Transformers Questions and Answers – Phasor Diagrams of Equivalent and Approximate Model

This set of Transformers Interview Questions and Answers focuses on “Phasor Diagrams of Equivalent and Approximate Model”.

1. Which of the following is not considered in the standard voltage scale for power supply in India?
a) 11kV
b) 33kV
c) 66kV
d) 122kV

Explanation: All the transformers that are used in power system analysis have their secondary voltages equal to 11kV, 33kV, 66kV, etc. As this voltage ranges are commonly used in transmission lines, 122kV is the wrong option.

2. The maximum load that a power transformer can carry is limited because of its ___________
a) temperature rise
b) dielectric strength of oil
c) voltage ratio
d) copper loss

Explanation: One can increase the dielectric strength of oil, by changing the oil. Similarly, temperature rise and copper losses can also be controlled by using various techniques. The only thing which is constant is voltage ratio which can’t be altered.

3. The voltage transformation ratio of a transformer is defined as ratio of _____________
a) primary turns to secondary turns
b) secondary current to primary current
c) secondary induced emf to primary induced emf
d) secondary terminal voltage to primary applied voltage

Explanation: Voltage transformation ratio is equal to secondary induced emf to the primary induced emf. While the secondary and primary voltage induced are different from emfs as these are emf minus the losses in the line.

4. If a transformer is made to run on to a voltage which is more than the rated voltage _______________
a) its power factor will deteriorate
b) its power factor will increase
c) its power factor will remain unaffected
d) its power factor will be zero

Explanation: Every electric device works in appropriate condition with maximum output and minimum losses when it is operated at rated conditions. Thus, if transformer is made to run at higher operating voltage its power factor will deteriorate.

5. Which of the following equation correctly represents the exact phasor diagram of transformer?
a) V1=E1+I1R1+jI1X1
b) V1=E1+I1R1+jI2X2
c) V2=E2+I1R1+jI1X1
d) V1=E1-I1R1+jI1X1

Explanation: According to the primary and secondary equivalent circuits of a transformer equation stated in option 1 correctly suits with the kirchoff’s voltage law for primary side of a transformer, similarly equation for secondary side can also be written down.

6. Approximate phasor diagram of a transformer is based on _____________
a) V1=E1+I1R1+jI1X1
b) V2=E2+I2R2+jI2X2
c) V1=V2+IR+jIX
d) V1=E1+I1R1+jI1X2

Explanation: Approximate equivalent circuit of a transformer neglects the middle branch of transformer equivalent circuit’ which makes transformer circuit even more simpler. Then, all resistances (though they are negligible) are summed up as R and similarly all reactance as X.

7. Hysteresis loss and eddy current loss is directly proportional to __________
a) f and f2
b) f2 and f
c) f and f
d) f2 and f2

Explanation: Hysteresis loss is directly proportional to frequency according to Steinmetz’s formula. While eddy current losses are directly proportional to square of flux density, thickness, frequency. Both losses are load independent.

8. What will happen to hysteresis loss if voltage is doubled, load is doubled and frequency is halved?
a) Will be twice
b) Will be halved
c) Will remain same
d) Will be four times

Explanation: Hysteresis loss is directly proportional to voltage and frequency as well. It is load independent. Thus by doing voltage twice and frequency half of the original value we will get same hysteresis loss.

Sanfoundry Global Education & Learning Series – Transformers.