Interview Questions Answers in Electrical Engineering

Befor synchronissation of Generator what test to be carried
out.pl.specify with reason.

Answer

Answer one is mostly correct. Two other factors are important. 1) the winding type (e.g. 2/3 or 5/8) as this can affect circulating neutral current. 2) phase angle at time of synchronising must be within limits to prevent high currents to pull DGs into synch.
For answer two - load sharing equipment can allow DGs of different ratings to be used in parallel. Frequency is important not RPM. The number of poles in an alternator can vary requiring a different RPM for the same frequency.
Another important factor iis governor response to load changes

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Question { 5543 }

Generally in d.c. circuits the direction of current is from
possitive to negative.In a.c.circuits what is the direction
fo current flow

Answer

Generally it is conventionally seen as flowing from the
phase to neutral.
In reality the electrons (or current) flows in one
direction for half a cycle and then in the opposite
direction for the other half cycle.

Is This Answer Correct ?

2 Yes

0 No

Question { Dalkia, 26685 }

whu does the power factor of diesel generator always be 0.8.
please explain

Answer

Most DG's are rated at 0.8 PF. They do not have to operate
at this PF but certain considerations must be taken into
account when the PF is changed.

1 - The kw available is only 80% of the kva rating. This is
limited by the output of the engine. If the load is at
unity PF the output is only 80% of the kva rating.
Otherwise the engine is overloaded.

2 - The maximum current the alternator windings are
designed to carry is the current at the kva rating. If the
PF is below 0.8 the kw load must be reduced to make up for
the extra kvar load so the kva is not above the rated
maximum.

Is This Answer Correct ?

1 Yes

0 No

Question { 3507 }

Whether capacitor loads can be connected for testing a
diesel engine at its full load capacity? Is there any
limitation of capacotir load? Which is the best load for
testing DG sets?

Answer

Answer 1 is correct.

Normally a resistive load is used for testing a DG. As most
DG's are rated at 0.8 lagging power factor the resistive
load is calculated at unity power factor load on the DG.
This is usually 80% of the kva rating. This tests the
engine to full load.

Where it is required to test the generator at full load
also an induction load is used together with the resistive
load to give the correct power factor.

Capacitor loads are not normally used as DG's do not
operate safely on leading power factors.

Is This Answer Correct ?

1 Yes

0 No

Question { Bhel, 23267 }

how to synchronize 4 nos. of diesel generator sets of
different capacity (kva)?

Answer

First of all it is possible to have different size of
generators synchronized. If the condutions of voltage
frequency and phase angle is correct. The load sharing must
be set for a percentage load. This means all set will have
the same load (example 60% of max load on all sets).
Issues can be where the load sharing is not set correctly
or where the governor response is too different. This can
cause hunting of the load from one set to the others.
Another inportant consideration is the wave form. If it is
different high current can flow through the neutral. If the
alternators are made by the same company this is not an
issue.

Is This Answer Correct ?

6 Yes

0 No

Question { Cushman Wakefield, 14146 }

what is effect of reverse c.t connection on metering.

Answer

It only has an effect where the direction of current flow
is important. For example monitoring import and export of
power.

Is This Answer Correct ?

0 Yes

1 No

Question { 7389 }

why motor burns ?

Answer

A motor will burn out because of the failure of the
insulation.
This can be because of heat from overload, high or low
voltage or not enough cooling. The insulation can also fail
because of moisture, age, dirt or salty environment.

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Question { 4074 }

Why don't we use ground free electricity to avoid electric
shock.As birds are do not get shock.

Answer

If you mean that we do not earth the neutral?

In a perfect situation this would be good and reduce the
risk of electric shock.

Unfortunately in real life it can only work in small
situations - using an isolation transformer for a handheld
light for example.

In large installations there is always a risk of failures
of insulation or machines. This would then cause a
connection of the neutral or phase to earth and therefore
it would no longer be ground free. The other thing is that
no insulation is perfect and there will always be leakage
with the same result.

Is This Answer Correct ?

0 Yes

1 No

Question { 4853 }

How would you supply a single house hold load from a 3-phase
supply coming out of a sub-station?

Answer

A neutral will be required.

A single phase and neutral can be used leaving the other 2
phases unused.Or the single phase loads be connected
between each of the separate phases and the common neutral.
The single phase loads can be balanced across the 3 phases
in this way.

Is This Answer Correct ?

1 Yes

0 No

Question { IBM, 10121 }

A 4-Pole D.C. Generator has a speed of 1500r.p.m. Then what
will be the frequency of D.C. Generator ?

Answer

The generator will put out a DC voltage with a slight
ripple. The ripple frequency will depend on the number of
coils on the armature and the number of segments on the
commutator.
It will not have a frequency similar to AC.

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Question { 6234 }

Why Cables are always circular and not square or rectangular?

Answer

There are a number of reasons but I think the main 2 are
easier to make and more flexible.

I have seen multi core aluminium cables where the
conductors were formed into segments of a circle to fit
together into a smaller area.

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0 Yes

2 No

Question { 4909 }

what will happen if neutal in AC ckt will get open

Answer

Both answers above are true but do not explain why.
With a 3 phase load an artificial star point is created to
replace the neutral. In a perfectly balanced load this will
have no effect.
If the load per phase is unbalanced the where the star
point is created will depend on the load unbalance. the
voltage phase to neutral may increase in one phase and
decrease in the others.
In one instance I have seen this where the voltage in one
phase increased to 300 in a 416/240 volt system.

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Question { HCL, 11691 }

In a 3 pin plug holder, what is the purpose of diameter of
ground pin is larger than phase and nutral pin

Answer

I think the simple answer is found if you look at the
ground pin on plugs from most other parts of the world.
Generally the positioning, size or shape of the ground pin
is different to the phase and neutral. This stops the earth
pin from being accidently put into the hole for one of the
other pins.

Is This Answer Correct ?

2 Yes

1 No

Question { TCS, 44640 }

why we use RYB phase sequence? and somewhere we use UVW ?
why?

Answer

RYB has been used because this is the normal colours of the
3 phases (Red Yellow Blue). Now the international standard
is RWB (Red White Blue) because the earth is Green with an
Yellow stripe. This change is because some colour blind
people will see red as green and green as red when the
colours are together.
UVW is the terminal naming standard.

Is This Answer Correct ?

9 Yes

11 No

Question { 3859 }

If genrator not build up voltage , How u can Manually
Excite ?

Answer

This is usually done by flashing the alternator. Turn of
load first.
Disconnect the AVR and connect the battery (9-12 volts)
across the field. Must be same polarity as the AVR. Run
generator and check output voltage. Voltage will usually be
higher then normal. if no voltage or unbalanced voltage the
windings or rotating diodes are faulty.
Stop generator and disconnect battery and reconnect AVR.
Try starting generator set again and check if voltage
present. If still not working replace AVR and try again.

Is This Answer Correct ?

1 Yes

0 No

kurt jorgensen