Sparky question

Can one of you in the know explain this to me:

I have a 24 volt genny, if i connect it to a 12 volt battery it then by magic become a 12v genny. If i connect it to a grid invertor, will it produce 24 volts or something else?????
Reason i am asking is that lots of the 100 to 200dc v grid ties around but assume that my genny, being a 24v will not work them, then my mind gest stuck in the whole its magic thing'.

Is it a power thing, so v=ir, r is constant p=iv, power is what it produces, no all lost, can someone explain?

Chris

My Swea grid tie is weird!
It says on the box that its 0-58v
Now what I think its done is clever,I connected my two solar systems to it at 32v and that has become its cuttin. :?
If I'd connected two 12v batteries first at at 24v I dare say that would have been its cuttin. :mrgreen:

V is related to RPM.

A 24V genny, that makes 24V open at 400 RPM:

1) Will make 12Vo at 200 RPM.
Which will hard stall if proper blades designed for 24V cut in at 6MPH.

2) Will make 240V (factor of 10) at 4000 RPM (factor of 10).
Which will happen at 60 MPH (factor of 10).

Will make 48V at 800 RPM at 12 MPH (factor of 2)

3) In real life, this is getting to the point of Off-the-Reality-Scale.
It may not make 240V, even well past 4000RPM.
If it does make 240, it will have issues with inductive reactance or the core material's B-H curve limiting the output.

Best to get a 100~120V genny for grid tie.
Exponentially less transmission losses.
Exponentially less conversion losses.
Moving amps is the killer. Volts move by themselves! LOL
P=IIR

Possibly something like Shawn's motor conversions?
"All volts and few amps" is not always bad.
We want Amps when charging a 12V battery (because AMPS x volts = Power).
We want volts when grid tied (because VOLTS x amps = Power).

Poor explanation, but the concepts are there.
G-

Ghurd, not a poor explenation, i am starting to understand, taking the specifics of my 24 v:
258rpm / 1A / 26V
400rpm / 10.2A / 29V
490rpm / 34A / 33.6V

Now connected to a 12V battery (as I took the other out recently for my caravan - needs must!)

Can I assume that it now produces 12 V at something around 125rpm, is the rpm capped by the battery so that I will never gut much current in it, thats what I would expect from your explenation, but dosen't quite fit with what i have seen, with new blades voltage remained low (about 15V) but sounded like a lot of power going into the battery. Is this where the moving current bit comes in, passed the cut in speed of 12V or so, produces current, voltage capped but speed increased so current increased, problem being I have forced load on the blades too early for efficient use?

So if I can find a grid tie to use, I want a cut in voltage of 26V (as that is a fixed and optimised voltage with the baldes spinning at an efficient speed for voltage / power production) going past 33V.

If I bought the futurenergy Windmaster 500 with an input of 35 to 150VDC then I wouldn't hit that voltage until the blades were at oer 500rpm, taking it past the design of the PMG, so absolutley useless for me, ie down voltage is possible but not efficient, up voltage is not probable or safe.

Close?

Chris

Additional bit of digging, the 1KW blades I have bought (Windsave) have an operating range listed as 100 to 800rpm, the Futurenergy have a range of 300 to 900rpm. Windsave genny produced 1KW at 12m/s, Futurenergy produces 1KW at 12.5m/s, so in the scale of everything else they seem similar enough for me to ignore the difference as the futurenergy range is within (all but at the top end) the range of the one I have, I assume a slightly different efficiency curve, but nothing to get hung up about.

Chris

Well after a lot of searching and struggling to find a grid tie for a 24v pmg (futurenergy use the 48v one) I just bought a grid tie, 600W with what appears to be the right spec for my turbine, hope so anyway, cost of £110:

Brand Power Jack
Model No. PSWGT-600
Output Power 600W
Output Wave Form PURE SINE WAVE
DC Input Voltage DC 20Volt – 55Volt
(solar panel/wind turbine)
AC Output Voltage AC 190v-250v
Frequency 50Hz
Efficiency 90%
No Load current Draw 0.7A
Distortion 2%
Temperature Protection 55C ± 5
Net Weight 3.0 kg
Made in China

Next thing is to stop it going over 600W, I have been advised by a very helpful gent on ebay that i should use a frequency switch to control the speed, sounds like a good idea from what i can understand, any thoughts?

Chris

You might be in the same situation as me with voltage I think you might find that your PMA is capable of generating more than 55v.I tested mine in windy conditions with no load and it hit 72v,you might find yours will do the same and fry your inverter.I'm in the process of limiting my voltage with sla alarm batteries. :(

Chris,

The trouble you are having in the conceptualization of Volts/Amps is understandable. You simply need to view it from several slightly different points of view and realize that there is a bit more to it than P=IE. There are numerous factors involved in power generation, and many ways of calculating the unknown variables. In theory an inductor (coil of wire) responds to a change in current with a change in voltage. Mathematically that is V = di/dt; more specifically V = L*di/dt. In the specific example of the coil in a PMA the relationship is further complicated by RPM and Flux Density, but the relationship between voltage and current in the coil remains pretty well intact w/o the added complications.

There are mitigating factors in any generator/alternator design that define its performance curve, but assuming you are in the fairly narrow region of its design specifications, the power output is going to be some reasonable percentage of the input power; as you move away from the design specifications, efficiency degrades, frequently a lot. Given the input power and the efficiency the two numbers that you are interested in are "open volts" and "short circuit amps". These numbers determine suitability for a particular purpose. If your "open volts" at 400 RPM = 29V, then this voltage can be "clamped" to any voltage less than 29V. The lower the clamping voltage, the higher the Amps. This assumes that the 400 RPM is constant, (which is rarely the case) and the input power is variable. In the real world the input RPM is frequently a function of the load and the load is also a function of the input power; that is in the case of a PMA HAWT, the swept area and the wind speed determine the maximum input power to the PMA. The design of the turbine determines the efficiency with which that power is translated to the PMA and the RPM at which the PMA will rotate. As the load on the PMA increases, it slows the turbine's rotation down, which in turn decreases the PMA's ability to supply the load. These interdependent forces are difficult to calculate, but fall very naturally into equilibrium, so we tend to use empirical measures to determine a wind turbine's performance. These empirical measures are "Open Volts", "Short Circuit Amps" and Watts (V*I) at a particular wind speed. It is easy to calculate a particular wind turbine's efficiency at a known wind speed by measuring the output power, very difficult to predict the output power or efficiency by analyzing the turbine's design.

I am not saying that careful design of a wind turbine system that performs close to design specifications at a particular wind speed is impossible, simply that the math is a great deal more complicated than the average DIYer is up for. It is far easier to make assumptions based on known designs and stick close to proven systems; then build the system and "tweak it".

One of the complications of PMA design for HAWTs involves the Volts/Amps?RPM relationships. Assuming our HAWT build is complete, and we know the no-load TSR and swept area, we can then attempt to maximize the PMA efficiency. If our goal is to charge a 12V battery, then we should design the PMA for 12V at a nominal cut in wind speed. If the goal is to produce 120V/240V 60hz AC then we should consider a much higher Voltage for cut in.

The open voltage of a PMA is a function of RPM, flux density and coil turns (and in all but the simplest designs, coil configuration). Assuming good magnets and a known RPM this leaves us with coil turns and configuration. The more turns per coil, and the more coils connected in series, the higher the voltage. For a fixed coil volume, more turns means smaller diameter wire. Smaller diameter wire means more resistance which in turn can mean more "copper losses". Additionally you need to be aware that as the number of turns increases the faster the voltage climbs with respect to RPM. Keeping all of these variables in line is a balancing act, and the solution is typically a series of compromises that leads us to a solution with the highest overall system efficiency. It does little good to design a super efficient HAWT/PMA if we then connect the output to an inverter that is only 45% efficient at our typical output power level! By the same token, if we design our PMA to be at peak efficiency at 200 RPM, but the prevailing winds typically turn it at 500 RPM then our PMA losses could be tremendous!

To achieve an efficient design requires careful consideration of each component in the system, not only by itself, but also in the way it interacts with the other components in the system. Of course, there is some credence to the philosophy: “Any power I get from the wind reduces my carbon footprint, so I am just going to assemble what bits I can salvage.” But, for me, the overall efficiency of a system is a noble pursuit; salvaging bits that likely have 10X more kWhs vested in their production than they will ever produce is a bit like Don Quixote’s approach, whom, by the way, was quite happy in his pursuit of windmills.

If our load is a simple resistor, then as voltage rises, current rises and power consumed increases by P=I^2R. If our load is a battery, essentially no current flows until it rises above the battery voltage. Once the battery voltage is reached current begins to flow into the battery effectively clamping the input voltage at some voltage above the battery voltage (determined by the battery’s internal impedance). As long as the current flowing into the battery is limited to a “safe level” for the battery, the input “open volts” (above the battery voltage) are of trivial import. The biggest hurdle most people face is attempting to get the voltage high enough at low RPM to charge their battery bank. If your battery bank is 48V you would want to design your PMA a bit differently than you might if your battery bank is 12V. Or, you might resolve the issue by charging your batteries in parallel (6V or 12V) and discharging them in series (48V). Both approaches are viable solutions.

Hrmmm, I do not know if I have clarified the Volt/Amp relationship for you at all, but I have to head off to work, so I will have to stop this for now.

Fish

Chris,
Looks like I had a LOT of thoughts.

"taking the specifics of my 24 v"...
Sorry, those numbers do not make any sense to me.
Guessing they came from a sales brochure.

The voltage is capped by the battery.
As the RPM climbs, the amps increase.
The battery voltage, amps, and losses sort of control the RPM to some extent (which is why an unloaded windmill goes so much faster).

You understand shorting it to a stop. That is 0V output.
Using a 24V as a 12V is basically running it 'half shorted'.

"with new blades voltage remained low (about 15V) but sounded like a lot of power going into the battery."
That is the sound of stall.
Uh... like the blades are not smoothly and efficiently interacting with the wind... the wind is whistling around them???
Might have a look at this guys experience, with special notice of comment #4 "SSSSSSSS".
He details some symptoms in the earlier posts he linked to.
http://www.fieldlines.com/story/2009/11/30/194624/37

Side note: Overspeeding blades tend to make a "ZZZZZZZ" sound. LOL
After you have heard both a few times, it is fairly easy to tell the difference. Honest.

I have no idea how that GTI is designed.
Best guess is the wide input voltage range allows it to peak at 600W output.
When the input voltage is 55V, output is 600W, input is 660W...
660W/55V=12A

Now the fun part. :twisted:
Your PMA numbers are a bit deceptive.
See my comment here:
viewtopic.php?f=32&t=3124

Take that Presto Crap thing for example. It can only make 7A at 1400RPM.
(after 1400RPM the amps actually go DOWN which I did not notice before)
So their 1683W windmill has 0% chance of overloading your 600W inverter.

Yours, at 490RPM, with 0A of output, the open V is only 33.6V.
Pull 12A out at 490RPM, and the voltage will drop like a rock. So will the watts. So will the amps, after the voltage drops. Many seem to be self balancing.
I do not intend to imply your PMA is junk, but all iron core PMSa will exhibit the same characteristics to some extent.

Most of the no-name unapproved Chinese GTIs limit the input amps. Maybe yours is limited to 12A.
When it reaches 12A, it does not allow more amps in. This allows the PMA to run at a higher voltage.
Reasonable chance your PMA would need to operate at 12A and 55V (at the SAME TIME) to overload the inverter. Reasonable chance your PMA can't do that.

The problem will be if the grid goes down.
The inverter has no load. The windmill has no load. The windmill has no load so the blades spin a lot faster. The windmill's Vopen goes up fast, maybe exceeds 55V and smokes the inverter.

It would be much easier to make some kind of snubber circuit to hold the volts down.
A frequency switch is Rube Goldberg at a minimum, and a bad idea at the worst. Who cares how fast it is going???
Shut it down at 55V, which is about 800RPM? Which should happen at about 3X the cut in or 18MPH. Seems self defeating.

Need to control the voltage. I made a few for larger windmills of the 17' class. Overvoltage conditions triggered a relay to shut the windmill down until manually reset. Fairly simple. They work well.
Might google Roger Stafford or Dan Lenox.

If this is a no battery system, there are simpler ways to do it. I prefer not to publish them publicly.
Email me if you want a sketch.
10,000 ways to skin a cat.

Shawn and Chris,
Limiting the input voltage using batteries is going to be a very bad idea. Especially if the grid goes down.
If the grid goes down, the batteries can go to a very high voltage in seconds.
If the battery is 48V, the GTI will run them down to like 5V each.
If the battery is 24V, then the inverter input voltage will be clamped to the battery voltage possibly limiting the output to less than half what it could be.
Naturally, that is related to non battery based GTIs. Most are somewhat based on loose MPPT algorithms which will be comprimized doing silly things the designers did not intend.

Shawn,
Might have a look at Mertz's GTI endeavors. That Chinese thing he is using is intended for a 12V battery bank. The retailers obviously have no clue how to use it and distribute very faulty info.
But the inverter's design is pretty good.
Wish they'd get it UL Listed. I'd buy one (or more).

Blisters on all both my typing fingers...
G-

[Gotwind Ben]

Wow,
You Americans enjoy typing, I personally like good images do the talking.
It's good to see such dedication, I appreciate it at this end :)