[RE-wrenches] voltage drop

Hugh hugh at scoraigwind.co.uk
Tue Dec 9 23:13:38 PST 2008


>I looked at the resistance values you have in that spreadsheet. Do 
>you have some kind of "fudge factor" built into it for temperature?

It's well worth being aware that temperature has a big effect.  So 
often I see people calculating this kind of stuff to 3 places of 
decimals based on some such table but unaware that this spurious 
accuracy only applies at some precise temperature.  In reality 10% 
accuracy is all you need and get.

Another thing to watch out for with volt drop in wires is that wind 
turbines feeding 3 phase rectifiers do not just have 1/3 of the 
current in each wire.  They actually have about 0.8 times the DC 
current in each wire (rms).  As a rough guess (which is about all you 
can do given the vagaries of temperature) the conservative thing to 
do is to say that the 'volt drop' (as used for power loss 
calculations) will be the same as it would be if using two wires and 
a DC current in them, just like it would be after the rectifier.  The 
difference is that the 3-phase circuit needs 3 wires, but most of the 
time it is only really using 2 of them.  In reality there is often a 
bit of sharing between wires but this doesn't make a huge difference 
to the result.

In Europe we have metric wire sizes (cross sectional area in sq mm) 
and this is a great help for mental calculations of volt-drop.  I 
usually estimate that 30 metres (one way) of 2-core one sqmm wire has 
a resistance of one ohm.  Then if the distance is more than 30 metres 
I increase ohms pro rata, and if the area is more I reduce it pro 
rata.  For example 60 metres of 10 sq mm would be 1/5 ohm approx. 
For hot wires the distance for one ohm would only be 22 metres or so. 
I use 30 because it's a nice round number, but in reality 28 metres 
is a better choice for normal ambiences.
-- 
Hugh Piggott

Scoraig Wind Electric
Scotland
http://www.scoraigwind.co.uk



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