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Electrical Machines & Applications 1 (DET 204)
Laboratory Module
EXPERIMENT 1
PARAMETERS DETERMINATION OF A TRANSFORMER
Objective:
The objective of the experiment is to perform open and short circuit tests on a singlephase transformer
a. to find out the core and copper losses.
b. to find out the parameters of the equivalent circuit.
c. to draw the equivalent circuit.
Introduction
A transformer is a static machine having two or more windings coupled by a mutual
magnetic field. Ferromagnetic cores are commonly used to provide tight magnetic
coupling and high flux densities. Single-phase transformers are widely used in low power
electronic or control circuits to isolate one circuit from another. They are also used as
instrument transformers to measure voltages and currents.
In a practical transformer there are core and copper losses due to the winding resistance
and leakage flux. These losses can be conveniently represented by an equivalent circuit
with resistances and inductances where all the quantities are referred to one side,
preferably high voltage side. The equivalent circuit of a single-phase transformer is given
in figure 1.
Figure 1
The resistance Rp represents the ohmic resistance of the primary winding and Rs – of the
secondary winding. The reactance Xp and Xs model the leakage flux of the primary and
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secondary windings, respectively. The resistance Rc is responsible for the core losses
due to hysteresis and eddy currents, and XM for the generation of the main flux
(magnetising reactance).
All impedances on the secondary side of the transformer can be recalculated for the
primary side. This is also know as the referring to the primary side and results in the
equivalent circuit shown in Figure 2.
Figure 2
The transformer equivalent models shown above are often more complex then necessary.
A more desirable equivalent circuit that works as well as the original circuit are shown in
Figure 3. In this model the excitation branch is moved to the front of the transformer, and
the the primary and secondary impedances are left in series with each other.
Figure 3
Experimental determination of all elements in the transformer equivalent circuit involves
2 tests:
a. open-circuit test
b. short circuit test.
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Measurement of the Primary Resistance
A DC ohmmeter should be connected across the primary terminals and Rp should be
recorded.
Open-Circuit Test
The transformer’s secondary should be open-circuited and primary winding supplied with
a full rated voltage (Figure 4). The input voltage (Voc), primary current (Ioc) and
power (Poc) are measured.
This test is sufficient to calculate the core resistance and magnetising reactance, using
Equations (1.0) to (1.7). The conductance of the core-loss resistor is given by:
Figure 4
Gc = 1/Rc
………………………………………….1.1
Bm = 1/Xm
………………………………………….1.2
Since these two elements are in parallel, their admittances add and the total excitation
admittance is
YE = Gc – jBm = 1/Rc – j1/Xm ……………………………1.3
The magnitude of the excitation admittance can be determined by the Equation (1.4)
……………………………………………1.4
The angle of the admittance can be found from knowledge of a circuit power factor.
………………………………………….1.5
and the power factor angle is given by Equation (1.6)
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………………………………………………. 1.6
The power factor is always lagging for a real transformer, so the angle of the current
always lags the angle of the voltage by
degrees. Therefore, the admittance YE is
.............................................1.7
Short Circuit Test
The transformer’s secondary terminals are short-circuited and the primary voltage is
supplied with the voltage, much reduced comparing with the rated value (Figure 5). In
practical situation, this voltage is adjusted so that the primary current is approximately
rated, and the primary voltage (Vsc), primary current (Isc) and power (Psc) are measured.
Figure5.
The following Equations (4.12) to (4.16) can be used to calculate elements in the primary
and secondary branches.
The magnitude of the series impedances referred to the primary side of the transformer is
…………………………………………….1.8
The short circuit power factor (PF) is given by:
………………………………………..1.9
and the power factor angle è is given by Equation (4.14)
…………………………………………...2.0
Therefore,
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……………………………………...2.1
The series impedance Zsc is equals to
………………………………………2.2
Equipment
1. Single Phase Transformer.
2. Variable AC Source (Variac)
3. Two Digital Multimeters.
4. AC Wattmeter (Use Voltech PM100)
Procedure
1. Rated Quantities
From the transformer nameplate, note the rated values of S1, V1, and V2. Calculate the
rated maximum currents for the ammeters on the primary and secondary sides. Do not
exceed these values.
2. Open Circuit Test
Connect the apparatus as shown in Figure 1.5. Apply the rated input voltage to the
primary winding and measure input voltage, current, power, and output voltage.
Do not turn the power on before your circuit has been checked by your Engineers!
Figure 1.5
3. Starting from VOC =20V turn the variac knob and slowly increase the input voltage.
Complete all the data in Table 1.1.
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4.Short Circuit test
Short-circuit the secondary winding through an ammeter as shown in Figure 6
Do not turn the power on before your circuit has been checked by your Engineers!
5.Slowly and gradually increase the applied voltage and carefully watch the primary
and secondary currents. Measurements and calculations ratio should be done for short
circuit current values specified in Table 1.2
Figure 6
6.Calculate the short circuit impedance as measured from the primary using Equations
(1.8) to (1.12). Complete all the measurements and calculations in Table 1.2.
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EXPERIMENT 1
OPEN-CIRCUIT AND SHORT-CIRCUIT TESTS OF A TRANSFORMER
Name : _______________________________
Group : ____________
Matric No: ____________________
Date : _______________
1. What are the rated voltage and current of the transformer?
______________________________________________
2
Table 1.1: Transformer Open circuit Test and Measurement
Table 1.2: Transformer short circuit test measurements and calculations.
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3. Show the calculation during maximum current and voltage to find the circuit
equivalent parameters YE, RC,PF, XM, ZSC, Req and Xeq. Draw the transformer
equivalent circuit.
4. What is the turn ratio between primary and secondary winding?
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5. From Table 1.1, Plot a graph Voc against Ioc. What can be concluded from the graph
obtain.?
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