The core loss in a transformer is constant
Open circuit and short circuit test on the transformer
The interruption and the short-circuit test are carried out to determine the parameters of the transformer such as its efficiency, voltage regulation, circuit constants, etc. These tests are performed with no actual load and therefore very little power is required for the test. The no-load and short-circuit test delivers a very accurate result compared to the full load test.
Quiescent current test
The open circuit test is used to determine the no-load current and losses of the transformer, on the basis of which its no-load parameters are determined. This test is carried out on the primary winding of the transformer. The wattmeter, ammeter, and voltage are connected to their primary winding. The nominal voltage is supplied to its primary winding with the help of the AC power source.
Circuit diagram for testing the open circuit on the transformer
The secondary winding of the transformer remains open and the voltmeter is connected to the terminal. This voltmeter measures the induced secondary voltage. When the secondary side of the transformer is open, the no-load current flows through the primary winding.
The value of the no-load current is very small compared to the full rated current. The copper loss occurs only on the primary winding of the transformer because the secondary winding is open. The wattmeter reading only represents the core and iron losses. The transformer core loss is the same for all types of loads.
Calculation of the quiescent current test
- W.0 - Wattmeter display
- V1 - voltmeter display
- I0 - Ammeter reading
Then the transformer iron loss is P.I = W0 and
The idle power factor is
Work component I.w is
Put the value of W0 from equation (1) in equation (2), you can get the value of the work component as
Magnetization component is
No load parameters are given below
Corresponding excitation resistance is
Equivalent exciting reactance is
The phasor diagram of the transformer with no load or when an no load test is performed is shown below
Vector diagram of the quiescent current test
The iron losses measured by the no-load test are used to calculate the efficiency of the transformer.
Short circuit test
The short-circuit test is carried out to determine the below mentioned parameter of the transformer.
- It determines the copper loss that occurs at full load. The copper loss is used to determine the efficiency of the transformer.
- The equivalent resistance, impedance and leakage reactance are known from the short circuit test.
The short circuit test is performed on the secondary or high voltage winding of the transformer. The measuring device such as wattmeter, voltmeter and ammeter is connected to the high-voltage winding of the transformer. Its primary winding is short-circuited with the help of a thick strip or ammeter connected to its terminal.
The low voltage source is connected to the secondary winding, due to which the full load current flows from both the secondary and primary winding of the transformer. The full load current is measured with the ammeter connected to the secondary winding.
The circuit diagram of the short circuit test is shown below
Circuit diagram for the short-circuit test on the transformer
The low voltage source is applied through the secondary winding, which is approximately 5 to 10% of the normal rated voltage. The flux is set in the core of the transformer. The size of the river is small compared to the normal river.
The iron loss of the transformer depends on the flux. It occurs less in the short circuit test due to the low flow. The reading of the wattmeter only determines the copper loss that occurs on your windings. The voltmeter measures the voltage applied to the high-voltage winding. The secondary current is induced in the transformer due to the applied voltage.
Calculation of the short circuit test
- W.c - wattmeter reading
- V2sc - voltmeter display
- I2sc - Ammeter reading
Then the copper loss is given when the transformer is fully loaded
Equivalent resistance based on the secondary side
The phasor diagram of the short-circuit test of the transformer is shown below
Vector diagram of the short circuit test
From the vector diagram
The equivalent impedance related to the secondary side is given by
The equivalent reactance related to the secondary side is given by
The voltage regulation of the transformer can be determined at any load and any power factor after knowing the values of Z.it and Rit.
In the short circuit test of the wattmeter record, the total losses including the core loss, but the value of the core loss compared to the copper loss are very small, so that the core loss can be neglected.
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