Short-Circuit and Open-Circuit Tests of A Transformer
Lab #7
Chris Visser
Mike Diliberto
Ray Rioux
November 1, 2006
Wentworth Institute of Technology
ELEC465-01/02
Department of Electronics and Mechanical
Prof. Ali Khabari
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Abstract:
The purpose of this lab was to study the operating characteristics of a single-phase
transformer. The most important results were the resistance and coil values for both
short-circuit and open-circuit tests.
Introduction:
The objective of this experiment was to measure the power, voltage and current of
both a short-circuit and open-circuit transformer. We also calculated the corresponding
resistance and inductance values along with the efficiency of the transformer.
Experimental:
Figure 1: The transformer circuit used for the short-circuit test.
Starting this experiment, we wired the transformer for a short-circuit test as seen
in Figure 1. We calculated the equivalent reactance on the high side. In order to
calculate Reqhs, we used the power and current readings from the short-circuit test. We
used the formula:
Psc = Isc^2 * Reqhs
Next in order to calculate the resistance in the inductor we had to find the total
reactance first. This was done using the current and voltage from the short-circuit test.
We used the equation:
Isc = Vsc / Zeqhs
We then used the calculated value and Reqhs in the following equation:
Zeqhs = √(Reqhs^2 + Xeqhs^2)
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Figure 2: The transformer circuit used in the open-circuit test part of the lab.
In part 2 we performed an open-circuit test on the transformer as seen in Figure 2
and recorded the values for power, voltage and current. First, we calculated the core
resistance on the low side Rfels. We did this by first finding Ifels with the equation:
Poc = Voc * Ifels
Using the calculated value for Ife, we then used the equation:
Rfels = Voc / Ifels
Next we calculated the magnetizing reactance Xm. We did this by first finding
the current Im with the equation:
Ioc = √(Ifels^2 + Imls^2)
We used the current value of Im to calculate Xmls as seen below:
Xmls = Voc / Im
In order to calculate the efficiency, we first needed to find the power loss. We did
this by adding the short circuit power and the open circuit power.
Lastly, we calculated the efficiency of the transformer. This was done using the
following equation:
η = [(Pinput – Ploss) / Pinput] * 100
Results and Discussion:
Below are our results and findings for the short-circuit test:
Psc = 70 W
Vsc = 6.8 V
Isc = 16.6 A
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Reqhs = 254 mΩ
Xeqhs = 324.96 mΩ
Open-circuit test:
Poc = 20 W
Voc = 120 V
Ioc = 1.2 A
Rfels = 720 Ω
Xmls = 100.98 Ω
The efficiency calculated for the transformer was 95.5%.
Conclusion:
By doing this experiment, we better know how to perform both a short-circuit and
open-circuit test on a single-phase transformer. We now know how to find the equivalent
winding impedance and core loss component of a transformer by taking measurements of
power, voltage and current. We also understand how to calculate the efficiency using the
power levels measured and given. We found our transformer to have very little loss
which is a good thing.
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