Question { 21828 }

why do we use magnetic contactors ?circuit breakers are not
enough?

Answer

The main function of magnetic contactor is to prevent
accidently operation after the power failure & again
recovery. Because as soon as power fail the holding coil
release the contacts and it will not operate untill someone
complete the hold circuit again .

Question { 5490 }

How can you test the vector group of power transformers?

Answer

(2u-2n)+(2n-1W) = (1U-1W)
( 2v-1W ) = (2w-1W)
( 1V-2v ) < (1V-2w)
( 1U-2v ) = (1U - 2w)

In DYn11 connect 1U & 2u .and apply the 3 phase 415 v ac to
HV side. Check for the above condition .if its ok then
group is DYN11.

Question { 9289 }

what is diff kav & kvarh?

Answer

THE DIFF. EXIST IN THE VECTORIAL RELATION.

KVA IS THE ACTUAL POWER SUPPLIED AND THE MULTIPLICATION OF
NO. OF PHASE ,PHASE VOLTAGE AND PHASE CURRENT.

WHEREAS THE KVAR IS THE REACTIVE POWER i.e. the
MULTIPLICATION OF ACTUAL POWER TO THE SIN OF ANGLE BETWEEN
VOLTAGE AND CURRENT.

KVA = 3*V*I
KVAR = 3*V*I*sin@

Question { 5273 }

IF 3 NOS.OF HEATERS,3KW CAPACITY EACH,1 NO.HEATER CONNECTED
IN R PHASE,1 IN Y PHASE AND 1 IN B PHASE.THEIR END POINTS
CONNECTED IN STAR.IF I CONNECT THIS STAR POINT TO NEUTRAL
OR DO NOT CONNECT TO NEUTRAL.WHAT WILL THE DIFFERENCE.

Answer

There will be no difference weather I connect the star
point to neutral or Not because ideally at the neutral
point the vector sum of the all three phase current is zero.

I=IR+IB+IY

Question { Honeywell, 3878 }

In 2500kva transformer how much is the withstand High
voltage power frequency between primary to secondary and
earth? And secondary to primary and earth?

Answer

The question is somewhat wrong. Because high voltage power
frequency depends upon the rated voltage not on the
capacity. Like As per IS 2026 for 12 kv level specified
value is 28 kv.So for detail refer to IS 2026 part I

Question { BSNL, 9055 }

1.After synchronising the alternator to the grid,how we r
delivering power to the grid?if the i/p increases does the
speed hence frequency increases?
2.y the power factor is changing when exitation changes?what
is the relation with p.f and flux?

Answer

The real power is goverened by Frequency of prime mover.
The relation between real power and frequency of prime
mover is inversly proportional.So as load increase on the
grid it is shared by the genset and increased load met by
reducing the speed .Then governer throat open more to enter
more steam to maintain the frequency.

The voltage at alternator is controlled by the reactive
power flow. So increase in excitation beyond normal provide
capacitive VARs and decrease in excitation less than normal
provides the Inductive VARs required by the system.
generally the system comprises of Inductive load like
motor,transformer so alternator operate mostly in Under
Excitation state.