What is the working principle of a transformer Questions and Answers

20 Questions :

1. If a transformer is not marked, how could you test it for polarity?
2. A 10-kVA rating, that are connected in a closed delta arrangement, you would have a capacity of 30 
K VA. If one transformer is taken out of the bank, what would be the output capacity of the remaining 10
K VA transformers?
3. When connecting transformers in parallel, what factors must be taken into consideration?
4. What is a split-coil transformer?
5. Where may autotransformers be used?
6. What is the purpose of the markings on transformer leads?
7. In a bank of three single-phase transformers that are connected in a delta, each transformer delivers 240 volts at 10 amperes. What are the line voltages and line currents?
8. If you have a bank of three single-phase transformers that are connected in a closed delta arrangement, and one transformer burns up, how would you continue operation on the remaining two transformers?
9. What special precaution must be taken when using a booster transformer?
10. What is a three-phase transformer?
11. If transformers with different electrical characteristics are connected in parallel, what will happen?
12. Describe a current transformer.
13. What precautions must be taken when working with current transformers? Why?
14. What is an induction regulator?
15.Is it possible to connect two single-phase transformers to secure a three-phase output from a three-phase input?
16. When connecting an ordinary transformer as a booster transformer, what important factors must be considered?
17. When you use a bank of two single-phase transformers in an open delta arrangement, do they supply their full output rating?
18. How are current transformers rated?
19. Describe a potential transformer.
20. What is the phase relation between the three phases of a three-phase circuit?

Answers Above Questions:

1.Connect the transformer as shown in Figure 6-7. If it has subtractive polarity, V will be less than the voltage of the power source; if it has additive polarity, V will be greater than the voltage of the power source.
2.Each transformer would deliver 8.66 kVA, and you would have a bank capacity of 17.32 k VA.
3.Their electrical characteristics, such as voltage ratio, impedance percentage, and voltage regulation.
4.A transformer that has the coils on the low or high side in separate winding's so that they can be connected in series or parallel for higher or lower voltages, as desired.
5.(a) Where the system being supplied contains an identified grounded conductor that is solidly connected to a similar identified
6.They are there for standardization, so that transformer polarities are recognizable for any type of use.
7.The line voltages are each equal to 240 volts; however, the line current in each phase would be the current of each transformer multiplied by 1.732 (the square root of 3), or 17.32 amperes.
8.By merely disconnecting the leads to the disabled transformer.
9.There must be no fusing in the high side, or primary. Because the booster transformer is similar to a current transformer, an extremely high voltage could be built up on the secondary side if the fuse should blow.
10.A transformer that is the equivalent of three single-phase transformers, which are all wound on one core and enclosed within one common case.
11.They won’t distribute the load equally; one transformer will tend to assume more of the load than the other. This leads to overheating and, in severe cases, the destruction of the transformers.
12.A current transformer has a primary of a few turns of heavy conductor capable of carrying the total current, and the secondary consists of a number of turns of smaller wire. The primary winding is connected in series with the circuit carrying the current that is to be measured.
13.The secondary must never be opened when the primary circuit is energized. If it is necessary to disconnect an instrument while the circuit is energized, the secondary must be short-circuited. If the secondary is opened while the circuit is energized, the potential on the secondary might reach dangerously high values. By short-circuiting the secondary, damage is avoided and the voltage on the secondary is kept within safe limits.
14.This device is similar to a booster transformer. It has a primary and a secondary winding, which are wound on separate cores. The primary can be moved in either direction; this is usually done by an electric motor. In turning, the primary bucks or boosts the line voltage, as required. The amount of bucking or boosting is anticipated by the current being drawn by the line.
15.Yes, they would have to be connected in an open delta.
16.The high side of the transformer must be able to handle the approximate voltage of the line; the low side must have a voltage of approximately the value by which you wish to boost the line voltage and must also have a current capacity that is sufficient to carry the line current.
17.No. Each transformer is only capable of supplying 86.6 percent of its output rating.
18.They are rated at 50 to 5, 100 to 5, etc. The first number is the total current that the transformer is supposed to handle, and the second figure is the current on the secondary when the fullloadcurrent is flowing through the primary. For example, a 50- to-5 rating would have a multiplier of 10 Kilo.
19.A potential transformer is built like the ordinary isolation transformer, except that extra precautions are taken to ensure that the winding ratios are exact. Also, the primary winding is connected in parallel with the circuit to be measured.
20.They are 120 electrical degrees apart.