Interview Questions Answers in Electrical Engineering

Questions / { k.prakashchandra }

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Answers / { k.prakashchandra }

only three phases are coming in step down tranformers.how
to get neutral?

Answer

You mean the secondary is delta and output is 3 phase 3
wire.Then to create neutral ,you have to connect the
terminals to 3 phase neutral forming inductor coils and use
the star point as neutral.The easiest way is to connect the
terminals to a 3phase auto-transformer of correct capacity
and use the star point for neutral

Is This Answer Correct ?

3 Yes

2 No

Question { 4736 }

why we are having many type of vector group in the
transformer ,what is the use of it?

Answer

They are required for phase shifting or matching wherever
required, especially for measurements in electronic relays
and meters

Is This Answer Correct ?

1 Yes

1 No

Question { 11169 }

why C T secondary are rating 5A generaly .

Answer

Earlier they were using electromagnetic relays and
meters.Hence for accuracy and workability higher current
was required. Now a days CTs of 1 A secondaries are
used ,as electronic meters and relays have replaced them

Is This Answer Correct ?

4 Yes

2 No

Question { 5987 }

Why Humming noise originated in Starter motor and how it
was generated ?

Answer

If it is inside the starter , check for hold coil( 0 volt) and its accessories

Is This Answer Correct ?

2 Yes

1 No

Question { 3505 }

Generally transformers has delta primary and star
secondary. But in some transformer i saw Star primary and
delta secondary. what is the reason for chosing like this?

Answer

If the secondary is connected to a 3phase load or another
transformer

Is This Answer Correct ?

2 Yes

0 No

Question { 3256 }

Transmission line design. Say, I have a 1MVA generator and
2 transformers that will have the same load. What would be
the proper rating of the trasformers? Is it safe to say,
that I will simply divide the total rating of the generator?

Answer

Each generator should have its own tr called UGT (unit gen
tr) for unit differential protection .The capacity of tr is
equal to or a little more than that of gen .

Is This Answer Correct ?

1 Yes

1 No

Question { 3628 }

why supervision relay is used in incoming feeders?

Answer

It is used for all the feeders ,especially for out-going
radial feeders

Is This Answer Correct ?

1 Yes

0 No

Question { 3532 }

How to connect a Restricted earth fault relay for
2000KVA,11/0.433 KV Distribution Transformer and What is
the neutral CT ratio should be selelcted for 2000 MVA
Transformer for estabilishing the Restricted earth fault
relay

Answer

The ratio of NCT should be equal to that line CTs and both
should be identical

Is This Answer Correct ?

2 Yes

1 No

Question { 3167 }

We are having a 2000KVA DG set. It trips with Over speed
and Engne Hot when the mahine is stopped in full speed. How
shall I analyse the block load and reverse power.Is the
problem related to power factor. The machine which runs on
this DG is having all Synchronous motors with incividual
Drives.

Answer

Throw -off at full load results in high speed before the
governor controls the speed and also overheating

Is This Answer Correct ?

1 Yes

2 No

Question { TATA, 11257 }

Can anyone explain how the vector diagram of a transformer
which doesn't have name plate be found whether it is
Dy11/Dy5/Yd11 and so on?

Answer

Whether Dy or Yd can be ascertained from the bushings.To
find particular vector group ,CRO can be used finding
relationship between the current vectors.e.g.In Dy 11 I2R
lags I1R by 30 degrees Number 11 denotes position of minute-
hand in a clock.It can also found out by taking various
voltage measuremnts and analysing

Is This Answer Correct ?

7 Yes

0 No

Question { MAHINDRA, 6197 }

why celing fan is running in anti clock wise and table fan
is running in clock wised?

Answer

Ceiling fan is made to run anticlockwise when it is used for ' cooling- air' purpose .

Is This Answer Correct ?

0 Yes

4 No

Question { 3048 }

MY QUATION IS ABT THE MAIN POWER TRANSFORMER IN STEEL
PLANT , I WISH TO KNOW THAT WHY TRANSFORMER PROTECTION NOT
TRIP AS CABLES ON THE SECONDAY SIDE IS WATER COOLED.

TRANFORMER DETAIL
NORMAL KVA--150 KVA
PRIMARY VOLTAGE --34.5 KV
SEC.NO-LOAD VOLTAGE---1.290-908-690V ( WITH PRIMARY DELTA
CONNECTED)
CONNECTION--D/d (EXTERNAL CLOSED)
CONSTANT RATEING FROM--1.290-908V
CONSTANT CURRENT FROM--908-690
VECTOR GROUP--D III

Answer

As pure water is a bad conductor of electricity

Is This Answer Correct ?

3 Yes

0 No

Question { ABB, 67009 }

what is the difference between the 94 lockout relay and 86
lock out relay. I have noticed that the 94 lockout relay is
activated when the distance realy is activated and 86 is
activated when the 87T differential is activated. but why
can't the same lockouts be used for both the protection
schemes

Answer

94 lockout relay is for line fault only. 86 lockout relay
is master trip relay. It trips both primry and secondary of
the transformer as the fault is of serious nature.

Is This Answer Correct ?

2 Yes

12 No

Question { ONGC, 2394 }

why ht alternators gland plats using brass?

Answer

It is rustproof . Being softer than iron ,adjusts to slightly uneven surface and fastening bolt & nut can be removed easily .

Is This Answer Correct ?

0 Yes

0 No

Question { 25212 }

why rating of motor in HP?

Answer

The output of motor is mechanical not electrical

Is This Answer Correct ?

0 Yes

0 No