Added resistance...What am I missing?

[Seekscore]

I posted my power graphs a while back. I'm trying to get a little bit more power in the higher winds. I know the cause of my problem, if you want to call it that, is that my blades were made too fast. I've got some new CAD drawings coming and another set of blades I am going to try. I've just got to make a new blade hub mount and balance them before I put them on.

Anyway, it was suggested to add resistance to the line. I purchased a garage door spring that was about 25 inches long. I checked the resistance and it came out to be around .5 to .6 ohm. I read the sticky post about adding resistance to the line and saw where Chris had a good idea about mounting the spring between two bolts on a board. My thoughts were that I could use a large clamp to adjust how much of the spring I used as resistance. I had a lull in the wind today so I stopped the turbine and installed resistance in the line. I put the resistance on the DC side after the rectifier and after the current shunt. Basically the resistance was between the current shunt and the battery.

I turned on the turbine and there was no wind to speak of. A couple hours later I looked outside and the wind had picked up pretty good. Around 20mph. The blades were moving pretty fast. I went out to the shed where my control panel is and it didn't smell good. The paint that was on the spring was burning off. I guess that could be expected. I looked at my data logger and the voltage was 38VDC. RPM was around 280, which is higher than I have seen to date. I watched it for a bit and the voltage came up when the wind did. I saw the voltage get up to around 42VDC. Not good for a 24VDC system. I hated to do it, but I hit the stop switch. So I am sitting there thinking WTF? I disconnected power to the TS-60 charge controller thinking that maybe it had a hiccup or something. I let it sit for a couple of minutes. Hooked it back up and turned the turbine back on. Voltage quickly came back up to 37VDC and the charge controller was dumping again. So at this point, I'm thinking the controller isn't working. I stopped the turbine, removed the resistance and now the voltage is around 27.8VDC and dumping into the load at around 30A. Controller is working.

Someone please school me. WTF is going on? How would adding resistance the way I did cause the voltage to go crazy? So I'm sitting here thinking and about the only thing I can think of is maybe I had a bad connection to the resistance. Could it have been a bad connection on the resistance not allowing the controller to dump the right amount of current?

Mike

[joestue]

it sounds like your dump load is connected to the battery but its voltage sense line is connected on the turbine side of the big resistor you installed... the same place you're reading 42 volts.

[ghurd]

It sounds like the volt meter / logger is on the wrong side of the spring.
Need to measure at the battery terminals.

Spring Volts = Charging Amps x Spring Ohms
Turbine Voltage = Spring Volts + Battery Volts.

Sounds like the spring resistance is too high too.
Takes a while to get a 24V battery to 30V, especially when it is dumping 30A,
So I have to think the spring has >12V on it.  That seems kind of high to me.

Might connect both ends of the spring together, clamp at the middle center.
G-

[commanda]

30 amps and 1/2 ohm = 15 volts.

27 volt battery + 15 spring volts = 42 volts.

Like others suggested, I'd say you're measuring your volts on the wrong side of your spring.

BTW; power = I^2 * R
= 30 * 30 * 0.5
= 450 watts.

I'm not surprised the paint on your spring was going pyro.

[Seekscore]

I didn't measure the voltage with a meter off the battery. I just looked at the data logger. The data logger gets the voltage off the shunt. That is probably what I am seeing. I may try to put the resistance in between the rectifier and shunt. I am probably going to get a big wire wound resistor to use instead of the spring. I didn't anticipate the heat that is dissipated. This is one of those weekend projects that I decided to do at the last minute and use what was handy. I'll see if I can get a big resistor tomorrow.

Here is a quick functional drawing of my setup. Top picture is normal and the bottom is the line resistance.


Mike

[Flux]

Put your resistance between the rectifier and the shunt. The data logger must see the true battery voltage. If the shunt is the voltage reference for the logger, it must be tied directly to the battery.

Flux

[ChrisOlson]

I didn't measure the voltage with a meter off the battery. I just looked at the data logger. The data logger gets the voltage off the shunt. That is probably what I am seeing. I may try to put the resistance in between the rectifier and shunt

Hi Mike,

As everybody said you need to move your resistor to the other side of the shunt and have the controller look at battery voltage, not turbine operating voltage.

That big spring is very handy to "tune" the turbine for best output because you can move the clamp along the spring until you find the place where the turbine runs the best.  Then measure with an ohmmeter to that point and that's what you need for a resistor.  I have also done this in the AC lines coming from the turbine with three springs, and it works well there too.

I have heated those springs almost to glowing doing that and have never burned one out.  The spring is less "springy" after heating it, but the resistive properties of the spring steel doesn't seem to change after being heated.  They make a very tough resistor.
--
Chris

[Seekscore]

The springs did get hotter than I was expecting. If I use it much more, I am going to have to do some redesign. I had it mounted on a 2x6 with some 3/8" bolts. It only had about an inch of clearance between the spring and the wood. Pretty scary to leave it that way unattended.

Thanks to everyone who responded. After looking at my drawing and hearing what people said, I see why the voltage was up. I didn't take the time when I saw the voltage that high to measure the battery. Not quite panic mode, but was thinking the worst. I am going to go see if I can get some resistors after lunch and make another attempt.

I looked at the three hours or so of data I got while it was attached. It did get my RPM up quite a bit. From 250 max RPM to around 320 RPM with the line resistance added. I can't tell what power improvement there was because of the inaccurate voltage level I was seeing.

I always said that you learn more from your failures than you do from your successes, so I guess I am learning an awful lot...

Again, thanks for the help.

Mike

[ChrisOlson]

The springs did get hotter than I was expecting. If I use it much more, I am going to have to do some redesign. I had it mounted on a 2x6 with some 3/8" bolts. It only had about an inch of clearance between the spring and the wood. Pretty scary to leave it that way unattended.

Yes, that works temporary to stretch the spring out so the coils aren't touching.  But definitely do not leave it unattended.

It would be very interesting to see your data on this.  In probably, I'd say, 90% or better of the times I've tried using a resistor the turbine puts out more power but it's all burned up in the resistor and no extra power gets to the batteries.  It will be quite nice to see if you are able to tune it so you get a net gain into the batteries.

If the resistor could be used for water heating or something, then you definitely get a net gain from the extra power.  As you found out, the resistor dissipates quite a bit of heat.  I've had large springs like that just about glowing, which is a lot of power being dissipated.  It's hard to find 1/2 ohm heating elements for a resistor.  I've had thoughts about building a Point-Of-Use water heater under our kitchen sink and putting a homemade 1/2 ohm resistor in it and running all the turbine power thru that resistor/heater.  But getting that to pass any sort of inspection for our insurance or anything else is probably not going to happen.

--
Chris

[galeforce jones]

On ebay you can get ohmite power rib edge wound resister with adjustable clamps, good for 1000w. Or you could use stainless steel wire rope with a grip for adjustment if I remember correctly 4mm diameter of 7/19 s/s wire rope 1m long gave resistance of 0.5 ohms, you could just let it hang into a bucket of water for cooling.