The performance (efficiency, regulation, losses) of a transformer is best judged if the following four parameters of its equivalent circuit are known,
- The equivalent resistance R01 is referred to as primary, and the equivalent resistance R02 is referred to as secondary.
- The equivalent leakage reactance X01 is referred to as primary, and the equivalent leakage reactance X02 is referred to as secondary.
- The core-loss resistance Ro (parallel circuit).
- The magnetizing reactance Xo (parallel circuit).
These four parameters or constants of a transformer equivalent circuit can be determined by the following two tests,
- Open-circuit test, and
- Short-circuit test.
These tests are very simple and cheap to perform because we can find the required information without actually loading the transformer.
Open Circuit Test of Transformer :
The aim of the test is to determine the iron-loss or core-loss and no-load current Io which is helpful in finding Ro and Xo. The amount of power wasted in the core of a transformer can be obtained by this test.
In this test one winding usually, high voltage winding is left open and the other (LV side) is connected to the normal voltage supply as shown in the figure. Since normal voltage is applied to the primary, the normal flux will be set up in the core hence normal core losses will occur which are measured by the wattmeter.
As no current flows in the open secondary, the no-load primary current Io is small (usually 3 to 10 of full-load current), and the primary copper loss is negligibly small and nil in secondary. Hence, the OC test gives core loss alone practically (i.e., wattmeter reading) and is the same for all loads.
Sometimes, a high resistance voltmeter is connected across secondary to read secondary induced emf which helps to find the transformation ratio K.
The transformer vector diagram is the same as the no-load condition of a transformer as shown in the figure. If Wo is the wattmeter reading. Therefore,
Since the no-load current Io is very small therefore pressure coils of the wattmeter and voltmeter should be connected such that the current taken by them should not flow through the current coil of the watt-meter.
Separation of Hysteresis and Eddy Current Loss :
The core loss or iron loss consists of two parts.- Hysteresis loss :
Wh = P Bm1.6 f watt / m3
- Eddy current loss :We = Q Bm2 f2 watt / m3
Where P, Q are two constants,- Bm = maximum flux density (T)
- f = frequency (Hz)
The total iron loss is given by,Wi = Wh + We
= P Bm1.6 f + Q Bm2 f2
The observation table to perform an open-circuit on the transformer is shown below. Where no-load current can be calculated from the ammeter, total power loss (i.e., iron loss in core) is calculated through the wattmeter, and the voltage applied is known by the voltmeter.
| S.No. | Primary Rated Voltage V1 (volts) | No-load Current Io (amp) | Wattmeter Readings (iron loss) Wo (watt) | Secondary Induced Voltage V2 (volts) |
|---|
| Readings observed from the test |
Why Open Circuit Test is Performed on L.V. Side only?
The main thing is that an open circuit of a transformer is performed on any side of the transformer i.e., either on HV (high voltage) or LV (low voltage). But there are some benefits while performing the OC test on the LV side of the transformer.
The aim of this test is to determine the no-load losses i.e., core loss at rated voltage. The term rated voltage means we have to apply the rated value of the voltage on the LV side. This rated value of voltage is smaller (440V, 11kV) when compared to the HV side (33kV, 132kV, and above).
So when we prefer the HV side for testing, it becomes harder, costlier, and care must be taken while performing the test. Because the equipment (measuring instruments) used for tests i.e., to apply high voltages of about 33kV, 132kV, and above becomes costlier. And precautions must be taken with proper insulation to the body to avoid shocks that may lead to death.
The turns on the LV side of the transformer are less than the turns present on the HV side. Hence the power loss (i.e., due to copper loss) is almost negligible on the LV side of the transformer when it is on no-load.
As we know that the open circuit test is to determine the core loss. Thus when a rated voltage is applied on the L.V. side power loss occurred is very small. Therefore, the wattmeter reads only the core loss component of the transformer.