ALTERNATING CURRENT MACHINES – I(Practical:)
3. Perform the short circuit test of a single phase transformer.
Perform the short circuit test of a single phase transformer.
3.1 Sketch the required circuit diagram.
3.2 List tools, equipment & materials required.
3.3 Connect the equipment according to the circuit diagram keeping high side to the
source.
3.4 Short circuit the low side by a low resistance wire.
3.5 Energize the circuit by applying reduced voltage and increase the voltage until it takes
full load of current.
3.6 Record the instrument readings.
3.7 Observe copper loss and calculate and find Re, Xe & Ze.
1. Short Circuit Test on Transformer
The connection diagram for the short circuit test on the transformer is shown in the figure below. A voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. A low voltage of around 5-10% is applied to that HV side with the help of a variac (i.e. a variable ratio auto transformer). We short-circuit the LV side of the transformer. Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the HV side.
After reaching the rated current of the HV side, we record all the three instrument readings (Voltmeter, Ammeter and Watt-meter readings). The ammeter reading gives the primary equivalent of full load current IL. As the voltage applied for full load current in a short circuit test on the transformer is quite small compared to the rated primary voltage of the transformer, the core losses in the transformer can be taken as negligible here.
Let’s say, voltmeter reading is Vsc. The watt-meter reading indicates the input power during the test. As we have short-circuited the transformer, there is no output; hence the input power here consists of copper losses in the transformer. Since the applied voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to the rated voltage, so, we can neglect the core loss due to the small applied voltage. Hence the wattmeter reading can be taken as equal to copper losses in the transformer. Let us consider wattmeter reading is Psc.
If, Ze is equivalent impedance of transformer.
Therefore, if equivalent reactance of transformer is Xe.
These values are referred to the HV side of the transformer as the test is conducted on the HV side of the transformer. These values could easily be converted to the LV side by dividing these values with the square of transformation ratio.
Hence the short-circuit test of a transformer is used to determine copper losses in the transformer at full load. It is also used to obtain the parameters to approximate the equivalent circuit of a
2.Short circuit or Impedance test on Transformer
The connection diagram for short circuit test or impedance test on transformer is as shown in the figure below. The LV side of transformer is short circuited and wattmeter (W), voltmere (V) and ammeter (A) are connected on the HV side of the transformer. Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current. All the readings are taken at this rated current.
The voltage applied for full load current is very small as compared to rated voltage. Hence, core loss due to small applied voltage can be neglected. Thus, the wattmeter reading can be taken as copper loss in the transformer.
Therefore, W = Isc2Req....... (where Req is the equivalent resistance of transformer)
Zeq = Vsc/Isc.
Therefore, equivalent reactance of transformer can be calculated from the formula Zeq2 = Req2 + Xeq2.
These, values are referred to the HV side of the transformer.
Hence, it is seen that the short circuit test gives copper losses of transformer and approximate equivalent resistance and reactance of the transformer.