Question { 5107 }

can you check the current and voltage of a 12 V battery by
a digital multimeter in this fashion that apply the digital
multimeter across the battery terminals directly ? say yes
or no ...
i sais yes ...........
is this true or false ?
plzz tell me and also explain ........ plzz thanks

Answer

false
true
you can measure the voltage by connecting the voltmeter
across the battery(PARALLEL)
you can measure current by connecting the multimeter in series to the circuit you want to measure.

Question { 6018 }

i heard that power factor(PF) is the main concern for
industries, if they are maintaining PF around 0.6 they will
be fined for that. now the question is, how can they improve
the PF ranging between 0.9 to 0.95 which is safe for them
i,e escaping from the fine?

Answer

The PF is low because of inductive load. You can improve IT by adding capacitive load. Luckily cap load is loss free.That means you dont pay kwh for it but you must buy and maintain the capacitor and on and off it as necessary at the right amount.
One way to know how much you need, is you must know the load in KW and the pf.
assuming the load is 500 kw and the PF is 0.6 and you want to improve it to 0.95. you can find out the kva demands at this two pf levels
500/0.6=833kva . 500/0.95=526 kva, 833 kva -526kva =307kva

If you add 307kvar into the system in paralel you would improve the pf to 0.95 you can divide this into 8 steps of 40 KVAR each connected accros the 3 phases through an auto PF regulator.Your cap on each step will be rated as 40 kvar at 415 volts (rated voltage) 3 PHASE.

I welcome any improvements or comment about this.

Question { 8982 }

in gen traf what is ONAN , ONAF , OFAF and DYN

Answer

Answers #1 & #2 is correct but i can add something for the the benefit of some.
In ONAN the oil in the TX former circulates on its own due to hot oil goes up & cooler oil goes down.The TX former is cooled by just the air around it.
In ONAF the oil circulates as above but a fan is used to cool it.
In OFAF the oil is pumped with a pump for circulation and the air is cooled by a fan.
In DYN the primary winding of the tx former is wound in delta and the secondary is wound in star with tapping for neutral connection.

Question { 8122 }

How do you protect your appliances from harm in case of
neutral failure in a three phase power supply set up.

Answer

Yes if neutral fails in a three phase power supply system
than the single phase voltage may increase. You can fix voltage sensitive suppressors across the supply line in your applances which will short circuit the excess voltage to earth and cause the RCCB to trip and save your appliances from further damage.

Question { Siemens, 144666 }

how we calculate the 30 KW 3phase electric motor current on
noload

Answer

I agree with suthan prabu. there is no formula to calculate.
depends on the manufacturers design, but full load curret can be calculated.

Question { 11129 }

why generator and t/f ratings given in kva & motor rating
is in KW?

Answer

We buy/fix a motor to do a specific job.Job done is measured in watts.So a machine say a pump will use up to 3 KW to run as designed than we fix a motor of 3KW or slightly bigger to run it.Now the motor is 3KW or bigger say 30 KW and the voltage is AC or DC operated is up to you.

generators are rated in KVA because we dont know at what pf will it be operated. Say you have fitted a pf correction board to maintain at 0.95pf, you dont have a guarantee it wont breakdown or take some time to adjust. What is important the generator voltage is constant and the generator dont trip or burn.So if the pf can change anytime the current also can change anytime this may make your generator to trip on overcurrent,overvoltage(leading pf)or even destroy your genset.By rating your generator by KVA with a constant designed voltage you can know the maximum current the genset can give irrespective of pf.It is a safer and sure way of protecting your generator.you can even load yor genset at 0.2 pf so long you dont exceed the rated amps or KVA.

Question { 23518 }

WHY TRANSFORMER IS MEASURED IN KVA?

Answer

Any AC electrical equipment that has conductors have to be rated in voltage and amps, so it is KVA if more than thousand. WHY.
SIMPLE . If more than the designed amps flows through the equipment it will over heat and burn.(destroyed)
IF a 500 kw transformer is loaded at 415 v at unity pf to the max it will carry 500000/415x1.732 = 695 amps
if the same transformer is loaded at 500kw at 415 v but at 0.5 pf it will carry 500000/415x1.732x0.5 = 1391 amps
can the transformer winding take this amount of current.You see on both occasions the load was 500 kw but the pf was different and the amps was double.What will happen if the transformer is loaded at a further lower pf. should the transformer be still rated as 500kw

Question { TNEB Tamil Nadu Electricity Board, 54459 }

what is the use of megger? and types

Answer

First meggar is actually a brand name of electrical instruments made in England.
One of their famous instrument is the insulation/leakage tester as stated above by others.

Question { 4494 }

after parallaing two generators current is shooting up in
both the generators.for ex: load is 400 kw..after paralling
and load sharing current in running gen is approx 900 amps
and incoming is 750 amps which is too high when trying
toshift the load current in incoming gen is coming more
than 1300 amps and both getting tripped and making
blackout..

Answer

it looks like one of the genset is having a big difference in voltage or
the droop kit if fitted something wrong with it or
check after paralling do you have balanced three phase voltage and current or one of the phase is open circuit or
the gen is running the other as motor
if you know the actual reasons later please let us know also

Question { 14364 }

what is the potential difference between line and earth?

Answer

i am of the opinion MR Sree & MR Prashant is right.

Pot Def = line voltage - earth voltage
earth voltage is 0 volts.
line voltage depends on what you are measuring
so line voltage minus zero is = line voltage.

Question { 4811 }

what is the formular for the calculation of CT'S when
installed as part of a electrical installation so the
correct meter is used

Answer

There is no formula for this you just choose the correct size from what is available.
usually size the ammeter the size of the circuit.example the circuit breaker is 200 amps than the ammeter would be 200amps.The CT would be 200:5 or 200:1 whatever is available so long the ammeter is also of the same rating.

Question { 6750 }

do we people experience any shock if we catch any one of
the three phases (R Y B)in air with out touching ground.

Answer

NO.
To experience any shock current must flow in and out.

Question { Anna University, 86603 }

Why CT secondary should not be kept open?

Answer

In the physical construction of CTs you can see the secondary
is made up of a winding but the secondary is just one cable or
busbar.So it acts as a step up transtrofmer when the secondary
is left open.It is real I have seen two people who have had their hands burnt due to careless handling of the CTs.

Question { TCE, 17827 }

What types of testing in 11Kv Ht switchgeat during testing?

Answer

I belive you mean commisioning tests.
some of them are,
A)INSULATION TEST
b)contact resistance test
c)HV holding test
d)primary and sec injection test on the protection cct
e)transformers insulation, ratio test,vector group, insulating
oils dielectric tests.
f)cables insulation, phasing test

Question { GMR, 3696 }

What will happen if i connect two 3-phase tranformers
secondary supply on one bus bar that is R-Y-B-N?

Answer

transformers can be paralleled.If the spec of both the
transformers are identical than there is no problem.