Interview Questions Answers in Electrical Engineering

Questions / { srinivasarao talasila }

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Answers / { srinivasarao talasila }

Question { 5496 }

wht factor are considered to design ACB( air Circuit reaker)
415V

Answer

Making capacity,Breaking capacity,Earth fault and short
circuit,Time and Impulse current rating

Is This Answer Correct ?

0 Yes

1 No

Question { 5042 }

Can VFD step up to 60 Hz if it is designed to receive 50 Hz
input?

Answer

VFD is designed to give a out put frequency of 1Hz to 600
Hz but our connected load has to with stand.

In our plant we are running the german motor name plate RPM
1775 at 60Hz but we are running the same at 87Hz RPM 2610.

In above case also bearing capacity to withstand such high
rpm and current of the motor should be with in the name
plate limits.

Is This Answer Correct ?

0 Yes

0 No

Question { 7898 }

How to increase home power factor? Can we connect capacitor
directly? How to select capacity of capacitor?

Answer

There is a formula to calculate capacity of capacitor
required for a certain inductive load in KW
KVAR=KW*(Tan of running PF angle-Tan of desired PF angle)
Per suppose my plant is running 0.84 i need to take it to
0.98 at a load of 800KW

KVAR=800*(Tan 32.86-Tan 11.48)
KVAR=800*(0.6459-0.2030)
KVAR=800*0.4429
=354.32 say 355KVAR

Is This Answer Correct ?

0 Yes

0 No

Question { Genco, 6329 }

Does power factor vary with speed?If yes, how?If not, why?

Answer

PF will not vary with speed.
For speed control of induction motors we will follow these
techniques 1) By varying the frequency
2) By varying the input voltage
3) By adding the rotor resistance
4) By consequent pole changing technique
So, power factor is cosine of the angle between the voltage
and current.it will be lag in inductive loads and Lead in
capacitive loads.

Is This Answer Correct ?

1 Yes

0 No

Question { 3758 }

generally the electrical equipments not to be operated at
leading power factor, why?

Answer

Our protection system will be setted at the certain
inductive current lag.If wer operated at leading PF our
relay system will not work perfectly and almost we by
passed the system from protection.

Is This Answer Correct ?

0 Yes

0 No

Question { Inorbit, 4669 }

While DG is on Load? we can start the capacitor bank?

Answer

We can use the capacitor while DG is on load but you should
maintain Lag PF only and any abnormal switching off of the
load means PF will shoot Up and the AVR will fail
immediately.

Is This Answer Correct ?

2 Yes

0 No

Question { 4377 }

in small capacitor we use uf( micro farad) and in big
quantity we used kvar. i want to know that in 1 kva how
much will b uf(micru farad).

Answer

KVAR = square KV * 2PIfc/ 1000

Is This Answer Correct ?

1 Yes

0 No

Question { NTPC, 6561 }

In explosive and polluted environments which type of motors
do we use?

Answer

In FLP areas generally we will use EXd and EXde type motors
and highly polluted areas we will use IP65 standard motors
we will use

Is This Answer Correct ?

3 Yes

1 No

Question { Jindal, 7625 }

The safest value of current which can pass from human body?

Answer

any thing beyon the 10ma is a problem to human body

Is This Answer Correct ?

6 Yes

1 No

Question { TNEB Tamil Nadu Electricity Board, 9335 }

What is the need of black rock stones under the transformer
yard?

Answer

It is used to prevent the growing trees and as well as any
snakes r lizards.

Actually because of this rock stones these animals are
unable to walk on stones.

Thats why we are using 2.5mts to 3 mts of surrounding of
the equipment r substation

Is This Answer Correct ?

2 Yes

4 No

Question { Siemens, 9808 }

what is difference between short ckt and overload?

Answer

Short circuit means the shorting of any two r moro phases.

Over load means any equipment is taking more than the rated
current means we will say it is over loaded.

Is This Answer Correct ?

28 Yes

0 No

Question { IEEE, 15786 }

Whether the LIGHTNING is AC or DC?

Answer

It is a high magnitude pulse in micro seconds discharge.

so many mega volts of supply will discharge in 10 power -6
seconds.

Is This Answer Correct ?

33 Yes

9 No

Question { Equate, 4550 }

What are the causes of motor vibration?

Answer

The possible causes of motor vibration are...

1) Improper fixing of the base boalts
2) Loose tighting of the bearing seating areas
3) Bearing problem
4) loose coupling between load and motor
5) rotor dynamic balancing problem
6) Cooling fan balancing problem
7) Bearing is skidding inside either inner race or outer
race in end cover.

Is This Answer Correct ?

8 Yes

0 No

Question { NTPC, 12022 }

How to know motor winding is star or delta connected without see the information table and winding connection ?

Answer

Star connected or star running designed motor winding
resistance is some what high compared to delta running
designed motor.

Exambple 5.5KW star designed motor is having a resistance
of 1.2 ohams,same in delta is 0.45 ohams like that

Not exactly the same but there is a difference in
resistance.

Is This Answer Correct ?

11 Yes

6 No

Question { Reddy Labs, 26869 }

what is the current rating of 2500 KVA,33KV/433v transformer
on secondary(LT) side

Answer

Primary current=2500000/1.732*33000
=43.74A
Secondary current is =2500000/1.732*433
=3333A

Is This Answer Correct ?

43 Yes

4 No