Bergey or Whisper [RE-wrenches]

[hugh piggott]

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Hi Ian,

it's a challenge to do this without diagrams.

Imagine you are supplying 2 homes with 12 volt power.  You can 
connect them to a 24 volt windmill with the 2 batteries in series and 
use a 24 volt wind generator and reduce the wire losses.  since each 
house uses approximately the same amphours, this works out OK.

The reason it saves power is that instead of having a 2 wires to each 
house carrying current you can connect one house to the next and the 
current does not have to return to the source.

3 phase AC supply to several loads is a little bit like that.  Only 
there are 3 wires. In Europe the voltage between any two of these 
wires is about 400 volts.  Each house uses just one 'hot' wire. 
There is a fourth wire called 'neutral', which each house is also 
connected to and this is also grounded, but it carries very little 
current, because the return currents from each house almost 
completely cancel each other  out.  The voltage between hot and 
neutral is 230 volts.  (230 times root(3) is 400)

The result is that the currents to each 230 volt house supply are 
supplied by just one wire.  The neutral wire is small and only has to 
carry any differences in current (imbalances in phases).  Seems like 
magic but actually there is no free lunch because in reality the line 
voltage is 400 volts.

In any circuit there has to be an outward flow and a return flow. 
Where you are feeding several loads, using 3 phase power, you can 
increase the voltage and reduce the number of wires required by using 
the fact that the loads are using the same amount of current.

In the case of a battery being charged off a 3 phase wind turbine 
however, the AC line voltage cannot peak higher than the DC battery 
voltage.  The free lunch is not available.  There is no neutral. 
There are no other loads to go on to.  Current has to flow out 
through one wire and return by another.  At any given time, at least 
2 out of 3 wires are conducting.

The result is that the average current in each wire is approximately 
2/3 of the DC current.  In fact because the current is pulsating and 
not constant, the rms current is higher than this average, reaching 
up to around 80% of the DC current.

consequently if you use the same wire size as you would with DC, each 
wire will lose about 0.8^2=0.64=about 2/3 of the power that you would 
lose in one of the DC wires.  And since there are 3 wires, the loss 
is almost as high using 3 wires as 2.  plus you have to provide an 
extra wire.

no advantage.

How was that?


>Hi Hugh,
>
>I'm still wondering why this is such a common myth.
>
>Care to explain why you keep having to counter this myth? And can 
>you more fully explain what you say below?
>
>Ian
>
>>>  it's dc transmission to the controls. Wire will have to be
>>>sized accordingly larger compared to the 3-phase ac of the Whisper.
>>
>>the idea that the wire loss is greater with dc than with 3 phase ac 
>>is a very common fallacy. the rms current in each wire is about 0.8 
>>times the dc current.
>>
>>3 phase ac is only advantageous where the line voltage becomes 
>>greater than the end user voltage.  this is not the case with 3 
>>phase supplies to rectifiers.
>>
>>dc is actually slightly more copper-efficient than 3 phase ac. 
>>each wire may need to be slightly larger, but overall you save 
>>copper because you only use 2 wires.
>>--
>>Hugh
>>
>>down in Birmingham for a seminar about alternators.
>>http://www.scoraigwind.co.uk
>>
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>
>--
>Ian Woofenden <ian.woofenden at homepower.com>
>Associate Editor, Home Power magazine
>Editors are Professional Idiots - We misunderstand text so our readers won't.
>
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>PO Box 520, Ashland, OR 97520 USA
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>

-- 
Hugh

http://www.ScoraigWind.co.uk

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