In order to determine the parameters of a transformer such as voltage regulation and efficiency, the following open circuit and short tests are carried out. These tests are very convient , as they furnish the required information without actualy loading the transformer. Also the power required to carry out these test is very small as compared to full load output of the transformer.
- Open Circuit Test
- Calculation of Open Circuit Test
- Short Circuit Test
- Calculation of Short Circuit Test
Open Circuit Test
The Open circuit test is carried out to determine the no-load loss or core loss or iron loss. No load current I0, which is helpful in finding the no load parameters R0 and X0 of the transformer. This test is usually carried out on the low voltage (LV) side of the transformer that is a watt meter (W), a voltmeter (V) and an ammeter (A) are connected to the primary side of the transformer as shown in the figure below. The primary winding is connected to the normal rated voltage V1 and frequency as given of the nameplate rating of the transformer. The secondary side of the transformer is kept open circuited.
As the high voltage side (HV) of the transformer is open circuited, the current drawn by the primary is no load current I0 which is measured by an ammeter connected.The value of no-load current is very small usually 2 to 10% of the rated full load current. Thus, there is no copper loss on the secondary side as it is open circuited and in the primary winding, the copper loss occurring are very small. Therefore, the wattmeter reading W0 only represents the core or iron losses. The voltmeter V’ connected to the secondary side measures the secondary induced voltage V2.
Calculation of open circuit test
- W0 – wattmeter reading
- V1 – voltmeter reading
- I0 – ammeter reading
The no-load power factor is
No load parameters are given below
The phasor diagram of transformer at no load or when an open circuit test is performed is shown below
Short Circuit Test
The short circuit test is performed to determine the copper losses at full load, which is further required for the calculation of the efficiency of the transformer. This test also helps in determining the equivalent impedance (Zes or Zep), resistance (Res or Rep) and the leakage reactance (Xes or Xep) of the transformer referred to the winding in which the measuring instruments are connected. By knowing these parameters the voltage drop can be calculated and thus voltage regulation can be determined.
The circuit diagram of the short circuit test is shown below
The short circuit test is usually carried out on the high voltage (HV) side of the transformer. The watt meter (W), a voltmeter (V) and an ammeter (A) are connected to the secondary side of the transformer as shown in the above figure.the primary side is short circuited. With the help of an autotransformer, a low voltage at the normal frequency is applied to the secondary side of the transformer so that full load current flows in both the windings of the transformer. The full load current is measured by ammeters connected.
As a low voltage is applied on the HV side, which is 5 to 10% of the normal rated voltage. Therefore, the value of the flux which is set up in the core is also small, about 1/30th to 1/8th of the normal flux. Hence, the iron losses are negligibly small due to the low value of flux and thus the wattmeter Wc only measures the copper loss in the transformer windings.
Calculation of Short Circuit Test
- Wc – Wattmeter reading
- V2sc – voltmeter reading
- I2sc – ammeter reading
The phasor diagram of the short circuit test of the transformer is shown below
The voltage regulation of the transformer can be determined at any load and power factor after knowing the values of Zes and Res.
In the short circuit test the wattmeter record, the total losses including core loss but the value of core loss is very small as compared to copper loss so, the core loss can be neglected.