61. You are required to produce an alternating current that is proportional to the input current. Which of the following devices would you use?
Power Transformer
Current Transformer
Potential Transformer
Instrument Transformer
Answer.2. Current Transformer
Explanation:
A current transformer ( CT) is a type of “instrument transformer” that is designed to produce an alternating current in its secondary winding which is proportional to the current being measured in its primary.
62. In a current transformers
The number of turns in primary and secondary windings are nearly equal
The number of primary turns is very small as compared to number of secondary turns.
The number of secondary turns is very small as compared to the number of primary turns
either (1) or (3).
Answer.3. The number of secondary turns is very small as compared to the number of primary turns
Explanation:
A current transformer is a device that is used for the transformation of current from a higher value into a proportionate current to a lower value.
The ratio of the primary current and the secondary current is known as the current transformer ratio of the circuit. The current ratio of the transformer is usually high. The secondary current ratings are of the order of 5A, 1A and 0.1A. The current primary ratings vary from 10A to 3000A or more.
63. A 1000/1 A, 5 VA, UPF bar-primary measuring current transformer has 1000 secondary turns. The current transformer exhibits a ratio error of 0.1 % and a phase error of 3.438 minutes when the primary current is 1000 A. At this operating condition, the RMS value of the magnetization current of the current transformer (in amperes, rounded off to two decimal places) is
Hence, the RMS value of the magnetizing current of CT is 1 Amp.
64. The cause of error in the current transformer is
CT draw Magnetizing current
Core saturation
Primary and secondary flux linkage
All of the above
Answer.4. All of the above
Explanation:
In an ideal CT, the actual transformation ratio should have been exactly equal to the turns ratio and the phase angle should have been zero. However, due to inherent physical limitations inherent to the electric and magnetic circuits of the CT, practical performance deviates from these ideal behaviors, and errors are introduced in measurement. The reasons for these errors are given here.
Primary winding always needs some magnetizing MMF to produce flux and, therefore, the CT draws the magnetizing current IM
CT no-load current must have a component I c that has to supply the core losses, i.e., the eddy current loss and the hysteresis loss.
Once the CT core becomes saturated, the flux density in the core no longer remains a linear function of the magnetizing force, this may introduce further errors.
Primary and secondary flux linkages differ due to unavoidable flux leakages.
65. The phase angle of the current transformer is taken as positive when secondary voltage is reversed _______ the primary voltage.
Lag
Lead
Either lead or lag
None of the above
Answer.2. Lead
Explanation:
Phase angle error:
In an ideal voltage transformer, there should not be any phase difference between the primary voltage and the secondary voltage reversed. However, in an actual transformer, there exists a phase difference between Vo and Vs reversed.
The phase angle is taken as +ve when secondary voltage reversed leads to the primary voltage.
The angle is -ve when the secondary voltage reversed lags the primary voltage.
It can be reduced by keeping the primary and secondary windings are wound as closely as possible.
