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> Close, Three phase systems actually are a carry over from the days of early induction motors. It's not just some historical accident. We continue to use mul

> Close, Three phase systems actually are a carry over from the days of early induction motors. It's not just some historical accident. We continue to use multi-phase power because it enables continuous delivery of power (unlike single-phase, where the delivered power drops to zero at least twice per cycle). And two phases require just as many wires as three phases, with more power delivered for a given mass of copper and insulation, so... we use three. > If the current on phase a is 20 amps and the current on phase B is 20 amps what is the return current on my neutral? That depends on the (complex) impedance in each branch. In the pathological case, the current on the neutral could be as high as 40 Amps.

kazinator 6y ago
The advantage of three phase over two is that three phase can energize the coils of a three-phase motor so as to create a rotating magnetic field. No extra circuitry is needed. Two-phase cannot do that when the phases are 180 degrees apart; opposite polarity is as good as a single phase. It would work if the two phases were 90 degrees apart. A two-phase motor operating on single phase (or 180 degree two phase) needs to generate the 90 degree phase signal with extra circuitry. Three phases is also smoother power delivery and even loading compared to two.
- deaddodo 6y ago
  This video actually explains the reasoning for tri-phase power and it's advantages in a succinct manner, for anyone still confused: https://www.youtube.com/watch?v=4oRT7PoXSS0 https://www.youtube.com/watch?v=4oRT7PoXSS0
crmrc114 6y ago
If both loads were on the same phase that would be a violation of the US NEC (NFPA 70) because use the load would be 40 amps..... And guessing we are taking 12awg, way over allowed current. In hindsight I did not specify circut ampacity, or wire gauge. Still the question stands, how much current would you read at the panel on the neutral with those two loads on the line :-) (In my example you have two phases 120 degrees apart. Think US Residential split phase. The actual result is pretty awesome. It's a common test problem for new electricians.) I won't spoil the result because it's too cool when you get it.
- crmrc114 6y ago
  Oops should have typed 180 degrees apart for residential.

kazinator 6y ago

The advantage of three phase over two is that three phase can energize the coils of a three-phase motor so as to create a rotating magnetic field. No extra circuitry is needed. Two-phase cannot do that when the phases are 180 degrees apart; opposite polarity is as good as a single phase. It would work if the two phases were 90 degrees apart. A two-phase motor operating on single phase (or 180 degree two phase) needs to generate the 90 degree phase signal with extra circuitry. Three phases is also smoother power delivery and even loading compared to two.

deaddodo 6y ago
This video actually explains the reasoning for tri-phase power and it's advantages in a succinct manner, for anyone still confused: https://www.youtube.com/watch?v=4oRT7PoXSS0 https://www.youtube.com/watch?v=4oRT7PoXSS0

crmrc114 6y ago

If both loads were on the same phase that would be a violation of the US NEC (NFPA 70) because use the load would be 40 amps..... And guessing we are taking 12awg, way over allowed current. In hindsight I did not specify circut ampacity, or wire gauge. Still the question stands, how much current would you read at the panel on the neutral with those two loads on the line :-) (In my example you have two phases 120 degrees apart. Think US Residential split phase. The actual result is pretty awesome. It's a common test problem for new electricians.) I won't spoil the result because it's too cool when you get it.

crmrc114 6y ago
Oops should have typed 180 degrees apart for residential.