Meter Behavior on Wind Turbine

[Tom in NH]

I hope some of you experienced wind people can tell me if my observations are what I should expect. My turbine is spinning nicely in the wind. I have an analog ammeter connected  to see how much current is flowing into the batteries. Even though the turbine appears to be turning at a fairly constant speed, my meter is jumping all over the place. Most of the time it's down around .5 to 1 amp, then it bounces up to 2 amps and back, then up to 5 amps and back. It never sits still. Is this the way its supposed to look? If so, how in the heck does one ever get an estimate of the average charge current? Thanks for any comments. --tom

[Flux]

Assuming you don't have an electrical fault, this is fairly typical of what you would expect on a turbulent site. I also suspect that you are running well stalled, the current fluctuates more under these conditions.

Current from wind is never steady, I often marvel at the precision of currents quoted here, that may be the case with a tower about 80 ft tall but I never see these conditions. Fluctuations from 2 to 30A within seconds are not uncommon.

It is this intermittent nature of wind that lets many of those badly overloaded machines survive. You may have peaks of 1Kw and an average power of perhaps only 300W.

This variable current is one of the things that make charge controllers less than ideal. The charge algorithms for batteries are based on charging from a constant source at regular intervals. When you are charging and discharging at changing rates as and when you can, it is difficult to keep track of state of charge.

Flux

[CompDoc]

Tom, are you measuring across a shunt or are you using the meter in-line with the charge current?  The best way is with a shunt and use the meter to measure volts across the shunt.  Ohms-law applied... I=V/R

[TomW]

tom;

Yes, it is completely normal to see the ammeter all over the place. At least here in my setup it is.

Cheers.

TomW

[Nando]

Ton in NH:

Indeed this is a natural behavior with winds with high variations in short time periods.

This is one of the problems why so many designers of MPPT charge controllers can not make them work properly and harvest the peak powers that may fluctuate in just few seconds, and many times fluctuating at the wind mill gyroscopic time constant and how good the tail controls this wild behavior.

In one of the 20 KW I worked on, we noticed variations 10 to 15 KW in 5 to 30 seconds elapsed time, which indicated a quite gusty behavior, and the other wind mill showed steady constant peak power of 10 KW even though the separation between the two wind mills was around 200 meters with a small mound between them and the mound partially shielding the arriving wind ( like 10 to 20 %), we were thinking in changing the placement of the larger wind mill until the data showed that the wind arriving at that angle was rare and short lived, we could not place the wind mills on top of the mound because there were towers for other purposes.

Nando

[Tom in NH]

I took the 5 amp peak reading on the meter (yes, it was a shunt), and worked backward through the calculations to estimate a windspeed. Figuring a 14 volt battery voltage, I was looking at 70 watts. Using Betz of 59%, area of 4.32 m^2, a perfect turbine would produce that much in a 7.78 mph wind. My anemometer showed gusts of about 13 mph so I guess the turbine is within reasonable limits, about 60 percent efficiency right now.

The formula I used was

vel.= [volts * amps) / (Betz * .05472 * area in sq. meters)^.33

or

v = [(14 volts * 5 amps)/(.59 * .05472 * 4.32 sq. m.)]^.3 = 7.78

7.78 / 13 mph peak gust = 60% Not too shabby IF the Betz limit applies the same for VAWTs as it does for HAWTs.

Thanks guys, for your help with the meter behavior.

-tom

[Ungrounded Lightning Rod]

By the way:  A charging application makes the phenomenon more extreme.  Charging current in a genny/wires/diodes/battery system is essentailly proportional to RPM over cutin, not to RPM.  So a small change in RPM can mean a big change in current.

[Tom in NH]

Lightning Rod, I always appreciate your interesting perspectives, such as this one and your other comment in my VAWT posting. I'll have to chew on both of them! --tom

[Ungrounded Lightning Rod]

Glad I could help.  B-)