Wire and regulation

[armcroft]

Wherever a wind generator is sited the power must be sent to where it will be used.

My plans call for 40' tower located 90' from shack plus another 30' to get to battery bank area, a total of about 160'.

Should I;

rectify current at top or base of tower and transmit as 12V DC. Not rectify and leave this till it gets to battery shed. Does generating single phase vs 3 phasse affect this decision?

What cable size should I use? Is multistrand insulated copper with area of 5mm squared  (about 0.04 square inches) OK? This is what I am using for all my other 12V wiring.

How can I divert power to a hot water heating element when batteries do not need charging. This must be automatic as we are only there once each week and could not always be able to switch power manually.

Slip rings vs twisted cable. Which is better?  

Hope you all had a great 4th of July.

Regards.

Robert

[DanB]

Hi Robert - here are my thoughts....

"Should I;

rectify current at top or base of tower and transmit as 12V DC. Not rectify and leave this till it gets to battery shed. Does generating single phase vs 3 phasse affect this decision?"

If you rectify it at the top of the tower, the rectifiers will cool better... but should they fail you'll probably be free- spinning (overspeeding).

If you rectify it at the bottom of the tower, you only need to run 2 lines from the tower to the batteries - which is nice, and DC covers the distance with a bit more efficiency than AC.

I prefer to rectify them near the batteries - this means running 3 lines from the machine, to the rectifiers (if you use 3 phase).  The main reason... I feel the rectifiers will stay dry, theyll be easy to replace and I can locate the "kill switch" next to them, but the kill switch will be on the windmill side of the rectifiers.  I find sometimes on a very windy day - it's rare that the machine slows enough so that I'd feel safe shorting the alternator through the rectifiers.  I just feel safer not shorting the output from the rectifiers, but rather shorting the AC straight from the alternator.  

Any way you go though should work fine.   Single phase vs 3 phase shouldn't affect your decision too much, but I'd avoid single phase - 3 phase has its advantages all around.

"What cable size should I use? Is multistrand insulated copper with area of 5mm squared  (about 0.04 square inches) OK? This is what I am using for all my other 12V wiring."

I dont know my metric conversions very well.  There should be a chart online though that will tell you voltage drop over a distance for any given current level - I'd work from that.  

"How can I divert power to a hot water heating element when batteries do not need charging. This must be automatic as we are only there once each week and could not always be able to switch power manually."

A Trace (Xantrex) C-40, or C60 would do nicely... and there are other brands that should be fine too.  Anything that will work as a diversion controller (which is what you need here)

Or...if you have a Trace SW series inverter, they have some Aux relays which can be adjusted to turn a load on, or off... depending on battery voltage.  It's a bit more primitive, but works OK - that's how my system works.

Slip rings vs twisted cable. Which is better?  

I like the simplicity and reliability of the "dangling cord" - up here, even on short towers in turbulant sites, they dont need to be unwound very often.  I like to dangle the cord, and put a big, locking plug/socket on the bottom of it so that it's an easy matter to unplug it, untwist it.. and then your good again.  My current machine went up in January... I've not had to untwist the wire yet.

[Ungrounded Lightning Rod]

If you're building your machine yourself you might consider winding it for a higher voltage and stepping it down with transformers at the battery shack.

This means more expense (for the transformers) so it presumes you can find something suitable within your budget.  But losses in a given size of wiring go with the resistance times the SQUARE of the current.  So even a moderate boost in voltage can save you a lot in either lost power or less expensive wiring.  (Insulation is cheap, copper and conduit expensive.)

[armcroft]

Thanks Dan,

I guess the wire is hung inside the tower and comes out the bottom of the tube/pipe?

What size & type of cable do you use for this??

Thanks

Robert.

[DanB]

I've never tried that, but I understand that transformers dont work very well at very low frequencies.  A 10' wind turbine might hit cutin at about 12 hz or so.  You could design the machine with lots of poles to rectify that problem, but I suspect it'd be more compllicated.  Axial flux dual rotor machines need fairly large poles because of the large airgap - which forces the frequency to be fairly low I think.  I also wonder how any losses in the transformer might affect startup - but have no idea 'cause I've not tried that.

[TrotFox]

Just one comment reguarding DC vs. AC efficiency.  There's a reason power transmission is done using AC in the grid.  AC transfers with much less line loss than DC.

Trot, the electrically trained, fox...

[Ungrounded Lightning Rod]

I've never tried that, but I understand that transformers dont work very well at very low frequencies.  A 10' wind turbine might hit cutin at about 12 hz or so.  Yo could design the machine with lots of poles to rectify that problem, but I suspect it'd be more compllicated.

Naw, it'd dead simple.

The problem with transformers at low frequencies is that the CHANGE in magnetic field in the iron is proportional to the INTEGRAL of the voltage (i.e. the sum of the products of the voltage times the time it's held - or (for constantly changing voltages) the limit of that sum as you break it up into vanishingly small time segments).  Hold the voltage for longer (i.e. same voltage, lower frequency) and the magnetic field in the iron increases.  Iron can only increase its magnitization som much and then it "saturates".  So you need more iron for a given voltage at a lower frequency.

But a magneto is exactly the same thing inside out:  A fixed magnetic field inducing current as it cuts through coils, i.e. the field through the coils goes up and down.  The voltage in the coils is the DERIVATIVE of the magnetic field through them, and the intergral of the derivative of a function is the function itself (excluding an arbitrary DC term, which is zero in a motor/transformer hookup).  Increase the frequency and you increase the voltage to the same extent that you decrease the duration - leaving the magnetic field in the transformer cloning the magnetic field through the coils in the genny.

So if you hang a transformer on a magneto, the peak magnetic field in the core, REGARDLESS of the frequency, will be the same:  The strength of the magnets generating the power that's generating the transformer field.  If a given transformer gets to 90% of saturation at 60 hz, it will get to 90% of saturation with the same genny at ANY  frequency.  (Or it would if you used superconductor wire instead of copper.  Resistive losses decrease the field in the trnasformer, so the slower the genny goes the more the losses in each cycle, and thus the more the field in the transformer falls short of the field in the genny.)

So if you get a set of transformers that can handle your magneto at its fastest speed, they'll handle it at ANY lower speed.  And since they're usually rated at 60 hz (where the resistive decay is very small) and have some margin, they'll probably handle any speed at all, certainly any that won't tear your mill apart and send the fragments bulleting away.

It's easy to figure out if you made your genny from a motor conversion:  Get a set of transformers with ratings that would let them supply the motor at its rated voltage and frequency.  Remember:  The motor coils/core have the SAME saturation issues as the transformer.  Even stuffing oversize magnets into the motor will just saturate it.  So it can never generate enough current to saturate your transformers and cause a current spike in the wiring.