TRANSFORMERS MCQs SET 06

TRANSFORMERS MCQs SET 06

1. What is the function of breather in a transformer?
a) To provide oxygen inside the tank
b) To cool the coils during reduced load
c) To cool the transformer oil
d) To arrest flow of moisture when outside air enters the transformer
 

Answer: d 

Explanation: Most of the power generation stations use silica gel breathers fitted to conservator of oil filled transformers. The most used purpose of these silica gel breathers is to arrest the moisture when the outside air is sucked in by the transformer during the breathing process.
 

2. Natural air cooling method can’t be adopted because of some unavoidable effects, beyond _______
a) 1.5 MVA
b) 5 MVA
c) 15 MVA
d) 50 MVA
 

Answer: a 

Explanation: Smaller size transformers are immersed in a tank containing transformer oil. The transformer oil because temperature properties, which is surrounding the core and windings gets heated, expands and moves upwards. It then flows downwards by the inside of tank walls which cause it to drop temperature and oil goes down to the bottom of the tank from where it rises once again completing the circulation cycle.
 

3. What is the no-load current drawn by transformer?
a) 0.2 to 0.5 per cent
b) 2 to 5 per cent
c) 12 to 15 per cent
d) 20 to 30 per cent
 

Answer: b 

Explanation: The no load current is about 2-5% of the full load current and it accounts for the losses in a transformer. These no-load losses include core (iron/fixed) losses, which contains eddy current losses & hysteresis losses and the copper (I2*R) losses due to the no Load current.
 

4. Purpose of no-load test on a transformer is ___________
a) Copper loss
b) Magnetising current
c) Magnetising current and loss
d) Efficiency of the transformer
 

Answer: c 

Explanation: No-load current is little bit greater than actual magnetizing current. Total no-load current supplied from the source has two components, one is magnetizing current which is utilized for magnetizing the core and other component is consumed for compensating the core losses in transformer.
 

5. No-load current in a transformer ________________
a) Lags behind the voltage by about 75°
b) Leads the voltage by about 75°
c) Lags behind the voltage by about 15°
d) Leads the voltage by about 15°
 

Answer: a 

Explanation: No-load current lags behind the voltage by an angle which is near to 900. Thus, angle between no-load current and magnetizing current is very small. No-load current has another component which is in phase with voltage.
 

6. Which of the following statement is true for no-load current of the transformer?
a) has high magnitude and low power factor
b) has high magnitude and high power factor
c) has small magnitude and high power factor
d) has small magnitude and low power factor
 

Answer: d 

Explanation: Since no-load current lags voltage by the angle of nearly 900, power factor being equal to cosine of the angle between current and voltage, it will be equal to value which is near to 0. Thus, power factor will be low.
 

7. In no-load test we keep secondary terminals __________
a) Shorted
b) Shorted via fixed resistor
c) Open
d) Shorted via variable resistors
 

Answer: c 

Explanation: In no-load test, as we don’t require any load, we are not allowed to connect any resistor (fixed/variable) to the transformer secondary. We don’t short the secondary terminals either.
 

8. Maximum value of flux established in a transformer on load is equal to _________
a) E1/ (4.44*f*N1)
b) E1/ (4.44*f*N2)
c) E2/ (4.44*f*N1)
d) Cannot define
 

Answer: a 

Explanation: E1/ (4.44*f*N1). The emf induced in the primary due to applied voltage to primary winding is equal to change in flux with respect to time multiplied by number of turns in the primary. So, by solving this equation we get, E1= (4.44*f*φ*N1).
 

9. Induced emf in the primary of transformer is equal to terminal voltage applied at primary.
a) True
b) False
 

Answer: a 

Explanation: Induced emf in the primary is approximately equal to the applied voltage. Ideally there lies a very small difference in the values, but it is neglected because winding resistance in the transformer is of very small order.
 

10. For a linear B-H relationship, which option is correct?
a) The exciting current is equal to core loss current
b) The exciting current is equal to magnetizing current
c) The exciting current is equal to de-magnetizing current
d) The exciting current is equal to cross-magnetizing current
 

Answer: b 

Explanation: For a linear B-H relationship it is assumed that, there are no losses present in the core like eddy current losses and hysteresis losses are neglected. Thus, core loss current is equal to 0, which ultimately confirms exciting current is purely magnetizing one.