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Warpspeed
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Posted: 15 June 2017 at 3:43am | IP Logged Quote Warpspeed

I agree, using stuff you already have always beats buying something new.

But others reading this thread may be starting out with nothing suitable readily at hand, and the PVI chips are a rather interesting component of which I only recently became aware. They may be useful for quite a few other "Back Shed" type projects.

I forgot you had the programmable load. Very handy thing to have.

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DaveP68
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Posted: 15 June 2017 at 7:22am | IP Logged Quote DaveP68

Good point re the PVI chips and that's why I said they keep the component count down. Yes they are the device of choice for those who take an interest.

The little transformers I'll end up using do require a few more parts make them work plus are now hard to find as F&P stopped using them 21 years ago.
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Posted: 15 June 2017 at 8:45am | IP Logged Quote Warpspeed

Those little transformers look to me like a standard common mode choke.
If you can measure the inductance, it would probably be fairly easy to find a direct equivalent.



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Posted: 16 June 2017 at 6:45am | IP Logged Quote DaveP68

I'll have to get back to you on that as the 'common mode choke' as you put it that I've got in big numbers just work from my perspective. Not much incentive for me to find out it's electrical characteristics.

Will let you know the results though when I get the switching circuit that you proposed up and running.

Edited by DaveP68 on 16 June 2017 at 6:45am
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Posted: 16 June 2017 at 6:59am | IP Logged Quote Warpspeed

O/k fine, its just that you said they were now hard to find.
If you already have quite a few salvaged from old circuit boards, then no problem.

It makes sense for F&P to have used something like that as they are cheap and pretty readily available.
They are made and sold to be used as common mode chokes, but they can also be used as a 1:1 high frequency transformer with pretty good voltage isolation.

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Posted: 16 June 2017 at 7:32am | IP Logged Quote DaveP68

I think the device of choice is still the PVI chips as they keep the component count down and that is what I was trying to promote.

Mean while got a heap these parts salvaged from old circuit boards so will put them to use to at least prove the concept of the proposed switching circuit.

The final production/working unit that we can be published on here will probably use the PVI chips to keep the circuitry easier to construct in our the back sheds.
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oztules
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Posted: 18 June 2017 at 6:29am | IP Logged Quote oztules

Davep68, I tried to play with my f&p, but i had wired it for low voltage high torque pump application on a mono pump... which works very well oddly enough..... anyway, I decided to pull it off and put it in the lathe, expecting maybe 15 amps or so, and at fairly low rpm... maybe 300 perhaps.

I don't know what stator it is, and have not even bothered to count the poles after the modifications some time back... memory is poor and pole count matters not any more, but you will probably want to know what it is... the pic follows.

I fired it up into some 220 amp 6v batteries, and this is what happened.

here is the stator with feeble wiring.




It needed over 500 rpm to get to 20 amps, and made it to 40 amps or more...on the 750 setting...... then the belts slipped.... the pic caught it at around 38amps.




So it was clear the driving device would not go much further.

I slipped a phase out to get an idea of what may happen when i connected the star point.... it went from 20 amps to 25, I was pulling up near 1000 rpm, belts were not happy, so I can say that in single phase , when you add the star point diodes as well, there is a marked increase, but i will need to do something better with the driver before i can discount the three phase hook up. It seems to be getting current limited after 750rpm, so the delta would not kick in until well over 1000rpm I suspect.

Anyway thats as far as I got today, and proved nothing, so at this stage i can't say the star delta diode system works at all ... never mind well, but the theory seems good so it needs better power inputs.

Very surprised at the 40 amps... didn't expect that much , and it would appear that a 5hp IC engine coupled to it would make a nice cheap little 2kw 48v charger ... even in just star.

I noticed the iron losses are very small too... another good thing.


EDIT..... on reflection, I should have tried it on 6v....dammit, i've put it away for the day .... maybe tomorrow...



.........oztules



Edited by oztules on 18 June 2017 at 6:54am


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Posted: 18 June 2017 at 11:14am | IP Logged Quote DaveP68

Hi oztules

The stator looks like an 80s 14x 1p star. Did you feed it into 2x 6 V batteries in series? That would make the battery terminal volts something around 14-15 VDC depending on how charged the batteries were.

The optimum current from that set up is 28.28 Amps at what ever volts/RPM level you want to run at. The maximum current is 40 Apms which is 1.414 x that optimum current for 80s 14x 1p star.

Interesting result removing 1 phase and noting a jump from 20 to 25 amps. RPM was very high though.

These stators are very predictable once you get to know them in their star/delta nodes.
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Posted: 18 June 2017 at 9:24pm | IP Logged Quote oztules

You know your stuff Dave.

Yes that was nominal 12v ( 2x6v )... don't know why I did not think of just paralleling the batts and run lower rpm to test the theory.... getting too old perhaps.

"Interesting result removing 1 phase and noting a jump from 20 to 25 amps. RPM was very high though."..... yes this makes no sense when you draw it out unless the star delta switching is working as described.... but then why not in 3 phase... will find out today.... is it just lack of rpm where the losses in the star legs were not high enough yet to allow the delta circuit to contribute???/ I'm thinking so, although not a shred of evidence to support me as yet.

"The optimum current from that set up is 28.28 Amps at what ever volts/RPM level you want to run at. The maximum current is 40 Apms which is 1.414 x that optimum current for 80s 14x 1p star."

... not sure I understand this... I would have expected optimim current in a reactance protected system to be flat stick, as no damage should follow whatever loading was presented.

The root 2 is more a peak/rms thing for me. the delta/star relationship is more a 1.7 x delta or star..... or are you thinking of restive I^2R loss in the wire?... Not being an F&P player, I am not sure what you mean by optimal I guess is what I am saying.

I can drive it into a direct short today too, that will show things up in the rpm range I have some torque in.

Hopefully I can put this to rest one way or another.

I will study your charts when i can find them to see if the engine generator is worth pursuing, 2kw for peanuts is worth looking at.... maybe 7p delta will work better... will look for your figures.


EDIT: found this on your other thread "Next test was with a 80s 42 pole stator wired 14x 1p Star got 1575 W at 1870 RPM with other readings of 57.4 VDC at 27.44 Apms."... and i don't understand it..... Why did I get 40 amps in the same sort of config.... did you not push it far enough?, or I am missing something else.... ie what happened at 3600rpm?


........oztules







Edited by oztules on 18 June 2017 at 9:36pm


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Posted: 18 June 2017 at 11:06pm | IP Logged Quote oztules

It's official.... don't know what I am talking about.

The best I can do is 45 amps on the dot. As I was only interested in the current, a short circuit was sufficient to test this setup. The leads are skinny, so it was nowhere near a real short I guess.

Result is NO DISCERNIBLE CHANGE with the extra diodes in place it was 45 amps versus 45 amps.. nothing... nix..

I can't see why this is the case... but it is.. Dave was right.

Still amused that 45amps is possible before full back mmf cancels the driving field.

At higher rpm it will also get inductive reactance due to the high frequency... but it takes very little rpm to get 45 amps at short circuit, so at least I can say the the frequency induced reactance is not why current limiting is occurring, but in fact back mmf is responsible for the most part at these low frequencies. Herbnz was right, flux was less right.... thats it for the synchronous impedance anyway.

I can see a generator being built soon using the 80s stator. 40 amps is plenty for a little petrol motor.


........oztules

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Posted: 19 June 2017 at 12:46am | IP Logged Quote DaveP68

Hi oztules

The optimum current refers to the most efficient power extraction at that RPM level. So in my example that turned out to be 1575 W at 1870 RPM with other readings of 57.4 VDC at 27.44 Apms. The current reading was over an approximated shunt, close but not as accurate so reading could have been as much as an amp too low.

Your maximum of 45 Amps is the limit again my one didn't quite get that high but I would have only been about 3 amps lower. I also didn't test my 80s 14x 1p at a very high RPM to reach 40 plus amps so my 1.414 times was just a projection. I your case you got about 1.5 times optimum current.

Some rotor caps can have less magnetism especially if there was a motor short on at the time of the washing machine failing which can generate an opposing field.

Will need to repeat my test to confirm your peak reading of 45 Amps with a different rotor cap. This time will do the new test at 24 VDC at around the same RPM level to check what my maximum current will be.

The continuous RPM limit of the rotor caps is 3000 RPM and they fly apart immediately at 5000 RPM!! They will do 3600 RPM for a short duration but if it has a defect could let go at 3600 RPM under load conditions, so be careful...
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Posted: 19 June 2017 at 4:08am | IP Logged Quote oztules

Thanks for the warning, didn't realise they were a bit delicate.

It comes down to amp turns for a given airgap and magnet arrangement. We either use less turns per phase to get the current up, or use better magnets.

One of the endearing things of the F&P is the current limiting, so more magnet is not my first preference...more rpm would have been......

45 amps at high rpm may be more difficult, as the synchronous impedance will then incorporate much more inductive reactance as the frequency rises, that may be why you have not seen 45amps perhaps. Mine was shortcircuit ( but for skinny leads), so the rpm was very modest to get the current up, so inductive reactance was very low i suspect.... see how it goes


...........oztules

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