Instrument Transformers MCQ

1. Which one of the following instruments is commonly used to measure the primary current of a transformer connected to mains?

1. Electrostatic meter
2. Current transformer
3. Moving coil type meter
4. Moving iron meter

2. A current transformer has a phase error of +3°. The phase angle between the primary and secondary current is

1. 177°
2. 3°
3. 183°
4. 180°

3. In using instrument transformers, care should be taken not to open circuit the

1. primary of a voltage transformer when the secondary is connected to the rated load.
2. secondary of a voltage transformer when the primary is energized with the rated voltage.
3. primary of a Current transformer when the secondary is connected to the rated load.
4. secondary of a current transformer when the primary is carrying the rated current.

4. In potential transformers, the secondary turns are increased slightly and the primary and secondary windings are wound as closely as possible to compensate for

1. phase angle and ratio error, respectively
2. ratio and phase angle error, respectively
3. any eddy current loss and hysteresis loss, respectively
4. the hysteresis loss and eddy current loss, respectively

5. Instrument transformers are known to introduce magnitude and phase errors in measurements. These are primarily due to

1. improper connections on the primary side.
2. measurement errors inherent in the meter connected to the transformer secondary.
3. open and short-circuit parameters of the instrument transformers.
4. none of these

6. In a constant voltage transformer (CVT), the output voltage remains constant due to

1. capacitor
2. input inductor
3. saturation
4. tapped windings

7. A 500A/5A, 50 Hz current transformer has a bar primary. The secondary burden is a pure resistance of 1 Ω and it draws a current of 5 A. If the magnetic core requires 250 AT for magnetization, the percentage ratio error is

1. 10.56
2. -10.56
3. 11.80
4. -11.80

8. A 50 Hz, bar primary CT has a secondary with 500 turns. The secondary supplies 5 A current into a purely resistive burden of 1 Ω. The magnetizing ampere-turns is 200. The phase angle between the primary and secondary current is

1. 4.6°
2. 85.4°
3. 94.6°
4. 175.4°

9. A 200/1 Current transformer (CT) is wound with 200 turns on the secondary on a toroidal core. When it carries a current of 160 A on the primary, the ratio and phase errors of the CT are found to be -0.5% and 30 minutes respectively. If the number of secondary turns is reduced by 1 the new ratio error (%) and phase error (min) will be respectively

1. -0.5, 35
2. 0.0, 30
3. -1.0, 30
4. -1.0, 25

10. Under the operating condition, the secondary of a current transformer is always short circuit because

1. It protects the primary circuits
2. It is safe for human beings
3. It avoids core saturation and high voltage induction
4. None of these

11. The current transformer that is used to measure a 100A current by 5 A ammeter is

1. step-up transformer
2. step down transformer
3. power transformer
4. distribution transformer

12. Primary current in a current transformer is increased to a very high value. It will cause

1. Increase in ratio error
2. Increase in phase angle error but a decrease in ratio error
3. Reduction in both ratio error and phase angle error
4. Increase in phase angle error only

13. In the current transformer

1. Secondary winding must be closed
2. Secondary winding must be open
3. Primary winding must be closed
4. Primary winding must be open

14. The potential transformers’ nominal ratio is defined as the ratio of

1. Primary winding voltage and secondary winding voltage
2. Rated primary winding voltage and rated secondary winding voltage
3. Primary windings turns and secondary winding turns
4. Any of these

15. A CT is connected in ______ with the line.

1. Series
2. Across
3. Both a and b
4. Not connected

16. Before disconnecting an ammeter from an energized current transformer circuit, the current transformer

1. Primary should be opened
2. Secondary should be opened
3. Primary should be shorted
4. Secondary should be shorted

17. A load is connected to supply. A current transformer (CT) and a potential transformer (PT) is used between load and supply. A power of 0.5 is measured at the secondary side of CT and PT. If the phase angle error of CT and PT are 0.4° and 0.7°, the power factor of the load is

1. cos 60.3°
2. cos 58.9°
3. cos 59.7°
4. cos 61.1°

18. In which of the following transformers, is the secondary winding always kept closed?

1. Current transformer
2. Potential transformer
3. Power transformer
4. Distribution transformer

19. What principle makes the instrument transformer working?

1. Self-induction
2. Lenz law
3. Mutual induction
4. Fleming’right hand rule

20. The burden of the current transformer is usually expressed in

1. Amperes
2. Volts
3. Volt amperes
4. Kilowatt

21. Current transformers for meters and relays usually have

1. 1 : 2 ratio
2. 5 : 1 ratio
3. 5 A secondary’s
4. 5 A primary’s

22. If we leave the secondary winding of the current transformer open 

1. Nothing will happen
2. CT will blast
3. There will be only a magnetizing component of the current flowing in the primary
4. There will not be any current in the primary

23. CT and PT are used generally to

1. Set up the respective quantities
2. Step down the respective quantities
3. Both
4. None

24. A PT is a device which is

1. Electro-statically coupled
2. Electrically coupled
3. Electromagnetically coupled
4. Conductively coupled

25. If the secondary burden of a current transformer is 15 VA and the secondary current is 5 A, then the impedance of the connected load will be

1. 0.6 ohm
2. 5 ohm
3. 6 ohm
4. 10 ohm