On Wed, 2 Mar 2005, Olin Lathrop wrote:

> Peter L. Peres wrote:
>> Afaik it takes two phases for that.
>
> Yeah, you're right.  I just repeated what I remembered from a brief lecture
> on power electronics a long time ago, but your math makes sense.  I do
> remember something about 3 phases required for constancy of something, but
> apparently not power.
>
> One advantage that 3 phase has the 2 phase doesn't is that all three phasers
> add to zero.  To get constant power with 2 phases they need to be separated
> by 90 degrees with the resulting vector sum definitely not zero.

I don't understand that but in any case power machinery will have torque 
output and that is a function of current. So you could argue that the 
ripple in the sum current between the instantaneous currents on 3 phases 
doing work is lower than on 2 phases doing work. I also know that some 
applications which require rectification for high poewer used 6 phases 
(30 degrees apart) for the same reason. Aluminium smelting was one (?). 
This may no longer be true.

Peter
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