Question { 4613 }

1. what is the power factor of a purely inductive load?
2. Which type of transformer connection is preferred in
case of power supply transformers?
3. In power transformer usually which winding is closer to
the core?
4. what will happen to the generated emf if the no. of
poles of an alternator is increased?

Answer

1. In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit. The power factor can get values in the range from 0 to 1. When all the power is reactive power with no real power (usually inductive load) - the power factor is 0.
2. Star Delta Connection is used in both transmission and distribution networks transformers.
3. LV winding is placed near to the core to reduce winding insulation requirements.
4. If no. Of poles increased in an alternator it's EMF generation increases, speed decreases.

Question { JK Cement, 4013 }

why there is a temperature rise at different metal to metal
contact at cable lug??suppose cable is aluminium and the lug
is copper,than why the temperature rise takes place??

Answer

Galvanic corrosion-

As moisture can easily enter to the termination point, aluminium conductor, as a reactive metal in
galvanic series, is susceptible to corrosion when aluminium comes into contact with a more stable
material, such as the relatively noble metal copper. Galvanic corrosion is a typical issue for

30
aluminium conductor when dissimilar metals are in contact with each other in the presence of
electrolyte. Such deterioration of aluminium can not only accelerate the depletion of conductor, but
also affect the contact behaviour and promote the contact failure.
 The use of dissimilar metals at the termination point is also not good at coping with thermal shock
or high temperature condition which generates incompatible thermal expansions of conductor and
terminal, since aluminium has a relatively high coefficient of expansion than common terminal
metals such as copper metal. Incompatible mechanical/thermal behaviours between terminals and
aluminium conductors deteriorate the connection and eventually cause a failure since dynamic
loads or vibration can be existing over a long time period

Question { 3678 }

Is there any decided or standard frequency to check the
meager testing of transformer, DG set, LT Panel in the
industries in a year.
I means how many times we should be check the same.

Answer

Yes as per NBC It's in summer and twice in a year.

Question { MAHINDRA, 5516 }

we have taken HV test of 2.5 k and Megger test on 11 kv HT
cable.here it shows no fault but when we are charging the
same cable with 11 kv supply why the VCB 11 kv gets tripped
can anybody define the possible cause of fault ?

Answer

Check out the control circuit. And every possible relevant thing in panel cable breaker.... Thoroughly.

Question { 4279 }

What is an Isolation Transformer? Where it will use?

Answer

Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass. Transformers that have a ratio of 1 to 1 between the primary and secondary windings are often used to protect secondary circuits and individuals from electrical shocks between energized conductors and earth ground. Suitably designed isolation transformers block interference caused by ground loops.

Question { 9890 }

If the load is 10Kw and power factor has to improve from 0.5
to 0.8,how can we select the capacitor rating
accordingly.Illustrate by showing formula?

Answer

Kvar = KW * [ tan (cos^-1* Current PF) - tan (cos^-1* Required PF)]

Question { 3669 }

in transformer input is d.c signal which type of emf is induced

Answer

Due to stationary flux produced in core No emf induced when applied a DC supply sugnal.

Question { EFS, 3749 }

what are the safety in transformer,diesel generator,ups, chiller plant,h.t. , L.t. , sewage treatment plant(stp),water treatment plant(wtp), fire.
plz ans. me in detail if you know any of the mention above.
i urgently need

Answer

Safety Of Transformer
1. Mog
2. Surge Arrester
3. Explosion Vent /spr
4. Breather

Question { EFS, 3749 }

what are the safety in transformer,diesel generator,ups, chiller plant,h.t. , L.t. , sewage treatment plant(stp),water treatment plant(wtp), fire.
plz ans. me in detail if you know any of the mention above.
i urgently need

Answer

Transformer Safety
Mog, Explosion Vent, Surge Arrester, Bucchholz Relay, Breather

Question { 7055 }

Why we not use the impedance relay in protection of short transmission line ?

Answer

1. During Short circuit current Step and Touch potential increases.Step potential : It is the potential developed between the two feet on the ground of a man or animal when short circuit occurs. This results in flow of current in the body leads to electrical shockTouch potential: It is the potential that is developed between the ground and the body of the equipment when a person touches the body during fault condition. When operating personnel touch an electrical equipment during short circuit condition, fault current flows through the human body. This is defined as touch potential.Gravel/Stones in the substation is provided to reduce the step potential and touch potential when operators work on switch yard.2. Power Transformers installed in the substations will have oil as cooling and insulating medium. Oil leakage takes place during operation or when changing the oil in the transformer. This oil spillage which can catch fire is dangerous to the switchyard operation. So Stones/Gravel is provided to protect from fire when oil spillage takes place.3. Stones eliminate the growth of small weeds and plants inside the switch yard4. Improves yard working condition5. Provides protection from wild life and snakes.

Question { 3609 }

why tungsten is used in incandescent lamp instead of nichrome though nichrome has high melting point

Answer

Pure tungsten has some amazing properties including the highest melting point (3695 K), lowest vapor pressure, and greatest tensile strength out of all the metals.Because of these properties it is the most commonly used material for light bulb filaments. It can reach high temperature before melting and therefore emit a brighter light than nichrome can. Nichrome on the other hand, although it has a pretty high melting point (1673 K) it is not as high as tungsten, but it is often used for heating devices, such as a blowdryer. An electric current can heat the filament around 2000-3300 K, which stays below the melting point of tungsten but above that of nichrome.

Question { 1577 }

my project load is 6 MW so please suggest the size of cable.. ALU 3c 400 aqmm cable can be sufficient.. is there any thumb rule for calculation of cable size please help me

Answer

Yes there's a thumb rule for cable size calculation for 1sqmm Aluminum cable can carry 2Amps and 1 Sqmm Copper wire can carry 4 Amps.

However the cable size increase above 100 Sqmm this thumb rule can not be considered. You need to refer the cable size calculation chart.

Question { 1744 }

why power factor is lagging and leading(i.e -1to +1)

Answer

Practically Power factor will be always lagging as there will be presence of more Inductive load.

Pf can be leading if load is capacitive in nature.

Question { 2162 }

Why do you use an isolator on both sides of the circuit breaker? ( shortly answer)

Answer

Isolators are provided with interlocks on the both sides of CB in outdoor panel, reason is to identify either circuit is open or closed. Accordingly worker can take decision either to work on system or not. if CB open then isolators are also open.