Interview Questions Answers in Electrical Engineering

how to calculate the size of cable for 100 kva load,?can
somebody tell me with a formula

Answer

Considering Powe factor 0.8
Kw = KVA*.08
KW = 100*0.8
= 80 KW
w = 1.73 * 415 * Il * 0.8
80000 = 1.73 * 415 * I1 * 0.8
I = 80000/ 1.73*415*0.8
= 139 Amp

You can choose the cable which current carring capacity per
phase is on or above 140 Amp

By considering the future expansion of load the 180 Sq.mm
AYFY cable is ideal.

Is This Answer Correct ?

18 Yes

2 No

Question { 8477 }

Form fator is the ratio of?

Answer

Form Factor = RMS Value / Average Value

Is This Answer Correct ?

3 Yes

0 No

Question { Voltas, 139099 }

how do you calculate IKW(consumption per TR) in HVAC?

Answer

In general Practice you can consider 1 Tr = 1200 Watt = 1.2
kw

1 TR = 3.5 KW is next to impossible.

Is This Answer Correct ?

35 Yes

26 No

Question { Pfizer, 16896 }

11KW Delta motor is taking about 44Amps in star and goes
upto 70Amps in delta and trips OFF. The rated current is
26Amps. Application is for the blower. The motor takes ( no
load without coupling impller ) only 2.5A and 5Amps in star
& delta starter test.Need to know why its taking high
current.

Answer

There could be two problems

1. the improper connection of the motor to starter mean the
phase sequence.

2. may be possible the one phase is missing & motor running
on 2 phasis means the single phasing phenomina

this is the only reason..........

Is This Answer Correct ?

1 Yes

3 No

Question { 8213 }

if we convert ampers into kwh or mwh ,which one formula use
by us,pls tell me?

Answer

KWH = watt x no of hour /1000

W = v x I for single phase
w = 1.73 x VL x IL x Cos@

Is This Answer Correct ?

6 Yes

0 No

Question { 4979 }

Need help. 3 phase system. Neutral line arced and melted
upon connection of connector with 3 phase power. Why would
that happen?

Answer

Pinky,,

be clear what kind of equipment , what is a supply voltage,
the singnal voltage etc.

so that the ans would be givan

Is This Answer Correct ?

0 Yes

0 No

Question { 9404 }

suppose we using 3 phase motor of 440 v means what range
of C.T. is require for measuring current ?

Answer

Rating of CT means maximum carrtent to be measure for that
voltage , powerfactor & Wattage capacity is required.
depending upon that you can calculate the total current to
be measured & also the rating of the current transformer

Is This Answer Correct ?

0 Yes

0 No

Question { 8261 }

suppose we using 3 phase motor of 440 v means what range of
C.T. is require for measuring current , the rating of motor
is 1HP ?

Answer

Navnit is Absulitly right for load of 1 HP the CT operated
meter is not required at all , direct loading is goodone

Is This Answer Correct ?

4 Yes

2 No

Question { 5235 }

how to calculate of which rating transformer should be
selected? take any example

Answer

usually transformer rating is based on power to be drawn by
the transformer. however few factor is also play a part
like LF, DF...

For Ex say total Connected load is 820 KW
Maximum Demand would be MD= Total connected Load x L.F (0.8)
xD.F (0.7) = 820 x 0.8 x 0.7 = 459 KW

now considering the load at 0.8 leg PF the kva rating of
the transformer is

KVA required = 459 / 0.8 = 574 Kva say .......600 KVA

Is This Answer Correct ?

2 Yes

0 No

Question { Suzlon, 9921 }

parallel operation for tansformer which condition fill fil
for that

Answer

1.polarity must be same ( Must be full fill )
2.phase sequence must be same (Must be full fill )
3.turns transformation ratio must be sane ( Desired )
4.frequency must be same ( Must be full Fill )
5.% Impedence must be same ( Desired as to ensure equall
load sharing of both the transformer
6.Vector group should be identical ( Desired )

Is This Answer Correct ?

6 Yes

1 No

Question { 8285 }

what is the equation of converting KW into HP ?

Answer

KW = HP * 0.746
HP = KW/0.746

Is This Answer Correct ?

7 Yes

0 No

Question { 9544 }

which transmission is cheaper ac or dc?

Answer

Both transmission have there own advantages &
disadvantages..

However for Lond distance say 1000 Km distribution the DC
transmission is cheaper because of Line losses but in short
distance AC is cheaper because the Cost of AC DC coverter
Substation is very high.

Is This Answer Correct ?

7 Yes

0 No

Question { 3922 }

plz tell me about detailof induction motor Slip

Answer

Above ans is correct but for better understanding s = Ns-
N where is a (synchronous) speed of magnatic flux & n is
the actual spee of the motor .

also %s = Ns - n / NS

Is This Answer Correct ?

2 Yes

0 No

Question { 4408 }

why are the transmission lines operated at 220 kv nly... nt
les dan dat...

Answer

High the voltage , current is less hence the area of the
conductor is less so also the losses are less.

Is This Answer Correct ?

2 Yes

0 No

Question { Jindal, 18925 }

why our indian system choose the 230 volt for single
phase&415 for 3-phase voltage,in forign side 110 volt is
used what is the difference

Answer

Basic rule of electricity is W= V*I now wattage would be
the rating of the quipment which remain constant means the
voltage is decrease then current will increase but if
frequency is increase current will decrease now as per this
in foreign the frequency is increase & voltage is decrease
so the required insulation level is decrease. in india the
voltage level is 230 & frequency 50.

Is This Answer Correct ?

7 Yes

2 No

virendra