, the outgoing voltage will be the same as what comes in. This is
the case for an isolation transformer.
·
In certain exceptional cases, one large coil of
wire can serve as both primary and secondary. This is the case with variable
auto-transformers and xenon strobe trigger transformers.
Types of transformers:
·
Power
transformers
1. Single-phase 2. 3-phase
(Used in power transmission and distribution systems)
·
Step-up
or step-down transformers
·
Current
transformers
·
Voltage
transformers
·
Auto-transformers
Transformers
are constructed so that their characteristics match the application for which
they are intended. The differences in
construction may involve the size of
the windings or the relationship between the primary
and secondary windings. Transformer types are also designated by the
function the transformer serves in a circuit, such as an isolation transformer.
Distribution
Transformer
Distribution transformers
are generally used in electrical power
distribution and transmission systems. This class of
transformer has the highest power, or volt-ampere ratings, and the highest continuous
voltage rating. The power rating is
normally determined by the type of
cooling methods the transformer may use. Some commonly-used
methods of cooling are by using oil or some other heat-conducting material. Ampere
rating is increased in a distribution transformer by increasing the size of the
primary and secondary windings; voltage ratings are increased by increasing the
voltage rating of the insulation used in making the transformer.
Power
Transformer
Power transformers are
used in electronic circuits and come
in many different types and applications.
Electronics or power transformers are
sometimes considered to be those with
ratings of 300 volt-amperes and below. These transformers normally
provide power to the power supply of an electronic device, such as in power
amplifiers in audio receivers.
Step-up transformers
A "step-up
transformer" allows a device that requires a high voltage power supply to
operate from a lower voltage source. The transformer takes in the low voltage
at a high current and puts out the high voltage at a low current.
Step-down transformers
A "step-down
transformer" allows a device that requires a low voltage power supply to
operate from a higher voltage. The transformer takes in the high voltage at a
low current and puts out a low voltage at a high current.
THEORY:
A voltage transformer
connected to a constant primary voltage source usually delivers nearly constant
voltage to the load. From the consumers point of view the question of how
nearly constant voltage output under different loading conditions is an
important question. A closely rated quantity is the term voltage regulation the
definition of which is given below;
Voltage
regulation = open circuit voltage –
load voltage x 100
Open
circuit voltage
Voltage
regulation = (V1 – V’2)
/ V1
From the phase diagram,
V1-V’2 = I’2.r.Cosθ + I’2.x.Sinθ
V1-V’2 = I’2.r.Cosθ
+ I’2.x.Sinθ
V1 V1
Since
Io is small compared to I’2,
Approximate Voltage Regulation = I2.r.Cosθ
+ I2.x.Sinθ
V1
Considering
only the hysteresis, eddy current and copper losses.
Efficiency = output power x 100%
Input
power
= 1 – losses x 100%
Input
= 1 –
losses x 100%
Output + losses
= V2. I2.Cosθ x 100%
V2. I2.Cosθ +
(I’2)2.r + PC
Where PC
= core loss (Note I1 = I’2)
For the determination of efficiency
from the given above equations are often used which needs the summation of
losses. This method is more convenient, economical and gives more accurate
results for efficiency.
PROCEDURE:
The
transformer was examined, and given a special attention to the construction,
rated voltage, kava, frequency, etc. The rated currents for each side were
calculated. Then the Terminal identification test, Polarity test, Open circuit
test and Short circuit test were done by using follow the instructions which
are given in the handout sheet.
CALCULATIONS
Rated Currents
Using the Data on the Name Plate
Rated current of High Voltage side = 4000VA / 400 V
= 10
A
Rated current of Low Voltage side = 4000VA / 230
= 17.39
A
Open Circuit Test
Voltmeter reading
(V)
|
Ammeter reading
(A)
|
Wattmeter reading
(w)
|
220
|
0.92
|
36
|
210
|
0.73
|
32
|
200
|
0.56
|
29
|
190
|
0.42
|
26
|
180
|
0.32
|
24
|
170
|
0.22
|
21
|
160
|
0.20
|
19
|
Multiplication factor for the
wattmeter is 2
Efficiency
Efficiency = Output Power X 100
Input Power
Input Power = Output
Power +
Copper Loss + Core Loss
Rated input voltage = 400
V
Rated output voltage = 230 V
Re/ = ( N2 / N1)2
X Re
= ( 230 / 400 )2 X 0.62
= 0.205Ω
Xe/ = ( N2 / N1)2
X Xe
=
( 230 / 400 )2 X 0.5622
=
0.1858 Ω
Output Power = VFLIFLCosӨ Cos
Ө = 1
Copper loss = (
IFL )2 Re/
Core
loss = ( Vin )2 / Rc = P
Efficiency =
VFLIFLCosӨ
X 100%
{VFLIFLCosӨ
+ ( IFL )2
Re/ + P}
= {230 X 17.39 X 8.37 X 1} x 100%
{(230 X 17.39 X 8.37 X 1)+(8.372 X0.2066)+62
= 97.7766% (at full load)
Efficiency = VFLIFLCosӨ X 0.5 X
100%
{VFLIFLCosӨX0.5
+ ( IFL X0.5)2 Re/ + P }
= {230 X 17.39 X 8.37 X 0.5 X 1} x 100%
{(230 X
17.39 X8.37 X0.5X 1)+(8.372 X0.25X0.2066)+62
= 92.31% (at half full-load)
The voltage regulation
= I1.r.Cosθ + I1.X.Sinθ since
θ = 0,
V1
= I1.r
V1
=
(10 x 0.62) / 400 = 0.015
RESULTS
Equivalent circuit parameters are:
R = 0.62 Ω
Xe = 0.5622Ω
Rc
= = 1344.44 Ω
xm= =
243.37 Ω
Efficiency of the transformer at
full-load is 97.7766%
Efficiency of the transformer at half
full-load is = 92.31%
Voltage
regulation of the transformer is 0.015
Efficiency of transformers
In
practice, real transformers are less than 100% efficient.
·
First, there are resistive losses in the coils (losing power I2.r). For a
given material, the resistance of the coils can be reduced by making their
cross section large. The resistivity can also be made low by using high purity
copper.
·
Second, there are some eddy current losses in the core. These can
be reduced by laminating the core. Laminations reduce the area of circuits in
the core, and so reduce the Faraday emf, and so the current flowing in the
core, and so the energy thus lost.
·
Third, there are hysteresis losses in the core. The magentisation
and demagnetisation curves for magnetic materials are often a little different
(hysteresis or history depedence) and this means that the energy required to
magnetise the core (while the current is increasing) is not entirely recovered
during demagnetisation. The difference in energy is lost as heat in the core.
· Finally, the geometric
design as well as the material of the core may be optimised to ensure that the
magnetic flux in each coil of the secondary is nearly the same as that in each
coil of the primary.
· Stray losses
· Mechanical losses
To increase the efficient of the
transformer it’s very important to use a good coolant method in order to take
away the heat which is generated by the transformer.
Coolant is very important because high
temperatures also Can damage the winding of the transformer. In some
transformers transformer oil act as a coolant and also as a winding insulation.
Methods to reduce the temperature in a transformers
1.
Indoor
transformers can be cooled by natural air flow but large transformers cannot be
cooled using this method. For large transformers above 200kVA we use forced
circulation of clean air.
2.
In
large distribution transformers external radiators are added to increase the
cooling surface of the oil filled tank. Oil circulates around the transformer
windings and moves through the radiator. That oil has a large specific heat
capacity.
3.
In
still higher ratings there are Cooling
fans to blow air over the radiators.
4.
For
transformers in the megawatt range Cooling may be effected by an oil-water heat
exchanger
