21. A current transformer has a phase error of +3°. The phase angle between the primary and secondary currents is
3°
177°
180°
183°
Answer.2. 177°
Explanation:
Given that
Phase angle error θ = 3°
Phase angle between the primary current IP and secondary current
IS = δ = 180° – θ = 180° – 3° = 177°
22. An industrial analyzer is NOT suitable for measuring which of the following?
Power
Resistance
Current
Power factor
Answer.2. Resistance
Explanation:
An industrial analyzer is used for single-phase or three-phase measurements of the:
Active power
Reactive power
Apparent power
Power factor
Phase angle
Energy
Voltage
Current
23. A current-carrying conductor is wrapped eight times around the jaw of a clamp-on meter that reads 50 A. What will be the actual value of the conductor current?
400 A
50 A
12.5 A
6.25 A
Answer.4. 6.25 A
Explanation:
Given, Np = 1, Ns = 8, Ip = 50 A
the secondary current is given by
Is/Ip = Np/Ns
Is = 50 × (1 / 8)
Is = 6.25 A
24. Burden of current transformer is usually expressed in
Amperes
Volts
Volt amperes
Kilowatt
Answer.3. Volt amperes
Explanation:
Load on Current Transformer is called as a burden because Current Transformer secondary winding can’t be kept open, high voltage can do the insulation failure. The burden of the Current Transformer can be specified as Volt-Ampere absorbed at a certain Power Factor.
25. Under operating conditions, the secondary of a current transformer is always short-circuited because of ________.
It protects the primary circuits
It is safe for human beings
It avoids core saturation and high voltage induction
None of these
Answer.3. It avoids core saturation and high voltage induction
Explanation:
The secondary side of the current transformer is always kept short-circuited in order to avoid core saturation and high voltage induction so that the current transformer can be used to measure high values of currents.
26. An instrument transformer is used to extend the range of-
Induction instrument
Electrostatic instrument
Moving coil instrument
PMMC instrument/PMMC
Answer.1. Induction instrument
Explanation:
Instrument transformers are of two types.
1. Potential transformer
2. Current transformer
These are used to extend the range of induction-type instruments.
27. A current transformer has a double turn primary and a 100 turn secondary winding. The secondary draws current of 6 A and magnetizing ampere-turns is 60 A. What is the actual transformation ratio?
100.5
101
50.24
25.12
Answer.3. 50.24
Explanation:
Magnetizing ampere-turns = 60 A = NI
Magnetizing current (Im) = 30 A
Number of primary turns (N1) = 2
Number of secondary turns (N2) = 100
Nominal ratio, n=N2/N1=100/2=50
Exciting current (I0) = Im = 30 A
The rated secondary current (I2) = 6 A
Reflected secondary current = nI2 = 50 × 6 = 300 A
28. The total impedance of the secondary winding, leads, and burden of a 5 A CT is 0.01 Ω. If the fault current is 20 times the rated primary current of the CT, the VA output of the CT is
100 VA
50 VA
150 VA
200 VA
Answer.1. 100 VA
Explanation:
Given that, impedance of secondary winding = 0.01 Ω
Rated current = 5 A
fault current = 20 times of rated current
= 20 × 5 = 100 A
VA output = I2z = 1002 × 0.01 = 100 VA
29. The current transformer is considered as _____
Parallel Transformer
Series Transformer
Either Series or Parallel Transformer
Auto transformer
Answer.2. Series Transformer
Explanation:
The current transformer is considered as a series transformer with virtual short circuit conditions whereas the potential transformer is considered as a parallel transformer with its open-circuited secondary winding, operating with no damage to the transformer or to the operator.
30. The voltage transformer is considered as _____
Parallel Transformer
Series Transformer
Either Series or Parallel Transformer
Autotransformer
Answer.1. Parallel Transformer
Explanation:
The potential transformer is considered as a parallel transformer with its open-circuited secondary winding, operating with no damage to the transformer or to the operator whereas the current transformer is considered as a series transformer with virtual short circuit conditions.
