higher voltage turbine + lower voltage bat bank = brake. Why?

[makenzie71]

why does running a higher voltage turbine on a lower voltage battery bank make the turbine act like it's running with the brakes on?  You guys told this was happening with one of my turbines but I never really got an explanation.  It's not a problem I'm trying to overcome...but someone asked me the "why" and I didn't know, but would like to.

[Adriaan Kragten]

Every rotor has a certain optimum cubic line which is the line which can be drawn through the tops of the P-n curves of the rotor for different wind speeds. The Cp is maximal for the optimum cubic line. A generator has a certain Pmech-n curve for a certain battery voltage. The matching in between rotor and generator is optimum when the Pmech-n curve of the generator has two points of intersecting with the optimum cubic line of the rotor which are lying not far apart (for matching see chapter 8 of public report KD 35). Next assume that the matching is optimal for 48 V battery charging. This means that the average charging voltage is about 52 V.

The Pmech-n curve for 24 V battery charging, so for an average charging voltage of about 26 V, is lying far left from the Pmech-n curve for 52 V. So this means that the Pmech-n curve for 26 V is lying far left from the optimum cubic line. So the tip speed ratio of the rotor will be much lower than the optimum tip speed ratio and the Cp which is realized by the rotor will also be much lower than the optimum Cp. The Pmech-n curve for 12 V battery charging, so for an average charging voltage of 13 V, is lying even more left from the optimum cubic line and so the real tip speed ratio and the Cp will be even more lower than for 26 V. The Pmech-n curve for short-circuit is lying left from the Pmech-n curve for 13 V but the distance isn't very large. Therefore, using a much too low battery voltage has almost the same effect as short-circuit. Measured Pmech-n and Pel-n curves for different voltages and for short-circuit are given in my public report KD 78.

An example of the P-n curves for different wind speeds, the optimum cubic line, the Pmech-n and Pel-n curves of the generator and the P-n curves for short-circuit in delta and in star is given in figure 4 of public report KD 484 of the VIRYA-3B3. The generator curves are given for 26 V star. However, this is for a generator with a modified 115/200 V winding. The same curves are valid for 52 V star for the original 230/400 V winding. The VIRYA-3B3 makes use of the generator for which the measurements are given in KD 78 for the original 230/400 V winding. So if you would draw the measured curves for 26 V star in figure 4, you will see that the matching is much worse than for 52 V star (for the original 230/400 V winding). The generator hasn't been measured for 13 V star but the Q-n curve for 13 V star is lying about in the middle of the Q-n curve for 26 V star and the Q-n curve for short-circuit in star. The Pmech-n curve can be derived from the Q-n curve if you use the formula Pmech = Q * pi * n / 30.

[MattM]

The battery essentially locks down the voltage.  Until the battery hits it's limit the battery will keep the voltage locked down.  The battery resists the higher voltage because any excess voltage becomes the charging voltage.  When battery is charging electrons flow from cathode to anode through the separator and current flow from anode to cathode.  When you discharge that battery, you are reversing the charging process and using the potential electric energy to drive other electric components.  The battery potential has a limit, so eventually it cannot clamp down a charging voltage indefinitely.  That is why forum members talk about resistors for a dump load to protect your batteries.

[MagnetJuice]

I am going to try to explain this in a simple way. With an image.

A battery is NOT a resistor but it has internal resistance.

Let us assume that a battery internal resistance is 1 ohm, then:

Since the batteries are in series, it is the same as having resistors in series.

The lower the battery voltage, the closer it is to a short circuit, or a brake.

I hope this image will help you visualize it.




I hope this helps.

Ed

[makenzie71]

Thanks Ed!  That does help...and, actually after reading your response I was able to make more sense of Adriaan's explanation lol...I have to build up to his level of intelligence

[MagnetJuice]

Mak, the image above can also help you understand what happens when you add resistance to the line.

Imagine that you add a one ohm hi power resistor to the line of the 36 Volts battery bank. Your turbine senses that as being a 48 Volts battery bank.

That would be useful if you want to speed up your turbine or keep the blades from stalling.

Keep up the testing. People are learning. 

Ed

[MattM]

And what about adding batteries in parallel?

[Adriaan Kragten]

If you connect two 1 Ohm loads (or batteries) in parallel, the resulting resistance is 1/2 Ohm.

[Adriaan Kragten]

Thanks Ed!  That does help...and, actually after reading your response I was able to make more sense of Adriaan's explanation lol...I have to build up to his level of intelligence

The picture of Ed may have helped you to understand the problem but assuming that a battery has a constant resistance is far away from reality.

If a battery is charged with a current I and a voltage U, the load resistance R of the battery can be calculated by: R = U / I (Ohm). If the charging voltage U is only a little higher than the open voltage of a 12 V battery, you will get only a small current I. This means that the load resistance R of a battery for a low charging current will be high. If the charging voltage U is only 2 V higher than the open battery voltage of a 12 V battery, you will get a very high charging current I. The load resistance R of the battery for a high charging current is therefore low. So the load resistance depends very much on the current.

If you want to compare the matching in between rotor and generator for different nominal battery voltages, you have to use real measured Pmech-n curves for different average charging voltages and find out at what the real tip speed ratio is for the point of intersection of the Pmech-n curve of the generator and the P-n curve of the rotor for a certain wind speed.

A problem with the determination of the resistance of a battery is that a battery generates its own voltage. If one takes only the voltage difference delta U in between the charging voltage and the open voltage, one can define the internal resistance of the battery Rint. Rint is given by: Rint = delta U / I. But also for Rint one finds no constant value as the open voltage depends on the charging state of the battery and also very much on the time which has passed after charging and after opening of the contacts. One has to wait at least ten minutes to find the real open voltage. Rint is determined by the resistance on the lead plates in the battery but also by the resistance of the sulfuric acid and this last resistance depends on the acid rate.