Question { L&T, 14630 }

WHY EARTHING NOT NEEDED IN BATTERY IN SUBSTATION?

Answer

Basically a device is earthed to avoid any stray
circulating current that appear on the body of the device
operating with an AC supply.

since AC creates an alternating flux. this changes in flux
induces stray currents in the metallic body of the devices.

When coming to DC, since it is a constant voltage it does
not produce flux change to induce any currents so earthing
is not required for a DC battery.

Question { JSW, 8972 }

as we know that N=120f/P
speed of an im can be controled only through changing no if
poles, can we control by changing the frequncy????

Answer

For the speed control of an induction motor if you consider
to change the number of poles, every time you want to
change a speed you cant remove or add the poles to a motor
coz this is done while manufacturing the machine.

So for the speed control of an induction motor we use the
Variable-Frequency Drive (VFD) which takes in the standard
voltage at 50Hz frequency and gives the output at any
frequency required. the frequency can vary from 0 Hz to
50Hz and we can also increase the frequency more than 50Hz
to make the motor run with speeds higher than the rated
speed of the motor.

Question { Ordnance Factories Board, 11276 }

what the different types of cables used for 11kv and 33kv
over head work and what is the size of cable in sqmm.

Answer

The Area of cross-section of a current carrying conductor,
depends upon the amount of current it has to carry.

1 sqmm of aluminium conductor has a capacity of carrying 1
Ampere of current and can be loaded upto 1.5A

1sqmm of Copper conductor has a capacity of carrying 1.2
Ampere of current and can be loaded upto 2.5A

So when Heavy loads are to be transmitted at long distances
we use high voltage so that the current becomes less, there
by decreasing the conductor size, losses etc.

Question { Welspun, 41951 }

How can we calculate ngr resistance for a generator and
Transformer ?
What type of transformer requires ngr ?

Answer

The value of the NGR to be inserted in the cicuit can be
calculated by the simple formula

R = V / (√3 * I)

Where V is the System Line voltage in Volts
I is the value of the fault current to be limited

In general practice the fault current is limited to the
full load rating of the largest equipment available in the
system.

Question { 4377 }

only some transformers and some generator requires NGR why?

Answer

NGR - Neutral Grounding Resistor is used to limit the fault
current of a sytem.

So generally a system with high line voltage of 3.3KV and
above normally comprises of high load equipment which
results in very high fault currents due to a Short-circuit
fault. Normally during a Earth fault the fault current
flowing through the earthed phase is to be carried by the
remaining phases which may be very much more than the
rating of the conductors. So we follow the Neutral
Grounding practice so that the fault current is carried by
the nuetral.

Now this current carried by the nuetral is to be limited
such that this high fault current does not damage the
grounded equipment so in the neutral we insert a resistance
which is the NGR.

So for any high voltage system to limit the fault current
NGRs are used. for the equipment working at lesser voltage
normally can withstand the short circuit currents for short
time.

Question { 5487 }

why mosquito bat makes noise when it hits with a insect?

Answer

A mosquito bat consists of three layers of mesh, the outer
two being one terminal and the inner being the other
terminal of a transformer that provides high current at low
voltage.

When a insect comes in between the two meshes it creates a
closed path for the current to flow. since it is a high
current it results in a sudden surge which creates a spark
that eventually ionises the air molecules in and around the
arc. this sudden ionisation of air creates the hissing
sound.

Question { Sterling Wilson, 7669 }

How to calculate the NGR value for a 2.7MVA Transfomer,
11KV system where the fault current has to be limited to
50A?

Answer

The value of the NGR to be inserted in the cicuit can be
calculated by the simple formula

R = V / (√3 * I)

Where V is the System Line voltage in Volts
I is the value of the fault current to be limited in your
case it is 50A.

In general practice the fault current is limited to the
full load rating of the largest equipment available in the
system.

Question { 8067 }

what is sf6 where it is use & working or function ?

Answer

SF6 is Sulphur Hexa-Flouride - A Chemical substance used
in the Circuit breakers as an ARC quencher.

Those breakers are specifically called "SF6 Circuit
Breakers" usually used at voltage levels of 132KV and higher

Question { IGCAR, 58720 }

what is the difference between electrical degree n
mechanical degree?

Answer

Mechanical degrees: the degree of physical or mechanical
rotation a single conductor in an alternator.

Electrical degrees: the degree or the cycle of emf induced
in a single conductor in an alternator

Consider a Two pole Alternator, when a conductor rotates in
the feild, the emf induced in the conductor will be maximum
when the conductor is at the center of the pole and the emf
induced will be minimum when the conductor is in the middle
of the gap between the two poles. so when we consider the
graph of the induced emf versus the position or degree of
rotation. For one complete rotation of the condutor in the
feild - One complete cycle of emf is generated in the
conductor (Positive half cycle at south pole and negative
halfcycle at north pole).

The same way when we consider a 4 pole alternator a single
conductor when completing one mechanical rotation will
cross the four poles there by giving the induced emf graph
with two positive half cycles and two negative halfcycles
which gives us two complete cycles of induced emf.

So for One Mechanical rotation (360 degrees) the induced
emf completes two cycles (360 * 2 = 720 degrees in terms of
the sine wave)

So the relation between Electrical and Mechanical degrees
can be written as

Elect. Degrees = (N0. of Poles / 2) * Mech. Degrees

Source: Elecrical Machinary by P.S.Bimbhra

Question { 20452 }

How to calculate battery back up time for a 400 AH battery
bank installed in telecom shelter for a load of 20 Amps and
time taken to fully charge this battery bank?

Answer

Part -I:

400Ah Battery means the battery can supply a DC load of 400
Amps for a duration of 1 Hour until the battery gets fully
discharged. so if you are using the battery to supply the
above mentioned load of 20 Amps the battery can work for
400/20 = 20 Hours. provided the 20Amps is a Direct load
connected to the Battery. if you use the battery for a
inverter or UPS to suplly an AC load of 20 Amps. then the
duration is different.

Part - II:

For charging any battery we have to provide a voltage that
is slightly (5% - 10%) higher than the Battery terminal
voltage. The duration of charging can be known by the same
process as above. A 400Ah gets charged in 1 Hour provided
the charging source is capable of supplying 400 Amps to the
Battery. But normally the current of the charging source
will be limited to a lesser finite value. so if you can
know the maximum load the source can supply you can easily
calculate the duration required for the battery to get
fully charged.

Question { Lucas TVS, 29935 }

what is the advantages of Lighting transformer?.. is it
possible to reduce the power consumption ?.

Answer

Generally the power is transmitted in 3-Phase DELTA
circuit. where as lighting is conneted to single phase. and
lighting load is totally unbalanced so a delta circuit
cannot be used in supllying a lighting load due to non-
availability of neutral conductor.

for this purpose a lighting transformer is used which
transforms the 3-phase delta circuit into a 3-phase star
circuit. which can be used to supply single phase
unbalanced loads due to the availability of neutral wire.

the transformer can be used in transforming any voltage
level.

Question { 4950 }

In CT's they mentioned 400/5 or 1250/5, 1000/1 here
400,1250,1000 is Breaker Capacity what is the /5 Explain me
briefly

Answer

CT is used for measuring high currents. even in a circuit
breaker it is used to measure the current and to send the
signal to the relay in case of any abnormality.

the 400/5 on the CT denotes the transformer ratio. that is
when 400Amps flows through the primary of the CT the
secondary of the CT is induced with 5Amps, which is easy
for measurement.

so the fraction on the CT denotes its ration which defines
the value of current flowing in the secondary side.

Question { 4347 }

In DC while we mention the voltage we are using -48V but in
case of AC we mention only 230V not -230V, why we using -48V
not 48V

Answer

in the measurement of voltage in any circuit we measure the
voltage of one conductor with respect to another.

In DC supply we have two conductors namely Positive(+) and
negative(-). if we consider 48V DC across the two
conductors, then if we measure the value on the positive
conductor with respect teo the negative conductor the value
is +48V. if we measure the same value on the negative
conductor with respect to the positive conductor then the
value will be -48V. this 48V is the instantaneous value of
the DC supply.

Where as in AC we have two conductors namely Phase and the
Neutral. always the Phase constitute the current path and
the neutral forms the return path. we say that the AC
voltage is sinusoidal. that is it varies from positive to
negative when seen with respect to the neutral.

its impossible to measure the instantaneous value of the AC
supply bcoz it keeps on continuosly varying. so for the AC
supply we consider the RMS value (Root Mean Square Value)
as the voltage of the supply. so in this case there is
always the same value when measured with respect to any
conductor. so the AC voltage does not have polarity.

Question { Sepco, 20941 }

What is the stander resistance value between the neutral and
earth?

Answer

actually the parameters between earth and neutral is
measured in terms of VOLTAGE. according to IS the voltage
between nuetral and earth shud be less than 0.8V.

Nuetral is taken from the source while earthing is done
near the device. so to check for a perfect earthing
conditions the above voltage criteria is to be satisfied.

Question { 4046 }

anybody can give the voltage value for 1M ohm

Answer

1M Ohm is the resistance value. other than this the
resistor is rated with wattage like 1/2 watt, 1 watt so on.

so the i*i*r value of that corresponding resistance shud
not be more than the wattage rating of the resistor. this
determines the current value. when u know the current value
eventually you get to know the voltage.