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Why Only Three Phase

09/11/2011 1:36 AM

why we are bound in three phase with 120 desplace we also can make 6 phase with 60 degree displace then why only 3 phase any special reson is there ?

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#1

Re: why only three phase

09/11/2011 2:08 AM

Three phase is about the minimal to create a reliable rotating field that generator and motor spin at a given frequency. Also the reverse rotation function is a stable element. You can make it work with as many phases as you like, it only becomes more complicated and you will need to connect (install and pay for) more wires. Look at some stepper motors with a bunch of wires.

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#2

Re: why only three phase

09/11/2011 6:22 AM

Old Mercury Arc rectifiers were made to work on 48 phases by the use of inter-phase transformers. How many phases do you want?

A 12 phase MAR

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#3

Re: Why Only Three Phase

09/11/2011 7:57 AM

Imagine the complexity of getting all six phases in order to get a motor running smoothly! With three phase, the motor runs forward or reverse. To change the rotation, you change two wires! Now lets try six!! Three phase is simple, reliable and cost effective without the problems of having a phase in the wrong place! Ie the motor will run smooth in both directions!

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#5
In reply to #3

Re: Why Only Three Phase

09/12/2011 2:56 AM

Ye gods, reversing a motor would be a nightmare!

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Guru
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#4

Re: Why Only Three Phase

09/11/2011 9:10 AM

The "special" reason is cost, for a given amount of copper and iron we can generate and transmit the maximum amount of power using three phases. Most of the other variations of frequency, phase, number of conductors, etc. have been tried but never found widespread use in bulk power utility systems.

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#6

Re: Why Only Three Phase

09/12/2011 2:56 AM

if you want to have a six phase system...go on...no one is stopping..but you should be responsible enough to carry the consequences there is...having all your equipments made to adopt a 6 phase system...and with all the complexity there is....try solving it out how much power will you get on a 60 degree angle...

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#7
In reply to #6

Re: Why Only Three Phase

09/14/2011 1:18 PM

There are 6 and 12 phase "systems" in use.

Special purpose of course.

I can think of Radar rotating motors, tethered un-manned sub power supplies to name two.

The main advantage to consider offsetting the costs described by other posters, is ripple voltage in an AC to DC converter.

With more phases, the crest factor goes down and filter capacitors banks and inductor pi filters size can be decreased.

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