Question { 10571 }

what is the disadvantage if earth is connected to neutral

Answer

The earth connection to neutral (or neutral grounding) has no disadvantage. In fact, it provides safety to the electric equipment. If neutral is solidly grounded then fault current for SLG fault will be greater than that of 3phase fault. So the protective devices should be of higher value. That could be the only disadvantage.

Question { 5066 }

what is the working principle of UPS?

Answer

UPS mainly consists of 3 things, a battery, a charge controller and a DC to AC converter (inverter). The battery gets charged by utility. When utility is out, it supplies the load with the energy stored in the battery. Battery stores DC only so it has to be converted to AC before connecting to load which is done by inverter. Since the inverter works on electronics and doesn't provide pure sinusoidal waveform, UPS output has a lot of harmonics.

Question { ABB, 5145 }

Difference between earthing & grounding.?

Answer

Grounding is used for referencing the voltage. A common point is taken.
Earthing is used for safety purpose, a conductor is connected to ground or any other safe medium, giving a low resistance path for current to pass as to avoid any effect of surges.

Question { 2693 }

why core is useful for the transformer and in its operation

Answer

Transformer works on electromagnetism that is mutual inductance. If the voltage of primary is varied, the effect is felt by secondary but this 'effect' (flux) has to be transferred to the secondary first. It means the core is responsible for the connection of flux from primary to secondary hence it is made of conductor sheets.

Question { Bhel, 2565 }

Why we use modulus with voltage in stability equation that
is P=[E][V]SIN@/X WHY VOLTAGE IS IN MODE VALUE WHAT IS THE
REASON?

Answer

Because the sign is taken care by the angle and we only need the magnitude so mod is taken.

Question { 13001 }

two bulbs of 100w and 40w respectively connected in paralel
across a 230v supply.which bulb will glow bright and why?

Answer

Bulbs have filament and its light depends on how hot the filament is. The greater the current, the hotter will be the filament, and the brighter will the bulb lit.
Since both the bulbs are connected in parallel, the voltage across them will be equal, 100 watt bulb will have (100/230)= 0.435 amp of current while 40 watt bulb will have (40/230) = 0.174 amp of current passing through it. Hence 100 watt bulb will be brighter.

Question { Prasad Group, 3201 }

Why should we use jelly filled armoured cables in underground
Communication system? What is the use of the Jelly? If we go
for unarmoured, then what happens?

Answer

I'm not sure but jelly is used to absorb moisture which means cable's insulation will have longer life.

Question { 3448 }

IN A SMALL D.C GENERATOR, IF WE REPLACE THE LAMINATED
CORE ARMATURE WITH A WOODEN BLOCK OF SAME SIZE & SHAPE
AND WIND WIRES WOUND ON THE SLOTS OF WOODEN ARMATURE AND
CONNECTED TO THE COMMUTATOR.COMMUTATOR IS FITTED WITH BRUSH
TO RECEIVE INDUCED E.M.F.WHETHER E.M.F GENERATED OR NOT?

Answer

It won't work because the motors/generators work on electromagnetism and a wooden block won't be containing any flux.

Question { 3702 }

show that power dissipated by a pure inductor exited by a
sinusoidal voltage source v = vm sin wt is zero

Answer

Let Vmcos(wt+X) be the voltage and Imcos(wt+Y) be the current through the inductor.
Now the power is given by
P=V*I=VmImcos(wt+X)cos(wt+Y)
By using a trigonometric identity:
P= 0.5 * VmIm * cos(X-Y) + 0.5 * VmIm * cos(2wt+X+Y)
This expression has 2 terms which both must be zero.
The first term is a constant (independent of time) but notice that in an inductor Voltage leads Current by 90 degrees so X-Y = 90 and cos(90) = 0 so first term is zero.
Now for average power integrate the above expression for a cycle and divide it by time period, since integral of every sinusoid for a complete cycle is zero the next term will essentially be zero too.

So P = 0 for an inductor.

Email me if you still have any doubt or query.
sarmadahmed@live.com

Question { NTPC, 9923 }

A ceiling fan moves Counter clockwise & a pedestal fan
clockwise.....y the difference????

Answer

I hope you know about the right hand rule, for example if you rotate a screw anticlockwise it will loosen and when clockwise it tightens. Let the movement be the output whose direction can be determined by the right hand rule (curl=movement, face of thumb=direction of output).
By keeping this in mind, let the air (by the fan) be the output, ceiling fan is required to throw the air downward so these're made to rotate clockwise and exhaust fan is required to throw the air in the other direction, these're made to rotate counter clockwise.

Question { NTPC, 9923 }

A ceiling fan moves Counter clockwise & a pedestal fan
clockwise.....y the difference????

Answer

That's a universal rule relating cause and effect, I screwed
this question from my daily observations, Fleming's rule is
the form of this universal rule for magnetic and electric
fields. But this law works everywhere, like screw, rolling belt, well bucket or alot of things.

Question { 4581 }

What are the main problems in distribution? I have to do an assignment so which problem should I choose? Like power factor problem etc.

Answer

Power Factor, I can understand but the other problems like losses and lightning is related to transmission right? Could you please tell me some problems like overloading, sparking, grounding etc.. I would really appreciate your help.

Question { 5064 }

why electron flow is opposite to current flow?

Answer

The direction of conventional current is always opposite to the direction of the electronic current. In history, it was assumed that the current is due to the flow of positive charges. When it was discovered that the current is actually the flow of electrons, already much work has been done by taking the opposite direction of current so scientists decided to go with the conventional current in future too.

Question { 3440 }

why the power factor is low at no load?

Answer

Because at no load, the only significant current component is Excitation current which is 90 degree out of phase with the supply voltage. Since pf = cos(@) and @ is around 90 here, power factor is very low.

Question { 5191 }

Dear All,
Can anybody please tell me which value(saturated or unsaturated) of sub-transient reactance(Xd") we take for the calculation of fault current of the system?

Answer

Fault current calculations are usually done to find the rating of protection equipment. For this we need the maximum fault current. Since unsaturated machines has lower value of sub-transient reactance, we should use unsaturated sub transient reactance for the calculations.