1. In a transformer zero voltage regulation is achieved at a load power factor which is
- leading
- lagging
- unity
- zero
Answer: 1
Explanation : At leading power factor the voltage regulation can be negative or zero. This can be found from this equation
% regulation = εxcosθ – εrsinθ
2. What is the magnitude of mutually induced emf, E2 in a transformer?
- directly proportional to rate of change of flux and number of secondary turns
- inversely proportional to rate of change of flux and number of secondary turns
- proportional to rate of change of flux and inversely proportional to number of secondary turns
- inversely proportional to rate of change of flux and proportional to number of secondary turns
Answer: 1
Explanation: Due to the supply voltage, a secondary induced emf is produced. V1 is in antiphase with E2 and the magnitude of E2 is proportional to rate of change of flux and number of secondary turns.
3. Which of these will happen in a transformer when the number of secondary turns is less than the number of primary turns?
- The voltage gets stepped up
- The voltage gets stepped down
- The power gets stepped up
- The power gets stepped down
Answer: 2
Explanation: According to the fundamentals of electrical engineering, the voltage transformation ratio is the ratio of the number of secondary turns to that of the number of primary turns. When the transformation ratio is less than 1, the step-down operation occurs.
4. Which of the following Transformer can have tap changer?
- Power transformer.
- Earthing transformer.
- Current transformer
- Potential transformer
Answer: 1
5. The constant losses in transformer is/are
- eddy current loss.
- hysteresis loss.
- copper loss.
- both 1 & 2
Answer: 4
Explanation: Eddy current loss and hysteresis loss are almost independent of load, significantly depending on supply voltage and frequency. As the flux density or flux is constant for a given voltage and frequency, eddy current loss and hysteresis loss remain constant at any load. Therefore, these losses are called constant losses.
Copper loss varies as the square of load current and called variable loss.
6. The efficiency of two identical transformers under loaded conditions can be determined by
- open circuit test.
- short-circuit test.
- back-to-back test.
- any of the above.
Answer: 3
7. What is the number of primary turns in a 200/1000 V transformer if the emf per turn is 10V?
- 5
- 10
- 20
- 40
Answer: 3
Explanation: According to the electrical engineering fundamentals, the number of primary turns is the ratio of induced emf in the primary to the emf induced per turn. Thus, the number of primary turns in a 200/1000 V transformer with emf per turn of 10V is 200/10 = 20.
8. What is the equivalent resistance of the primary of a transformer with a voltage transformation ratio of 10 and primary resistance of 0.05Ω when referred to secondary?
- 0.005
- 0.5
- 5
- 50
Answer: 3
Explanation: According to fundamentals of electrical engineering, the equivalent resistance of the primary of a transformer with a voltage transformation ratio of K and primary resistance of RΩ when referred to secondary is equal to K2*R. Thus, the answer will be 102*0.05 = 5.
9. In a transformer, hysteresis and eddy current losses depend upon
- load current
- maximum flux density
- supply frequency
- both 2 and 3
Answer: 4
10. In a transformer operating at constant voltage if the input frequency increases, the core
- increase
- decrease
- remains constant
- none of the above
Answer: 2
