On Tue, 1 Mar 2005, Olin Lathrop wrote:

> Mike Hawkshaw wrote:
>> The real advantage of three phase is that it the most efficient way of
>> transmitting power, taking into account the copper losses, and the cost
>> of more conductors to go to more phases.
>
> The real point behind 3 phase power is that total power delivered is
> constant and does not vary with the 60Hz or 50Hz power cycle.  Three phases
> is the minimum required for this to be true.  Among other things, this means
> that the torque on the generators at power plants is constant.  This is very
> important for such high power machines.  This also means that 3 phase motors
> draw a constant power and put out a constant torque.

Afaik it takes two phases for that. Proof: instantaneous power for a 
sin, cos 2-phase system, at 90 degrees is:

Pi = (U1*sin(wt)*I1*sin(wt)) + (U2*sin(wt+pi/2)*I2*sin(wt+pi/2))

with U1=U2 and I1=I2 this becomes:

Pi = 2*U*I*(sin(wt)^2 +cos(wt)^2) = 2*U*I*1

I think that the real real reason why 3 phases are 'better' is the 
better use of copper and iron. For 2 phases one still needs 3 wires and 
one of them would have to be thicker. With 3 phases they can be equal. 
Also the torque of a 3-phase machine is more constant than a 2-phase 
machine's although I doubt that this applies to any sizeable generator, 
those having in the hundreds or even thousands of poles and 
phasing/equalizing gear downstream to prevent imbalance between the 
phase loads.

Peter
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist