This set of Transformers Multiple Choice Questions & Answers (MCQs) focuses on “SC Test on Transformer”.
1. While conducting short-circuit test on a transformer which side is short circuited?
a) High voltage side
b) Low voltage side
c) Primary side
d) Secondary side
Explanation: It’s a common practice to conduct SC test from HV side, while keeping LV side short circuited. Thus, short circuited current is made to flow from shorted low voltage terminals i.e. LV side.
2. During short circuit test why iron losses are negligible?
a) The current on secondary side is negligible
b) The voltage on secondary side does not vary
c) The voltage applied on primary side is low
d) Full-load current is not supplied to the transformer
Explanation: Very small amount of voltage is given to the transformer primary thus the magnetic losses which are dependent on magnetic flux density will get minimum value, hence iron losses are negligible.
3. Short circuit test on transformers is conducted to determine ______
a) Core losses
b) Copper losses
c) Hysteresis losses
d) Eddy current losses
Explanation: Short circuit test is used to determine the copper losses taking place in the transformer under operation, while open circuit test gives us the value of core losses taking place in the transformer.
4. When a short circuit test on a transformer is performed at 25 V, 50 Hz, the drawn current is I1. If the test is performed by 25 V and 25 Hz and power drawn current is I2, then
a) I1 > I2
b) I1 < I2
c) I1 = I2
d) Can’t be defined
Explanation: Current by ohm’s law is equal to voltage divided by impedance. So, I=V/Z. Here Z is inductive load, thus Z= 2πfL. So as the frequency decreases the impedance also decreases and ultimately it reduces the denominator term causing increase in current.
5. Why SC test is not conducted on LV side?
a) Difficult to arrange low voltage supply
b) Difficult to arrange high current supply
c) Difficult to arrange low voltage and high current supply to the LV
d) SC test on LV does not give correct results
Explanation: If rated voltages and power is considered we need only 5% of rated voltage to be applied at on HV side, while by calculations current requirement is also less. For the same test on LV side though voltage required is less compare to HV side, current required is very high.
6. SC test gives ______________
a) Series parameters of equivalent circuit
b) Parallel parameters of equivalent circuit
c) Both parameters of equivalent circuit
d) Neither series nor parallel parameter of equivalent circuit
Explanation: Short circuit test gives the copper losses; these losses are taken into consideration by series parameters of the equivalent circuit. While, Open circuit test gives us iron losses; which are shown by parallel components of equivalent circuit.
7. For 200 kVA, 440/6600-V transformer, short circuit test on the LV side would require ______
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.
8. For a transformer given of 100 kVA, 220/6000-V transformer, short circuit test is performed. What current rating is needed?
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.
9. 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.
10. We only get copper losses from the short circuit test.
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).
11. 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.
12. 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.
Sanfoundry Global Education & Learning Series – Transformers.
To practice all areas of Transformers, here is complete set of 1000+ Multiple Choice Questions and Answers.