Sound like normal operation?

[makenzie71]

So I have my larger turbine running through a bridge rectifier and to a 24v battery bank.  No load on the bank the turbine spools up and is regularly kicking out 250~350 watts, voltage varying between 26 and 32vdc.  The battery bank is wired to a 20~45v micro grid tie inverter.  When the inverter comes on and loads the batteries turbine output drops to 15~50 watts (we only have 8mph of wind right now anyway) and voltage drops to 24.2~24.8vdc where it stays stable for quite some time (2 hours now since I flipped the switch this morning).  If the wind dies completely the inverter tends to error out when battery voltages gets close to 23vdc.

Does this sound like at least somewhat normal operation?  I'm mostly asking because it seems, to me anyway, that the wattage drop under the load of the inverter is pretty steep.

[Adriaan Kragten]

The large difference in power is probably caused by the different voltages. The rotor has an optimum cubic line and the generator must be loaded such that the Pmech-n curve of the generator is lying close to the optimum cubic line of the rotor. The Pmech- curve shifts to the right if the voltage is higher (see report KD 78). If the generator is loaded by only a 24 battery, the voltage at high power can become very high if there is no battery charge controller with dump load which limits the voltage. The voltage becomes that high that the Pmech-n curve has been shifted that much to the right that it lies close enough to the optimum cubic line to realize an acceptable matching and so an acceptable Cp and power. However, if the inverter is coupled to the battery, the charging voltage is that low that the distance in between the Pmech-n curve of the generator and the optimum cubic line of the rotor becomes too large. So the generator will be loaded too strong to get an acceptable matching and so an acceptable Cp and power. I think that the power will go up strongly if you use a 36 V battery, at least if your inverter can be used for a nominal battery voltage of 36 V.

[makenzie71]

36v is well within the operating range.  I'll try it out and see...I think my third battery is a little weak but should be enough to at least test the theory.

[makenzie71]

I switched the bank over to 36v and the whole setup did seem to work a bit better today.  I did 1.6kwh of production back into the grid from the two mills vs the .6kwh yesterday with more wind.  Seems to be the right direction.

[Adriaan Kragten]

36 V is an unusual battery voltage. May be your wind turbine is meant for 48 V battery charging and in this case the output will probably be higher than for a 36 V battery. But the rotational speed and the tip speed ratio for a 48 V battery will be higher and the rotor may therefore be noisier.

[makenzie71]

Luckily they're 100 yards away so the noise isn't too concerning.

One of the turbines did better charging batteries at 48v but pretty much had the same output and performance running the inverter.  The bigger of the two free-wheels when I tried running it to batteries at 48v, but when I let it spool up unloaded output is 60~70vdc.  My current inverter won't support a 48v battery bank.  I have another one coming, though, that has a 20~60v input range.  I'll try running both rigs off a 48v bank when that one gets here and see if there's any real difference.