single phasing is the fault in that one of three phase will
break or not proper then machine or equipments will get
unbalance.

my mean to say is suppose r,y&b is three phase & y is not
showing voltage for some reason than it is single phasing.

if it is happens then machine will not work proper it will
get heated because of phase unbalancing or burnt.

if 1 phase is out then machine works with two phase with 
heavy losses. it takes very high current..

dear abhishek,

if it will take high current at single phasing then
equipment damage because of that only single phase preventer
devloped.

Main purpose of using transformer is to convert the power, 
Either from high to low or vice versa....i e step up or 
Step down....for more deatils go through Power plant or 
power system books.. Why transformer is neccessary in 
transmission net work..n How the power will converted from 
high voltage to low voltage..etc

Transformers are in such a way bcos they are either step up or step down. In case of Step down the configuration is Star to delta & the other way if it is Step up. It is to protect the transformer & the equipments safety.

earthing means zero potential grounding means which 
equipment we want keep the zero potential it is connected 
to earth.

grounding:
this means keeping that particular terminal as common point.
example-1.
consider the case in electronic circuitry(as with a music 
system) where dc supply is used, the negative terminal is 
usually grounded to other metallic parts of that system.
example-2.
take any circuitry where a number of different voltages are 
used, 0-12v, 0-24v, 0-110v, 0-230v etc. here all the ov 
terminals are grounded and taken it as common.
earthing:
this means making the common point to earth.the potential 
of this point shall be Zero volt.
example-1.
transformer neutral is earthed to keep the potential as 
zero.
example-2
equipments are earthed for better safety of living beings.

power facter - it is the sine angle between voltage vecter
and current vecter. 

suppose you have 1.5 kw watt motor running on 230 volts and
power facter of your motor is .65 than your current requirement 

i =p/volts*power facter
i =1500/230*0.8
i =8.15 amp.

and if you will improve power facter of your motor up to
0.85 than current requirement is 

i=p/volts*power facter
i=1500/230*0.85
i=7.67 amp.

as you can see current taken by your motor is less if you
will improve your power facter.

advantages in big inductive machine & load.
1) current is less so losses should be less.
2) less current decrease your cable size & cost
3) voltage regulation should be good.
4) rating of switch gear will less.
and finally you will get some advantages from government if
you have big factory.

hai mehul how  we can improve power factor from 0.8 to 0.85
or 0.9 & its either lag or lead ?please clear me thank you

hai ramya.

there is a lot of way to increase power facter of your
machine or factory. 

just read the book of b. l. thareja.

Three phase motor differential relay testing can be done 
with single phase test kit.

There are two type of relay testings:
1) Primary injection : In this testing entire system 
(including relay, control wiring and power wiring get 
checked),
2) Secondary injection : In this testing generally we check 
relay only. 

By using single phase test kit:
1) Primary injection : 
a) Carryout shorting of all three phases of motor winding 
by using short link for individual phases seperately. 
b) Inject current by using current injection device (for 
inzone and outzone fault)
3) Check stability of relay for outzone fault and operation 
of relay for inzone fault.

2) Secondary injection :
Directly inject fault current in relay terminal and check 
proper operation of relay. 

Santosh Kawade 
9224 69 3116

433 IS LILNE TO LINE VALUE OF 250

415 IS NOT DIVISIBLE BY 1.732

Appliances rated voltage is 415V. If Trasformer is specied 
at 415V, it will further drop down at receving end and 
appliances may not work

To compensate Voltage drop at receiving end

Appliances rated voltage is 415V. If Trasformer is specied 
at 415V, it will further drop down at receving end and 
appliances may not work

Power distribution network is supposed to have a voltage 
drop of around 4%.  As the utility provider has to assure a 
three phase voltage of 415 V at consumer's metering point, 
the secondary phase to phase voltage of the distribution 
transformer is specifed at 4% higher than standard three 
phase-phase to voltage of 415 V, which works out to 433 V.

20.86v.8khz

Assume that the transformer is typical iron-cored stepdown 
transformer.
The actual output result will not be 20.86 volts calculated 
with straight ratio and the transformer will likely be 
destroyed due to the following factors:
1. Iron core has lower frequency response and that makes it 
good only for 50-60Hz.  8,000Hz frequency will not get 
transformed properly because the magnetic flux in iron core 
will be saturated.  The primary coil will receive 
equivalent to direct current voltage therefore is likely to 
overheat.
2. The insulation of a 230v transformer will not withstand 
400v therefore ground leakage is very likely.
3. The size of copper wire and number of winding suitable 
for the current rating designed for 230v will draw more 
power into the coil when connected to 400v, again, 
overheated.

20.869v,8kHz  is the voltage and frequency of secondary.

o/p voltage will be 20.8 volt, but due to very high 
freguency 8 Khz hytresis loss & edd current will be 
increased because this loss is directly related to 
frequency.

e1/e2=e3/e4
230/12=400/e4
e4=400/19.16667
e4=20.869v
so ans is : 20.869v
freq remain same.

3/23volts or appro .13 volts. assume xmer to be ideal one 
with no resistance in windings,nohysteresis loss etc, at 
80khz the frequency is 160 times 50khz ,so current in 
primary is 160 times less than at 50hz as inductive 
reactance of primary ,ie 2 pi f l is 160 times more. as 
current is 160 times less so is magneti flux in core,and so 
is induced emf in secondary. if hysteresis loss and eddy 
current loss r considered , then it is even less

Selecting a cable is based upon the need of consumer.DC 
cable is preffered for transmitting power at high voltages 
because at increase of voltage level the size of conducter 
increases in case of AC but the crossection area of 
conductor can be decreased if it is transmitted in DC.IR 
losses are low in case of transmission through DC.

in ac the current supply are in alternating in nature
whereas in dc the current direct in nature.due to this ac
takes more current so it doesn't be widely used.in dc its
current consumptions are less,so its most probably used.

current rating and insulation property are the main
constrain between ac & dc

The average curent of DC is higher than AC .Same thumb rule 
is used for selecting cable either DC or AC but insulation 
depends on voltage level.
 The loss in DC transmision is less than in AC transmission.

ac current has high intensity ,during the flow of current it
produces heat rather than the dc current

In selecting a cable it is based on the rms (root mean
square) value of the current. The fact is that rms value
produces the same amount or almost equal of energy like that
dc. 
 
20A of dc= 20A of AC rms...

its very simple 
Ac power=VIcos@ w ,,
DC power=VI  w

It does not matter much for AC or DC. The rms value of AC 
is equal to DC. However if AC voltage rating is high then 
you should cosider additional heating of cable due to AC 
loses.