Big Wire? [RE-wrenches]

[Hugh Piggott]

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I agree that it is important to look at the average power loss in 
wires (energy loss) rather than the power loss at peak power.  In 
off-grid systems, peak power times are when the power is usually 
least valuable to us so I am not overly concerned about loss at those 
times either.  In grid-tie systems the arithmetic changes.


>
>If anyone can suggest how a factor for applying it to 3-phase, please do.
>
>Windy

Many people think that the current in 3 phase wires is 1/3 of the DC 
current.  This is not the case.  In fact the rms current in one wire 
leading to a rectifier will be between 0.74 and 0.82 times the DC 
current.  If you use the same wire size as DC (but with 3 wires) the 
loss will be between 82% and 100% of the loss in the DC wire run.

3 phase uses more copper than DC for the same loss.

People get confused because there is a big saving with AC systems 
using 3 phase connected star.  However in a wind system feeding a 
rectifier the situation is very different.  Peak voltage is about 
equal to battery voltage and currents are not much less than DC, 2 
wire currents as noted above.
-- 
Hugh

On Guemes Island WA, USA
http://www.scoraigwind.co.uk/

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