A number of tests are carried out on a transformer. These tests are made to determine the voltage ratio, reactance of the windings, magnetizing current, core losses, copper losses, efficiency and temperature rise etc. Some of these tests are:
- Open circuit test
- Short circuit test
- Back to back test
- Insulation test
- Ratio test
- Polarity test
Open Circuit Test
The purpose of this test is to determine the core loss, no-load current, magnetizing reactance, core resistance, no-load p.f. This test is performed in the following way. One winding of the transformer (usually high voltage winding) is kept open. The other winding i.e. low voltage winding is connected to the supply of normal voltage and frequency. A watt-meter, a voltmeter and an ammeter are connected on low voltage (i.e. primary) side.
When normal voltage is applied, normal flux is set up in the core. The watt-meter records the normal losses. The no-load current (Io) measured by ammeter is very small as compared to the rated current. Hence copper losses, which are dependent on current, are negligibly small in primary and zero in secondary. The watt-meter reading, therefore, represents practically the core losses. See figure 1.
Let W = Watt-meter reading i.e. core loss in watts
= input power.
Vi = input voltage in volts
Io = no-load current in Amps
Φo = P.f. angle of input power
So W = Vi Io Cos Φo
Cos Φo = W/ Vi Io
Where Cos Φo = input P.f.
The components, Im and Iw of Io becomes
Iw = Io Cos Φo
Im = Io Sin Φo
And Vi = Iw Ro => Ro= Vi / Iw
Vi = ImXm => Xm = Vi / Xm
Short Circuit Test
This test is carried out to determine the copper losses, equivalent resistance, equivalent reactance, voltage drop and hence regulation of the transformer. This test is performed as: One winding usually the low voltage winding is short circuited through an ammeter. Ammeter is used to indicate the rated load current. A low voltage (usually 5 to 10%) is applied to the primary. This voltage is increased till full load current flows in the winding. The applied voltage is very small, so mutual flux is also very small. Therefore, core losses which depends on flux is negligibly small. The wattmeter reading, therefore, represents full load primary and secondary losses. These losses can be found at any load. See figure 2._
Z = equivalent impedance of the transformer referred towards primary side.
Vse = Short circuit input voltage
Ii = Input primary current
Back to Back Test
This test is also called regenerative test and sumpner’s test. This is test is performed to find the efficiency regulation and heating under the load conditions. The loss of power in this case is equal to losses (i.e. iron and copper). This test can be performed if two identical transformers are available.
The insulation materials of transformer are tested to know how much higher voltages they will with stand. The insulation materials are tested in two ways:
- By power frequency voltage test
- By impulse voltage test
In power frequency voltage test, a high-test value of voltage is applied to the transformer. While in the impulse voltage test, an impulse wave is applied through an impulse generator.
The ratio (i.e. turn or voltage ratio) is found by giving a normal or low voltage to high voltage side of the transformer. The voltage of the other side is noted by voltmeter. The ratio between voltages is found. There are other methods as well, which are very accurate form this one.
The polarity of transformer at any instant and at any terminal shows indicaton of current flow. The polarity of transformer are tested to know whether the voltages show additive or subtractive property.