Capacitors across each phase - more power

[domwild]

Hi,

A scientist with the name of Dr Tom Chalko wrote a paper which proved that F&Ps with caps across each phase increased the power.

The F&P has 42 coils or 3 x 14 for each of the three phases and therefore generates AC at a much higher frequency. Tom drove the F&P at hydro RPMs; from memory about 600 RPM and too high for wind.

From the little I know such high AC frequencies are not experienced with wind + Pigott PM alternators with far fewer coils.

Does this mean Pigott style PMs will not benefit from caps and are not generally seen on them??

[zeusmorg]

 Proven,, on paper,, so has it been proven in the field?

[commanda]

The value of (ideal) capacitance required varies with the RPM. In a hydro setup, where the rpm is essentially constant, it works well. On a wind turbine, where the rpm is varying constantly, it becomes much more problematic. A number of switching schemes have been proposed; I don't know whether anything has ever come of it.

The axial flux alternators generally don't benefit from it much, not because of the frequency, but because the F&P has iron-cored coils. The iron core gives rise to much higher internal inductance. I presume, from this, that it could possibly be beneficial to the motor conversion brigade.

Amanda

[TomW]

Zeusmorg;

Pretty large distinction there. Still curious which it is. I often thought caps could be used to advantage in these applications but never had time or resources to do much real world testing.

Those down under folks get to play with FnP's a lot. I had one awhile, but in the interest of experimenting I swapped it to Jerry for some other stuff.

Anyway be curious to know if it was a test in hardware or on paper.

BruceDownunder talked about the Aussies playing with caps on those motors in IRC this weekend, and it sounded like they were working to boost output. Just hearsay.

TomW

[Flux]

The F & P was designed as a motor, it is about as unsuitable a construction for an alternator as you could imagine, but it is cheap and easily available.

Running alternators into leading power factor loads causes rising rather than drooping characteristics. With a type of winding that inherently has a high inductive reactance the benefit of capacitors will be considerable in that in will let your output increase with speed whereas it would normally tail off at constant current.

For hydro this would be a great advantage. For wind it may or may not depending how the components of reactance balance the stall issues with the prop. I suspect it would be an advantage as long as you use modest capacitor values to just increase the slope of the power curve in the high wind region.

For the Hugh Piggott type machines just forget capacitors, they never become reactance limited and there is no benefit from maintaining a leading power factor that increases the rms stator current, adding extra copper loss. Unfortunately the capacitors can't even correct the rectifier power factor as it is a waveform rather than phase phenomena.

In some cases there may be an advantage for motor conversions but in general the frequency is too low for it to be practical for low voltage machines.

Flux

[domwild]

Zeus,

Yes, he dove those alternators via a bench drill.

[domwild]

Here are details but not his (Tom's) scientific paper. Once I find it I will post it.

http://www.thebackshed.com/Windmill/FORUM1/forum_posts.asp?TID=762&PN=3&TPN=2

Thanks for replies.

[domwild]

Here is the paper:

mtbest.net/SD_modification.pdf

Watch it, it is a 200 kb pdf file!

[MyCattMaxx]

You say that it is unsuitable yet many are making power out of an f&p. Go figure!!!!

[Flux]

I didn't say that it is unsuitable. I said that it was not the ideal way to design an alternator.

If something works and is cheaply and freely available there is absolutely no reason not to use it and I am perfectly aware that many do. Most people will be in the position where it is more practical to adapt something else than start from scratch. Much depends on skill and facilities available.

That doesn't change the fact that it has many things that are not ideal. The same can equally well be said for motor conversions as most of them are based on high speed motors that are not ideally suited to conversion to low speed PMAs. That doesn't prevent large numbers of people from converting them and using them satisfactorily.

The discussion was about adding capacitors and I was trying to explain why it could be beneficial for the F & P by removing some of the worst effects of one of its limitations.

Motors and generators work on the same principles and largely they are reversible and interchangeable but they are optimised for their intended use and if you adapt it for a different purpose then it may not be ideal ( in a few cases it may even fail to work)

Flux