Xantrex C35 Dump Resistor?

[RonReagan]

I am at the final stages of planning my wind generator using a Ametek 38 (geared for more rpm's). The motor has a max of 12A and it probably will put out 100 watts. I plan to use the Xantrex C35 which should be plenty for what this motor can put out. I was just wondering if I were to use a air resistor what is a safe bet in terms of wattage and resistance without pushing the C35's limits and blowing it? For example if the C35 switched to dump at 14 volts would I need to run a parallel setup to achieve the correct resistance like 3 2ohms which would be 21A? Let me know if I am way off course. So far I have found that Digi-Key has these Ohmite ones, but if you can suggest better ones that would be much appreciated, thanks.

[RonReagan]

Sorry about the repost I clicked back accidentally, you can scratch that other one...

[elt]

Hi Ron,

The numbers don't make sense to me. If your mill will make 100 watts, then at 14 volts you only need to dump 100 watts / 14 volts = 7 amps

14 volts/7 amps = 2 ohm resistor.

I suppose you could dump at a higher rate, like 21 amps or even 35 amps ... then your resistor would either be 14 volts/21amps or 14volts/35amps.

... if I were you, I'd finish the generator and then worry the dump controller once you knew you're actual power output.

Hope that makes sense.

- Ed.

[RonReagan]

Yeah, I was thinking about just taking it down if the batteries started getting high by having a break-over pole type of tower or something. I guess you are right in the aspect of getting data first then adding anything needed if I can't always be there to watch it. Its just with school coming up I wouldn't be able to babysit it. I guess I will see soon.

[richhagen]

In general, you dump load power consumption should be larger than you maximum generation power.  It must of course also be less than what your controller's maximum current is as well.  I would probably look for a dump load which would draw about 15 or 16 amps for your setup, in other words about 250 Watts at a typical dumping voltage in sustained high winds.  My experience with dumping is more with solar, but the principles are the same.  

from Ohm's law:

Resistance = square of voltage / power  

If you wanted your dump to consume about 250 Watts at 15 volts then you would aim for about .9 Ohms total resistance.

If you are using resistors, then you can connect several resistors in parallel to achieve the desired total resistance.

Total Resistance = 1 / ((1/(1st resistance) + 1/(2nd resistance) + 1/(3rd resistance))

where the (1st resistance) represents the value in Ohms of the resistance of the 1st resistor.  The example is for three resistors parallel, but it can be calculated for any number of parallel resistors in a similar manner.  

For example four resistors each of a value of 4 Ohms in parallel would have a total resistance of one Ohm, which would be in the ball park of what you need.  

Keep in mind that each of those resistors would have to be capable of dissipating (rated at or above) a sizable amount of power.  If the maximum possible voltage was to be 16V or less this would be:

P = square of voltage / resistance = 16 squared / 4 Ohms = 64 Watts

That would give you a total power dissipation of 256 Watts at 16 Volts, and the current at this voltage, which would also be the maximum would be 16 Amps.  

Hopefully I didn't make any basic math errors, and hopefully this helps a bit for calculations based upon whatever resistors you have available.  Rich