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Open Circuit Test of Transformer – Results and Warnings

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An important function of the electrical system is to make the electrical energy usable by the consumer and deliver it to their doorstep. And this extremely important work is done by the transformer or power transfer device. So open circuit test of a transformer is a very important task.

In order to transfer the same power from one circuit to another circuit while keeping the frequency fixed, the transmission and distribution lines are constantly connected to this device of fixed nature. For which transformer occupies a place as an integral part of the electrical power system. Due to constant power transfer and extensive use of power systems, various faults are observed in transformers. Any kind of fault, small or big, causes damage to the electrical power system, which can subsequently cause huge power loss and loss of life. Therefore, every EEE engineer needs to know what information can be obtained from the open circuit test of transformers and what precautions should be taken.

What is open circuit test of transformer? What information is available? What are the precautions? A detailed discussion on this is given below.

What is open circuit test of transformer?

A test conducted by leaving one side of the transformer (usually the high-voltage side) open and connecting the required number of measuring instruments to the other side, applying the rated normal full voltage and frequency of the transformer, is called an open circuit test. It is also called a no-load test.

Ammeter, voltmeter, and wattmeter are connected on the low-voltage side. Apart from that, a variac or voltage regulator is also connected to maintain the rated value of the voltage transformer applied to the low-voltage side. A rated supply results in the rated voltage of the voltmeter connected to the circuit, the no-load current (I) in the ammeter, and the power loss in the wattmeter. This no-load current is very small. It ranges from 3% to 5% of full-load current.

Open Circuit Test or No Load Test of Transformer

Fig: Circuit diagram of open circuit test or no-load test of transformer

Also Read: Advantages and Disadvantages of AC & DC Transmission

The reading shonwn by the wattmeter in the open circuit test is considered a core loss. Because in this condition there is no load on the secondary and normal mutual flux (Φ_m) is established in the core. Core loss is roughly proportional to the square of the mutual flux. Because of the high impedance in the primary, the no-load current is very low, so copper losses can be neglected.

So the wattmeter reading is taken entirely as a core loss. These losses include hysteresis loss and eddy current loss. In such a connection, however, the wattmeter’s own voltage coil and the power loss in the voltmeter coil are also reflected in the wattmeter reading. Therefore, to get the correct core loss, the initial reading of the wattmeter should be recorded. Then isolate the place marked X in the figure and take the wattmeter reading again. Now subtract the second reading from the first reading to get the correct core loss.

Such errors are more common in small transformers. Therefore, the correct core loss X should be determined in the above method.

What information can be obtained from open circuit test or no-load test of transformer?

The information obtained from the open circuit test or no-load test of the transformer is:

1. Core Loss (Eddy Current Loss and Hysteresis Loss)

2. No-load current

3. Magnetizing current ( I_µ )

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4. Working current ( I_ω  )

5. Core loss resistance ( R_0  )

6. Core loss reactance (  X_0 )

7. No-load power factor (  Cosθ_0 )

8. Transformation ratio (a).

The above information is obtained from open circuit test.

Why does wattmeter indicate only core loss from open circuit test?

The high-voltage side remains open during the open circuit test. Also, the transformer coils are inductive and the high impedance in the primary causes the no-load current to be very low, 3% to 5% of the rated current. The resulting loss is very small compared to the core loss and is almost negligible. Normalized mutual flux (Φ_m) is established for the rated supply voltage in the core. Core loss is again roughly proportional to the square of the mutual flux. So the wattmeter only indicates the core loss.

Precautions for Open Circuit Test of Transformers

Following precautions should be taken while doing an open circuit test of the transformer:

1. Low-range measuring equipment should be connected to the low side leaving the high side open for ease of operation.

2. The rated voltage of the transformer must always be maintained with a variac or voltage regulator on the low-side.

3. The terminals on the open side should be well insulated. If a voltmeter is connected to the high side, it should be in a high range.

4. The pressure coil of the wattmeter and the voltmeter circuit should be connected in such a way that the current flowing through them does not under any circumstances flow through the current coil of the voltmeter and wattmeter.

5. In the case of small transformers, the core loss must be determined properly for the power loss in the wattmeter’s own voltage coil and the voltmeter coil connected to the circuit.

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