1. For 200 kVA, 440/6600-V transformer, short circuit test on the LV side would require ______
a) 22V
b) 330V
c) 44V
d) Can’t be calculated
Explanation: For a given transformer SC test is conducted on LV side, thus we’ll use 5% of rated voltage on the low voltage side. Hence, 5% of 440V calculation gives the value of 440*5/100= 22V on LV side.
2. For a transformer given of 100 kVA, 220/6000-V transformer, short circuit test is performed. What current rating is needed?
a) 30A
b) 445A
c) 60A
d) Can’t be calculated
Explanation: For a given transformer here, test is performed on low voltage side, thus we need the value of current on the low voltage side, by dividing the reactive power by the rated voltage value, i.e. 200*1000/100= 445A
3. What will be the value of voltage and current for a given transformer of 10 MVA, 220/4400-V which we are about to perform the Short circuit test?
a) 220 V, 30 A
b) 220 V, 2.27A
c) 440 V, 30 A
d) 440 V, 2.27 A
Explanation: Since short circuit test is always done on the HV side unless mentioned specifically, thus values are calculated with HV side parameters. Voltage required on HV side = 4400*5/100 = 220 V and 10*1000/4400= 2.27A
4. We only get copper losses from the short circuit test.
a) True
b) False
Explanation: Since the transformer is excited at very low voltage, the iron-loss is negligible (that is why shunt branch is left out), the power input corresponds only to the copper-loss, i.e. PSC = PC (copper-loss).
5. With the help of short circuit calculations we get value of ____________
a) Individual resistance and inductance of both sides
b) Resistance and inductance of primary side
c) Resistance and inductance of primary side
d) Combined resistance and inductance of both sides
Explanation:Short calculations include the ratio of short circuited voltage to the short-circuited current which gives Z value, similarly the R value is calculated by dividing the Short-circuited power with short circuited current square. Then, X is calculated for whole circuit.
6. Short circuit test is performed on a transformer with a certain impressed voltage at rated frequency. What will happen if the short circuit test is now performed with the same magnitude of impressed voltage, but at frequency higher than the rated frequency?
a) The magnitude of current will increase, but power factor will decrease
b) The magnitude of current will decrease, but power factor will increase
c) The magnitude of current will increase, power factor will increase
d) The magnitude of current will decrease, power factor will decrease
Explanation: Since frequency has been increased, the leakage reactance will increase. Which will increase the impedance. Thus, current will be less due to inverse proportionality and power factor will be poorer
7. Sumpner’s test is conducted on transformers to study effect of ____________
a) Temperature
b) Stray losses
c) All-day efficiency
d) Cannot be determined
Explanation: Sumpner’s test is the test which is used to determine the steady temperature rise if the transformer was fully loaded continuously; this is so because under each of these tests the power loss to which the transformer is subjected is either the core-loss or copper-loss but not both.
8. Which of the following tests are enough to find all the parameters related to a transformer?
a) OC test
b) OC, SC test
c) OC, SC, Sumpner’s test
d) Sumpner’s test
Explanation: While OC and SC tests on a transformer yield its equivalent circuit parameters, these cannot be used for the ‘heat run’ test wherein the purpose is to determine the steady temperature rise if the transformer was fully loaded continuously.
9. Sumpner’s test is performed on _________
a) Single transformer at a time
b) Only two transformers at a time
c) Minimum 2 transformers at a time
d) Many transformers at a time
Explanation: Sumpner’s test is used to determine the effect of transformer on loaded condition. Thus, two transformers are tested simultaneously, where one simply acts as a load to another transformer
10. In Sumpner’s test __________
a) Primaries can be connected in either way
b) Primaries are connected in parallel with each other
c) Primaries of both transformers are connected in series with each other
d) No need to connect primaries
Explanation: Sumpner’s test is also called as back-to-back test, where two transformers are used where one transformer acts as a load to another transformer. Primaries of both of the transformers used in a test, are connected in parallel with each other