A power transformer is usually employed for the purpose of converting power, at a fixed
frequency, from one voltage to another.
If it is used for converting power from a high voltage to a low voltage, it is called
a step-down transformer.
The conversion efficiency of a power transformer is extremely high and almost all of the input
power is supplied as output power at the secondary winding.
For confirming the specifications and performances of an electrical power transformer it has
to go through Open Circuit Test (No load test) on Transformer.
In this video we will learn about Open circuit test and their procedure.
Objectives of Open circuit test.
To conduct standard open circuit tests in order to find the parameters of the equivalent
circuit of a transformer.
Evaluate the regulation and efficiency of the transformer at a given load.
Check the excitation characteristics of the transformer.
Open Circuit Test on Transformer.
The connection diagram for open circuit test on transformer is shown in the figure.
A voltmeter, wattmeter, and an ammeter are connected in Low Voltage side of the transformer
as shown.
The voltage at rated frequency is applied to that LV side with the help of a variac
of variable ratio auto transformer.
The High Voltage side of the transformer is kept open.
Now with the help of variac, applied voltage gets slowly increased until the voltmeter
gives reading equal to the rated voltage of the Low Voltage side.
After reaching at rated Low Voltage side voltage, all three instruments reading (Voltmeter,
Ammeter and Wattmeter readings) are recorded.
The ammeter reading gives the no load current Ie.
As no load current Ie is quite small compared to rated current of the transformer, the voltage
drops due to this current that can be taken as negligible.
Since, voltmeter reading V1 can be considered equal to secondary induced voltage of the
transformer, the input power during test is indicated by watt-meter reading.
As the transformer is open circuited, there is no output, hence the input power here consists
of core losses in transformer and copper loss in transformer during no load condition.
But as said earlier, the no load current in the transformer is quite small compared to
full load current, so copper loss due to the small no load current can be neglected.
Hence, the wattmeter reading can be taken as equal to core losses in transformer.
Let us consider wattmeter reading is P0.
Where Rm is shunt branch resistance of transformer.
If, Zm is shunt branch impedance of transformer.
Therefore, if shunt branch reactance of transformer is Xm, These values are referred to the Low
Voltage side of transformer as because the test is conducted on Low Voltage side of transformer.
These values could easily be referred to High Voltage side by multiplying these values with
square of transformation ratio.
Therefore it is seen that the open circuit test on transformer is used to determine core
losses in transformer and parameters of shunt branch of the equivalent circuit of transformer.
Thanks for watching this video.
We will be published continue transformer tests and their procedure one by one my next
video.
Do you have learned more about transformer, check out my video Short circuit or Full Load
Test of Transformer.
More update please subscribes my channel Learning Engineering.
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