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Warpspeed
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Posted: 06 June 2017 at 8:58pm | IP Logged Quote Warpspeed

That uprated alternator worked fine. I had no idea it was such a simple conversion, I thought it must have been rewound.

Anyhow, more thoughts on how to electronically switch this voltage doubler in and out.

This idea is completely untested, but should work as far as I can see.
The mosfets will need to have isolated gate drive, but when they both turn on together it will force the the three diode bridges into a series connection.

Once the wind turbine is up well above cut in speed and producing serious power, all the current is carried only by diodes, both mosfets being off.

I expect that voltage doubler mode would only be used below usual cut in speed, so the mosfets should only be seeing minimal current, so they and their heatsinks need not be huge.



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Posted: 06 June 2017 at 9:31pm | IP Logged Quote Warpspeed

Just had an even more interesting idea.

Our F&P has multiple coils per phase, so we are not limited to only switching three phases series/parallel for just voltage doubling.

If individual coils, or groups of coils have their own independent bridge rectifiers, we can stack more than three bridge rectifiers in series, and switch them in and out in groups using the above mosfet switching system (assuming that it works as expected).

It may be possible to get much higher voltage multiplication than just doubling, and more voltage steps.

The advantage is that at low wind speeds we only have low power, so the current in the series connected windings will not be high, but we can get a much higher voltage at very low rpm.

As rpm and power increase, we are running more of the the windings in parallel which not only reduces the output voltage, it multiplies the output current without overloading individual coils.

If this can be done, and properly controlled, it could be an excellent solution to matching the alternator to the turbine over a much wider operating range.

I do not have a turbine or F&P here to play with, but if someone else here can pick up the ball and run with it, that would be great !



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DaveP68
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Posted: 07 June 2017 at 12:48am | IP Logged Quote DaveP68

The sum of all 3 phases at the star point with respect to all 3 phases going to the rectifier is always at half of what is feeding into the rectifier. This occurs even during excessive load so going into saturation makes no difference. The star point is at the middle of all three phases electrically.

I've measured the AC current between the star point and the extra 2 diodes and it comes to ZERO even under max loading conditions. Maximum current that I got out of the test 80s standard star stator was 2.84 Amps at 600 RPM. That is 40 % greater than the optimum of 2.04 Amps when load matched correctly.

Tried fitting caps in different configurations to move the phase angle but everything shifts as it's a closed loop circuit.

Tony on the 'Star/Delta Controller' topic I proposed something along similar lines to what you have added to this discussion re a switching system.

DaveP68 wrote:


Electronic Switching is best done with delta wiring from my experiments.

The idea is to say cut a 36 pole stator into quarter or sixth depending on how much switching is required over a wide RPM range.

Each delta set of winding's is terminated into it's own rectifier which can then be switched using FET's to put all outputs in series or parallel depending on RPM at the time of the switching for a given voltage output.

Example;
At 100 RPM with a 36 pole stator 4x delta winding's all in series would produce 50V, .62A, 31W
At 200 RPM with same stator 2 sets of 2x in series put in parallel would produce 50V, 2.42A, 121W
At 450 RPM with same stator all delta winding's in parallel would produce 50V, 9A, 450W

Note the big jump in current range, this is due to the load better matching the stator so it's not going into saturation.

The FET's need to be fast switching and very low on resistance, with a make before break switch over so stator doesn't see an open circuit condition.

The switching can be done at different RPM's to produce lower or higher voltage.

The 36 pole stator is the best to use with a black rotor cap as stronger magnets. Also it can be cut into more configurations 1x 12, 2x 6, 3x 4, 4x 3, 6x 2 allowing a wider range of switching points. The stator can also be used in star mode for higher output voltage but less current. Difference is 1.732 times so delta volts 200 RPM uncut 100V 1.21A vs star 173.2V 0.7A same RPM.

I prefer delta wiring as better current output for Voltage I'm wanting at a given RPM.



The above is now way out of date and thinking of a different concept yet again using the same principle above but fitting cap doubles to up the output power.

Second aspect is to use a dual stator set up to start on one at cut in and do the switching over at offset RPM level's. This allows one stator to always be online while the other is switched into a more efficient mode. Switching occurring at a point close to that stator running into full saturation.

This gives an added advantage of no big voltage/current changes at the time of switching.

Edited by DaveP68 on 07 June 2017 at 12:49am
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Warpspeed
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Posted: 07 June 2017 at 1:23am | IP Logged Quote Warpspeed

There won't be any large voltage changes at the point of switching in a real system, because a large battery virtually fixes the output voltage.

The problem of what happens at the exact point of changeover (no momentary shorts, no briefly running open circuit) is solved by using a combination of diodes and mosfets as suggested. But I agree switching should be fast and clean to avoid high power dissipation operation of the mosfets.

If the switching points are well chosen there should not be any large jumps in current.

Think of it this way, if the system is running below cut in with all the windings connected in parallel there would be no current.

As we reach cut in speed the parallel windings would reach sufficient voltage to begin working. We do our switching when the current in parallel is equal to what it is already producing in series. From that point onward the current rises with rpm.

So when we change over there may not be such a large jump in output power if the changeover point is well chosen.

If that can be done, while also matching the power curve of the turbine I really do not know. Getting more grunt out of the alternator at very low rpm is only half the problem. We need to do that without stalling the blades.

Running with a real wind turbine and charging a battery is very different to bench testing into a resistive load.


Edited by Warpspeed on 07 June 2017 at 1:31am


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oztules
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Posted: 07 June 2017 at 5:21am | IP Logged Quote oztules

Disappointing Dave, the theory is good , but the results are strange..... not sure why


........oztules

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DaveP68
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Posted: 07 June 2017 at 10:10pm | IP Logged Quote DaveP68

Yes disappointing is the word, as it showed such potential...

Tried fitting an extra full wave rectifier between one of the output phases and the star point. When the 3 phase star output rectifier was at 100 VDC under maximum loading the output of the extra rectifier was at 70 VDC. This tells me that those 2 extra diodes in the other configuration are not able to go into forward conduction.

I would need to know more about the electrical characteristics of the other alternators that this technique has been applied to successfully.
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Posted: 07 June 2017 at 10:49pm | IP Logged Quote yahoo2

Warpspeed wrote:
That uprated alternator worked fine. I had no idea it was such a simple conversion, I thought it must have been rewound.


It was designed with the quick change to delta for easy fitment to match the revs of diesel engines probably made back to 1970 or maybe even earlier. I think with a twin v belt pulley that was slightly larger diameter as well. The bosch was OK but the early lucas was a pain, the voltage regulator and other bits were constantly failing from vibration and dust.

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DaveP68
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Posted: 14 June 2017 at 7:36am | IP Logged Quote DaveP68

Hi Tony

Have tried wiring up an 80's stator in the configuration you suggested. Removed the star point and wired each set of 14 poles per phase to it's own rectifier as per your drawing below.

Warpspeed wrote:
This is the general idea:



Got the exact results as you described.

One advantage to taking this approach is the voltage levels are better than the standard star output.

Standard Star on an 80s is 0.685 VDC per 1 RPM vs Delta at 0.395 VDC per 1 RPM.

With your set up we get a slightly higher volts per RPM in the series mode of 0.787 VDC per 1 RPM. The parallel set up is the same as Delta at about 0.394 VDC per 1 RPM.

I can see merit in the way this works as the cut in RPM can be reduced by about 18 % if required (18 % higher for same RPM).

Good work and keep those new ideas coming through.

Next step is to try out your switching idea with the view to publishing the results. This could take me a week or 2, as got a bit of other stuff on my plate at present.

Cheers

David

Edited by DaveP68 on 14 June 2017 at 7:37am
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Posted: 14 June 2017 at 3:20pm | IP Logged Quote Warpspeed

Yes, you get slightly more voltage than in star, but also slightly less maximum theoretical current capacity. It does put out a nice clean rectified dc voltage though.

But at very low wind speeds we cannot load the alternator to anything like its full current capacity anyway, without stalling the blades.
The driving torque to do it is just not there, so its not a practical limitation.

Its about the best we can do for max dc voltage at low rpm with just an alternator and some diodes.

The previously suggested mosfet switching scheme should work to give a hazard free changeover, but I have not actually tested this myself. Isolated gate drive power could be provided (initially for testing) from a couple of 9v batteries.


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Posted: 14 June 2017 at 9:22pm | IP Logged Quote DaveP68

Isolated gate drive can be achieved using small high frequency transformers driven by low power CMOS gates. The output of which can be rectified to provide full gate drive voltage to the MOSFET's. Have them by the hundreds as that was the technique used by F&P to drive the high side MOSFET's on the motor control module boards.

Here is a photo of one.



The current drawn at very low wind speeds is of no concern if it's being feed into a battery system. This is due to the driving torque of the shaft providing a natural limit on this anyway. This is done by getting the cut in speed voltage correct in the first place and will only allow the current to flow proportional to the power provided from the shaft.

The tricky part will be the switching pint going from series to parallel configuration without going to far outside the optimum TSP of the blades. I can work this out as Know the exact electrical characteristics of any F&P stator. This would also involve making sure the stator doesn't go to close to saturation before switching over.

MPPT sorts this out through a mathematical algorithm that hunts for the peak power output for a given RPM/TSR. Hence the complex electronics required.

Edited by DaveP68 on 14 June 2017 at 9:24pm
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Posted: 14 June 2017 at 10:35pm | IP Logged Quote Warpspeed

You might like to take a look at an International Rectifier PVI-N chip.
http://pdf.datasheetcatalog.com/datasheet/irf/pvin.pdf

These are a fairly unique opto isolator, in that the LED illuminates a small photovoltaic "internal mini solar panel" capable of turning on a completely isolated mosfet gate directly.
They are not ultra fast switching, about 100uS on and off, but ideal for something like this as it really simplifies things.

Its what the expensive high power mosfet dc solid state relays modules use, although the bare opto chip by itself is not expensive.

I only suggested using the battery for a quick bench test of the concept.

I think you might get much closer to how a real battery system behaves if you build a suitably powerful shunt voltage regulator to use as a load, to feed dc power into, and use that instead of a plain resistive load to test all of this.

One last idea.
Connecting three bridges in series produces exactly twice the dc output voltage of a delta connection, as you have already verified.

That could be further increased by connecting an electrolytic directly across the dc output of each individual bridge. Depending on the value of the electrolytic, the rpm, and the loading, it should be possible to get from at least double, to something almost approaching three times the usual delta dc voltage.

Again just another untried idea that should work.
But as its pretty easy to do, it must be worth a try.

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DaveP68
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Posted: 15 June 2017 at 3:18am | IP Logged Quote DaveP68

Thanks for the heads up on that International Rectifier PVI-N chip but I'll stick to what I've got as they are free vs $10.00 per chip.

One big advantage in using them though is they keep the eternal part count low.

Have already made you aware of the fully controllable load that can simulate a battery. It's adjustable from 40 VDC up to 460 VDC, but need to make a modification for it to take more than 5 Amps up to say 20 Amps. This way can set it to 48 V or up to 59 V to simulate different charge states of a battery.
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