Interview Questions Answers in Electrical Engineering

Questions / { pankaj }

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Answers / { pankaj }

why transformer rating in kva ?

Answer

TRANSFORMER LOSSESS ARE INDEPANDANT OF POWER FACTOR BECAUSE
COPPER LOSSES DEPENDS ON CURRENT AND IRON LOSSESS DEPENDS
ON VOLATGE. HENCE, RATED IN KVA

Is This Answer Correct ?

18 Yes

2 No

Question { ABC, 48681 }

Why we should use delta connection in primary and star
connection in an distribution transformer, which converts
400v a.c. voltage to 33kv.

Answer

For distribution transformers, loads are connected to
secondry side. hence, requirement of neutral can be
obtained by connecting secondry of the transformer in star.
Secondly, to reduce or minimise the third harmonics in the
transformers, primary side connection generally done in
delta.

Is This Answer Correct ?

18 Yes

4 No

Question { 27931 }

why current increases when voltage decreases in ac
system

Answer

transformation ration k = n1/n2=v1/v2=i2/i1.
or k = v1/v2 = i2/i1 which is inversly proportional

Is This Answer Correct ?

6 Yes

6 No

Question { ABB, 36890 }

how much loading in terms of ampere can put on 5 MVA,33/11
KV TRANSFORMER having rated current 263 Amp ?????????

Answer

Normayy transformer gives max efficieny at 75% of its rated
capacity because in that case cu loss = iron loss.

Is This Answer Correct ?

1 Yes

9 No

Question { Cummins, 20562 }

What is Automatic Voltage regulator(AVR)?

Answer

AVR generally used in generators The function of AVR is :
due to residual flux, initialy residual voltage build up in
the alternator, this voltage given to avr which compare the
output voltage of the alternator with the setr efrance
voltage and controls the output voltage.

Is This Answer Correct ?

14 Yes

1 No

Question { 6989 }

Why current Transformer is rated by 100A/5A/5A?

Answer

It means 100A for primary, 5A for metering & 5A for
protection

Is This Answer Correct ?

29 Yes

1 No

Question { Bhel, 24929 }

What is the thumb rule for calculating the P.F correction
Capacitor?? Say for 100 hp motor how much micro farad
Capacitor is needed .... Thumb rule so quick answer wanted

Answer

For 100 HP load kVAR required will be half i.e 50 kVAR. If
you go for proper calculation, ans would be closer.

Is This Answer Correct ?

8 Yes

18 No

Question { 21510 }

why we commonly use delta star winding in power and
distribution transformers

Answer

The best way to minimise 3rd harmonics in the transformers
there are two ways. Either transformermer has low flux
density which is not economical and not in use or to make
atleast one winding of the transformer in delta because 3rd
harmonics are absent in delta. Since the requirement of
three phase four wire system in distribution, we commonly
use delta star winding

Is This Answer Correct ?

25 Yes

3 No

Question { 7235 }

What is Skin effect in Cable?

Answer

When DC supply is given to the conductor, current flows
uniformly throughout the cross section of the conductor.
But in case of AC, Current does not flows uniformly
throughout the cross section and concentrate near the
surface of the core. The concentration of current near the
surface of the core is the skin effect.

Is This Answer Correct ?

13 Yes

2 No