Wiring and Resistors to turbine

[quest0111]

I have a Dan built 10' 48 volt hd turbine at 75'. First used #8 from tower to rectifier 140' away and #10 for 80' up the tower. It was stalling badly so I switched to #10 wire to the tower. It produces better, but still stalling. I want to change back to #8 wire and add resistors after the rectifiers.

Do I install only on positive cable which is #8?

I want to start with 1 ohm.

How many watts will the resistor need to be?

Should I plan on using 2 ohms?

Thanks,

Barry

[imsmooth]

Let's say you want to try a 1ohm resistor.  First, determine the maximum current you expect on the wire.  Let's say 20A.  20A^2 x 1 ohm = 400W

You can get this wattage by adding resistors in series and parallel.  Two 2ohm/250W resistors in parallel will give you 1ohm/500W.  You will need equal resistance on the POS and NEG leads of the rectifier.

Ebay has some nice power resistors that you can usually get at a decent price.

[quest0111]

So at 50 volt 20 amps I would only need 500 watts?

[imsmooth]

You need the voltage drop across the resistor.  Your resistor wattage is determined by its resistance and the current going through it: W = R*I^2

If you have a load across your generator and have 50v @ 20A (this would be a 2.5ohm load) then this is 1000w.  However, the generator is actually putting out more: wattage is lost in resistive losses in the stator and through the tower wires.  Let's say for this example you have a resistance of 2ohms for the stator and wire.  This would mean the generator is actally making 2*20^2 = 800w more power than you are harnessing.  You are making 1800w, but you are only able to utilize 1000w. THe generator would have an open voltage of 90v (20A*2.5 + 20A*2 = 90v).

If you added 1ohm of resistance to your wiring the total resistance is now 2.5ohm (load) + 3ohms (wiring/stator/resistor) = 5.5ohms.  Your current is now 90v/5.5 = 16.36A for the same RPM.  This is a smaller load on the generator.  The power dissipated by the resistor is 1ohm*16.3^2 = 265W

I would take the maximum current you expect and use this to size your resistor wattage requirements.  Using the 20A example, a 400w resistor would work.

[ghurd]

I don't get the logic in equal resistance in the Pos and Neg leads.

G-

[quest0111]

Should my amp meter be before or after the resistor or does it not matter? Thanks for the information. I'm looking at 1.3 ohm adjustable resistors 500-1000 watts.

[Madscientist267]

Sometimes scary aint it? People working with batteries that store significant amounts of power that don't even understand the very basics... sigh Makes you wonder what else they have going on. Not understanding that resistance in series is resistance in series is minor, but I just hope that the words 'fire extinguisher' are in their vocabulary to deal with the things that aren't...

[TomW]

G-;

Yeah, the equal resistance in both had me wondering, too. Maybe its like "new math" ?

I would also add:

If it needs to dissipate 500 watts, give it some headroom. I like 100% but even 20% might be good.

In laymens terms "Double the wattage or at least add 20% [600 watts in the 500 watt scenario] or more".

Tom

[imsmooth]

I guess I should have been more explicit and mentioned the practice of oversizing.  I thought 400W, being 50% more than 265W was sufficient.  Using the 400W requirement in the beginning, I thought 500W was sufficient, being that it is 25% greater.  Maybe this is not enough.

With regards the the equal resistance, that is just a mistake on my part and I apologize.  I was somehow thinking of balancing phases, even though I was talking about the DC after the rectifier.  

[umagrama]

It doesnt matter where you put the amp meter. The same current that flows into the resistor will be flowing out of the resistor. You will get the same measurements.

I'm trying to understand your issue, why are you adding resistors by the rectifier? Wouldn't that consume some of the power generated?

I might be missing something...

[quest0111]

Because it is stalling, as soon as the amps rise the alternator over powers the blades and slows it back down, almost immediately.

.

[ghurd]

Have you tried opening the air gap, just a hair?

G-

[quest0111]

No but in the manual it says I will need resistance with this set up. So I will try that first.

[Flux]

The difference in resistance with your change of wire from #8 to #10 only comes to about .11 ohms. It sounds as though you need more to get away from stall. You can try 1 ohm if you have a suitable resistor but it is a gamble to go out and buy one at this stage.

Far better you find out what resistance you really need then go out and buy the correct one. Use some scrap cable or something to get a feel of the resistance you really need.

When you find the value you need then from your tests you know your maximum current then you can work out the resistor wattage from I^2R. You can put the resistor anywhere in the dc circuit but it is logical to put it in dc positive as you will probably have grounds associated with dc negative.

If it is a 10ft machine and does about 1kw peak then 1 ohm will loose you about 400W and that may be enough to get you clear of stall so if you have to dive in with a guessed figure 1 ohm is as good a starting point as any.

Flux

[Boss]

Sometimes you guys nitpick on details and neglect to delve into a potentially interesting subject such as this started out to be.

For instance: Why would we want to lose wattage from our alternators?

Surely correcting the stalling  issue can be done before the alternator makes too much power.

I like the air gap adjustment, but it means removing the outside rotor again. This is difficult and time consuming project,

I'd like to see some of the experienced builders here suggest some tricks for increasing the air gap specific distances.

If I add a standard flat washer (if there is such a thing) to my 10 footer, making my air gap, say .10 inches greater than it was with just the nuts on the all-thread holding the two rotors apart, what kind of results can we expect?

Of course you can't tell me for certain, because you don't know what level of tolerances we have in our fabrication, but what was your experience?

   

[umagrama]

I agree with the above as far as trying to eat up power for the stalling problem. An increase on the air gap sounds much more appropriate solution.