 designing a PMA
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TheMerovingian
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Fri May 04 2007, 11:48AM
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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
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I'm building a windmill to run an alternator.
I'm designing a single phase, permanent magnet axial flux alternator .
For this i have 8 coils (80 turns each of) AWG24 wire, wired up in antiseries potted in resin. THe cross section area is about 8cm^2 (as much as the neodymium magnets i have ordered 4cmx2cmx0.5cm N35).
Femm shows that in aircore simulation the voltage will be only 1.41VRMS / 100 RPM with an air gap of 3 mm (while using full core it is 8.42VRMS / 100 RPM, but i'm trying to avoid iron core for losses and clogging).
To estimate the voltage i assumed that the flux varies sinusoidally
where Flux(t) = Fluxmax * SEN ( pi* RPM/60*N*t) where N is the magnet number (8 in this case), max flux coupling is got from FEMM
V = -dFlux/dt -> V= - FluxMax * pi*RPM/60* N * COS(pi*RPM/60*N*t)
Vpeak = FluxMax * pi*RPM/60* N * 1
VRMS = FluxMax * pi*RPM/60* N * 0.707
getting the aforementioned values.
Anyway i don't like the way femm models permanent magnets. In particular on the pole surface the field is 3 , 4 times weaker than expected (from measure values from magnet manifacturers of magnet of the same grade and dimensions)
Does this affect the simulation?
If yes how i can model the magnet correctly?
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