Interview Questions Answers in Electrical Engineering

Question { 3214 }

reason behind why the reverse power relay of incoming
machine was not acted even when no load was shifted on
incoming machine after synchronising the machines ?

Answer

The reverse power relay operates when power is flowing into
the machine only.
If the machine is not taking any load it is not in a
reverse power situation. Therefore the relay will not
operate.

Is This Answer Correct ?

0 Yes

0 No

Question { L&T, 4968 }

what is the function of short circuit maintenance ct and
droop ct.

Answer

These are both used on a DG.

The short circuit maintenance CT is used to keep the output
voltage and current high enough to clear a fault by
tripping an overcurrent device (circuit breaker).

The droop CT is used to cause the voltage to decrease as
the kvar load rises. This is used to help DG's share the
kvar load.

Is This Answer Correct ?

2 Yes

1 No

Question { 2490 }

WHY NOT CONSIDER PROTONS IN ELECTRICITY LIKE ELECTRIC CURRENT?

Answer

This is because the protons stay in the atoms where they
are and do not move from one to the next atom like
electrons.

There is a flow of electrons along a conductor but not a
flow of protons.

Is This Answer Correct ?

1 Yes

0 No

Question { 6918 }

How to improve insulation resistance of Motor

Answer

If due to age or exposure to moisture have caused the
insulation to be lower than normal it can be fixed in most
cases.

The process is to use a solvent which does not harm the
varnish to wash the windings of all dirt. Next bake the
windings in an oven to dry out any moisture. Next
revarnicsh the windings.

Is This Answer Correct ?

5 Yes

0 No

Question { TLC, 5949 }

we use 3 phase supply for induction motor why not use two
phase supply .

Answer

A 3 phase supply is the lowest number of phases where a
rotating field is produced. No extra starting controls are
needed (e.g. capacitor start or start windings)
Two phase systems have been used and can produce a rotating
field but it has to use a setup with separate neutrals for
each of the 2 phase. It therefore requires 4 wires to each
motor instead of 3 and it is also not as efficient.
Three phases also has the advantage of the currents in the
nuetral not being the sum of the 3 phase currents. In that
way only a single neutral can be used as the return for any
single phase load on all the 3 phases.

Is This Answer Correct ?

1 Yes

0 No

Question { Cummins, 7102 }

why DG sets power factor is 0.8 lagging.

Answer

Most installations are have a PF between 0.8 and Unity
lagging. By rating the DG to 0.8 PF it can operate in most
situations without problems.

Is This Answer Correct ?

0 Yes

3 No

Question { 4120 }

Transformers with dc supply

Answer

Induction requires a conductor to move through a magnetic
field.

In a transformer the conductor is stationary and the
magnetic field is moved by the rise and fall of voltage
from the sine wave. This requires a change in voltage. With
DC the only change would be when it is turned on and turned
off. So pure DC will not work with a transformer.

The second thing is that with AC there will be a winding
resistance and an impedence as well to limit the current
flow. With DC there will only be winding resistance. This
could cause the windings to over heat if the same voltage
is applied.

Is This Answer Correct ?

0 Yes

0 No

Question { 3750 }

define one ampere?

Answer

1 ampere is the current that will flow a 1 ohm resistance
if 1 volt is applied across it.

Is This Answer Correct ?

0 Yes

1 No

Question { 3563 }

Why mostly conductor made by 7 nos wire?

Answer

A centre strand and 6 strands fit neatly around this
strand. Next is 19 strands where a layer of 12 starnds on
top again. And so on as the cable gets thicker.
The cable is then flexible.

Is This Answer Correct ?

1 Yes

0 No

Question { Kirloskar, 6386 }

what is the main parts of DG? how it is working?

Answer

Also the engine and governor

Is This Answer Correct ?

2 Yes

0 No

Question { 4596 }

725KW Three DG set Synchronised but DG no-1 power showing
750KW DG n0-2 power showing 500KW DG no-03 showing 500KW.
DG is Shynchronised but why this different power showing?

Answer

If the DG's are the same rating then the load sharing is
out of adjustment.

Load sharing can be by speed droop (see answer above) or by
load sharing controller.

Adjustment of either of these can rectify problem.

Is This Answer Correct ?

0 Yes

0 No

Question { 8803 }

What is the use of Exciter in generator? & what will be the
current of Exciter in 500 KVA DG Set?

Answer

The above answer is true for DG set also. But sometimes on
small DG the output voltage is used instead of the Pilot
Exciter.

the current will vary with load and also the manufacturer
of the DG.

Often now 1-4 amps but some are 20 to 30 amps.

Is This Answer Correct ?

0 Yes

0 No

Question { 5206 }

In the UVR(Under Voltage Relay) there is a knob for the
hysteresis setting. what is this setting? its threshold is
given from 5% to 50%. please help me....

Answer

The hystersis is the difference between the voltage at
which the relay operates at and resets at.

For example the UVR is set to open the contacts at 200
volts and it can be set to close at 211.5 volts (for 230
volt sensing). This would be 5% hystersis setting.

This is to stop the relay switching on and off continuously
when the voltage is around 200 volts.

Is This Answer Correct ?

0 Yes

0 No

Question { 10510 }

If the neutral wire go pass through the core balance current
transformer? If yes then why? and no then why?

Answer

The neutral is requiered to pass through the core balance
CT together with the phase for the same circuit. It
operates on the current going through the pahse being
balanced with the current returning through the neutral. If
these are balanced the protection will not operate. If a
fault occurs (a short circuit to earth in either the phase
or neutral)some current will flow through the earth to the
neutral and not go through the CBCT. This will then operate
the protection.

Is This Answer Correct ?

6 Yes

1 No

Question { Ecil, 11646 }

Sparking occur when a load is switched off because the
circuit has high –

a.) Inductance

b.) Capacitance

c.) Resistance

d.) None

Answer

d.) None

Sparking occurs because a flow of current is broken. When
the 2 contacts are very close the air is ionized and will
conduct. As the contacts open further the arc cools and the
air loses the ability to conduct.

With AC it may be seen that sometimes a small switch sparks
and sometimes it does not. It will depend on the voltage at
the time of opening. At the peak of the sine wave it will
be a big spark at the zero line no spark.

Is This Answer Correct ?

3 Yes

3 No

kurt jorgensen