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Capacitors across each phase - more power | 9 comments (9 topical, editorial)
Re: Capacitors across each phase - more power (3.00 / 0) (#2)
by commanda (alwynne at unwired dot com dot au) on Mon Apr 7th, 2008 at 04:06:31 AM MST
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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



Re: Capacitors across each phase - more power (3.00 / 0) (#4)
by Flux on Mon Apr 7th, 2008 at 09:08:15 AM MST
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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

[ Parent ]



Re: Capacitors across each phase - more power (3.00 / 0) (#8)
by MyCattMaxx on Mon Apr 7th, 2008 at 10:14:40 PM MST
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You say that it is unsuitable yet many are making power out of an f&p. Go figure!!!!

[ Parent ]


Re: Capacitors across each phase - more power (3.00 / 0) (#9)
by Flux on Tue Apr 8th, 2008 at 01:15:14 AM MST
(User Info)

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

[ Parent ]



Capacitors across each phase - more power | 9 comments (9 topical, 0 editorial)

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