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edison s invention of direct current ( dc ) preceeded tesla s invention of alternating current ( ac ) . however , once there were two possible modes of power available , an inevitable debate on the merits and demerits of the two started that led to what is called the `` war of currents '' .
the inventor s became adversaries with edison promoting direct current ( dc ) for electric power distribution over the alternating current ( ac ) .
the `` war of currents '' got so bitter that both inventors lost a lot of money and rumors have it , their nobel prize @xcite .    today
the debate is more or less resolved with ac being accepted as the best method for electric power distribution , especially where the power requirement is large .
it can be appreciated that since direct current can not be trivially stepped up or stepped down , the same voltage level is transmitted as required by the load .
this resulted in large transmission losses .
transmission loss takes place due to heat dissipation along the current carrying wires used for delivering power from generation point to consumer .
the initial transmission networks laid were of copper , which is one of the best conductors with low resistivity . even with low resitivity , since the length of the transmission wires involved are large , they offered finite and non - negligiable resitance . thus introducing power loss during transmission .
mathematically power dissipated is gien as @xmath0 where i , @xmath1 , l and a are the rms ( root mean square ) current , wire s resistivity , it s length and cross - sectional area respectively .
thus , for transmitting a given power with minimum power loss , one would have to reduce the current while increasing the voltage .
this is exactly what a transformer does for ac .
thus for dc , transmission loss can only be minimised by using thicker copper wires . in turn
, only fairly low dc power can be transmitted . since ,
ac power can be stepped - up and down easily using transformers , the issue of transmission loss over thinner wires can be economically addressed .    with industrialization and increasing demands for higher levels of power ,
even a ac distribution is not enough .
hence , today distribution has moved to polyphase ( m ) distribution .
polyphase voltages are also ac voltages but made up of multiple sinosudial varying voltages .
of the possible polyphases , the three - phase supply is the most popular .
it refers to three voltages that differ in phase by @xmath2 degrees from each other .
the voltages go through their maxima in a regular order , after every @xmath3 .
the phase sequence are named ` a ' , ` b ' and ` c ' .    0.5 cm 0.5 cm
the three phases are generally distributed using three wires .
the phases are separated or collected at the load side using special transformers , called `` star '' or `` delta '' transformers ( fig  1a ) .
each wire has the same current carrying capability as in case of single phase supply .
however , the power delivered at the load can be far greater depending on how the two potential levels are selected .
consider , in the star transformer the load is connected between point ` a ' and neutral ( fig  1b ) the output waveform would be that of phase a , with the neutral point acting as ` zero potential ' point .
the power delivered to the load is @xmath4 .
however , if the load is connected between point ` a ' and ` b ' , the net potential difference can be found ( first in general ) from    @xmath5    where @xmath6 is the peak voltage and @xmath7 the phase difference between the two phases . for a 2-phase supply ,
where the phase difference between the two waveforms is @xmath8 , the potential difference would be @xmath9 this mathematics is graphically represented in fig  2 .
it can be appreciated that the power delivered to the load now would be @xmath10 .
this example shows how polyphase transmissions delivers more power .    while these are important issues to be appreciated by a physicist setting up a laboratory with instruments that require high power , schools and colleges do not help develop this concept with hand - on experimentation for fear of possible accidents that could even prove fatal .
while kits are present in engineering colleges for electrical engineering , not much is found in the literature in way of simple experiemnts for physics and electronics students . in this direction , we have designed and tested a simple circuit , where two phases are generated from a sinusodial wave taken from a function generator .
the proposed circuit requires three opamps , of which the first opamp is assembled in an inverting configuration @xcite . the sinusodial input from a function generator ( shown in fig  3 as ` 1 ' ) is given as an input to this inverting amplifier .
this signal also acts as one of the 3 phases , namely phase a. the gain of this amplifier is kept as unity with @xmath11 .
this circuit acts as a buffer preventing any loading by successive circuits and introduces a phase change of @xmath8 between output and input waveforms ( as indicative of the name `` inverting '' amplifier ) .
the rc circuit is designed to introduce a phase difference of @xmath3 for the selected frequency . however ,
a single combination of rc can at the most introduce a phase difference equal to or less than @xmath12 .
hence , two sections are used , with each rc section introducing a phase shift of @xmath13
. the values of r and c are selected using the formula @xcite @xmath14    _ in our study , for an 5khz wave provided from the function generator , we selected @xmath15 and @xmath16 .
_ hence points ` 3 ' and ` 4 ' of the circuit would be @xmath17 and @xmath18 respectively out of phase with respect to the input .
the impedence networks act as potential dividers and hence the voltage levels at ` 3 ' and ` 4 ' would be lower than that given as input . since the three phases would have the same amplitude , the signals at ` 3 ' and `
4 ' would have to be amplified .
we use as inverting amplifier again for point ` 4 ' .
the voltage gain of this amplifier is give by @xmath19 .
hence , @xmath20 has to be varied till the voltage signals amplitude is identical to the input signal .
also , this inverting amplifier introduces a phase change of @xmath8 . the net phase difference
hence would be @xmath21 which is nothing but a phase difference of @xmath22 .
the output of this op amp circuit would be phase ` b ' .
similar amplitude correction is required for the wave collected at point ` 3 ' . however , here we select a non - inverting amplifier circuit whose gain is given as @xmath23 and as the name suggests does not introduce any further phase shifts _ ( @xmath20 and @xmath24 used were @xmath25 pots and typically had values greater than @xmath26)_. hence , the net shift at the output remains the same as that at point ` 3 ' at @xmath27 or @xmath28 .
this is phase c of the simulated 3-phase supply .
fig  4 shows the waveforms ( phase a and c ) as captured by picoscope ( model 2202 ) .
since the model only gives dual trace , only two waves can be shown simultaneously . however , the data captured by picoscope shows all three phases simultaneously .
curve fitting these data points give the phase difference between the various phases to be 1.95radians or @xmath29 . also , the peak voltage ( @xmath6 ) also works out to be 1.58v .    while the circuits achieves the purpose of mimicing a 3-phase supply ,
experiments can be done to further understand applications of 3-phase supply .
for example , one can study the rectification and advantage of generating dc from 3-phase supply .
for this , all one needs is 3 diodes and a load resistance ( see fig  5 ) .
the output waveform is dc and also notice even without any filtering circuit , the output is continuous with little ripple .
this is because , as seen by the load , the input frequency is three times that of a single phase supply .
the ripples in a rectified output is inversely proportional to the frequency @xcite , hence the low ripples here can be understood .
also , this output gives visual idea and helps in easier understanding that each phase in a 3-phase supply is separated by @xmath3 . in this rectifier circuit , only that diode conducts , for which the phase connected to it has the highest instantaneous potential with respect to the neutral .
fig  4 shows one phase to have highest potential with repect to the other phases for @xmath3 .
thus , in one cycle ( @xmath30 ) there would be three peaks of the output dc wave .
this is visiable in fig  5 .
a simple circuit has been proposed to demonstrate the behaviour of 3-phase power distribution .
the simple circuit discussed in this article cost rs ( indian rupee ) 20/- to assemble ( less than a dollar ) and can be a very usual experiment in schools and under - graduate laboratorys to learn more about 3-phase supply and it s application .
the financial support of u.g.c ( india ) in the form of minor research project no.f.6 - 1(25)/2007(mrp / sc / nrcb ) is gratefully acknowledged .
the authors would like to express their gratitude to the lab technicians of the department of physics and electronics , s.g.t.b .
khalsa college , for the help rendered in carrying out the experiment .
the power distribution of nearly all major countries have accepted 3-phase distribution as a standard . with increasing power requirements of instrumentation today 
 even a small physics laboratory requires 3-phase supply . 
 while physics students are given an introduction of this in passing , no experiment work is done with 3-phase supply due to the sheer possibility of accidents while working with such large powers . 
 we believe a conceptual understanding of 3-phase supply would be useful for physics students with hands on experience using a simple circuit that can be assembled even in a high school laboratorys .