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This question already has an answer here:

Why 3 phase is preferred over any other phase power transmission ?

My instructor was explaining this topic in my Electrical machines class. And he mentioned the following points

  1. In single phase, system there is double frequency oscillating component. So, it is not preferred.

  2. In 2 phase system, there is no double frequency oscillating component. So, it is ok to use it. And it can be used to transfer 1.414 times output rating in the same frame size as single phase. Also, we have return neutral current which is 1.414 times the peak value which is not good.

  3. In 3 phase system, we can transfer 1.5 times output rating in the same frame size as single phase. Also, we have zero return neutral current which is also a good thing.

  4. In infinite phase system, we can transfer 1.57 times output rating in the same frame size as single phase. There is not much improvement from 3 phase so we went with 3 phase.

Now, I am confused with the thing how he got the values 1.414 for 2 phase, 1.5 for 3 phase and 1.57 for infinite phase and what does it mean by saying in the same frame size. Please someone help me with this concept.

EDIT: I want to understand how the 3 phase is 50% more efficient than 1 phase whereas infinite phase will be only 57% more efficient than 1 phase. I am not asking why 3 phase over 1 phase as that I have already mentioned in my question.

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marked as duplicate by Olin Lathrop power May 10 '18 at 11:15

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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  1. single phase: two conductors. Rotating magnetic field is not possible unless you add a second phase, shifted by 90 degrees by means of additional capacitor.
  2. two phase : needs 4 conductors, two for each phase.
  3. three phase : needs 3 conductors, as the sum of all currents is zero at any time.
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  • \$\begingroup\$ I want to understand that efficiency thing. My instructor said "Going from 1 phase to 3 phase gives efficient improvement by 50% increase (which can be utilized toward reduced copper by 75%). Increasing the number of phases arbitrary more than that (infinite) can only increase efficiency very little (some thing of the order of 7%)." \$\endgroup\$ – Nikhil Kashyap May 10 '18 at 9:53

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