my first PMA design

[mab]

Hi,

I've been trying to get my head around the calculations for desinging a axial flux PMA. I think I'm there but I'm hoping an expert might glance as my figures to see if they are plausible before I start building.

I'm using 16off  20mm dia x 10mm thick N42 'cos that's what I've got.

as I want max volts for min RPM & winding turns I'm thinking a 16 pole single rotor is better than a 8pole dual rotor.

Single phase for the same reason.

if I want 28v @300 rpm (5rps) then
with a coil thickness of 6mm, magnetic path length in 'air' from pole to pole is 14mm (allowing 1mm clearance), which gives a gauss of 1922 in the gap using this handy calculator I found online:-
http://www.magnetsales.com/Design/Calc_filles/FluxDensity2PlainDiscs.asp

using: Number of turns = Volts / ( poles x Rps x Tesla x Area x 2 ) = 28/(16 x 5 x 0.19 x 0.00031416 x 2) = 2932 turns - (I think I got this equn off this board?)

gives  no. of turns/coil =2931 / 16 coils= 183turns/coil

Am I close? - or have I gotten lost?

thx

mab

[jlt]

The only way to see how turns of wire you need is to wind a test coil.And don't forget the  dc volts will be about 40% more than the ac volts,
       To make the most amps I would use 16 mags on one disk  and a second rotor without mags.And make it 3 phase. forget the single phase it will vibrate a lot and wont  put out  more amps.

[electrondady1]

hello map.
perhaps you are simply testing the formula.
183 seems like a lot of turns.
all i could suggest is to lay the mags out on a suitable iron disk and form  a test coil.
 paying strict attention to the spacing between the magnets.
using rectangular mags i normally do 50/50 spacing .
my understandings that round mags can be placed a little tighter.
i would mimic the spacing danb uses on this round magnet alternators.
 

[electrondady1]

using a second rotor as jlt has suggested would be very beneficial for pulling the flux through the coils rather than sideways from mag to mag

for maximum power out, consider a two phase overlapping wave wound stater.

[mab]

Hi, thanks for the feedback

I was about to do a test coil next  . I've got the mags on the disk (only held on by their own magnetism & a long ziptie ATM so they won't stand many rpm).

I was going to try to make a fixed laminated iron ring (like volvofarmer's design?) for the other side of the coils - is a 2nd rotor better? - as it happens I have got a 2nd disk already.

And make it 3 phase. forget the single phase it will vibrate a lot and wont  put out  more amps.

I thought single phase would put out more volts/rpm though (for a given no. of turns)? - I'm trying to be lazy and make it work for a minimum number of turns on the coils (wound by hand  ); at least to begin with - I do like electrondady's idea about overlapping a 2nd phase, though.

Electrondady - I have made rotors large enough for 20mm between the mags  - so I can have a heavy gauge wire for the coils, but will read what danB has posted before I set my mags in place...

cheers

mab

[ghurd]

An empty 2nd rotor is better than laminations.

3-phase is a lot better than single phase.  Less vibration.  More amps / less resistance (usually?).  More efficient.

Overlapping coils sounds good on paper.  In my experience, actually making it is like getting the starter recoil spring back in a lawn mower... using only 1 hand.
G-

[mab]

Overlapping coils sounds good on paper.  In my experience, actually making it is like getting the starter recoil spring back in a lawn mower... using only 1 hand.

 

point taken, I've actually done that (with 2 hands) - just.

OK, looks like the next thing is indeed to do a test coil, then work out coils turns size & spacing for 3ph

cheers

mab

[Flux]

You are right in thinking that using 16 poles is the best way to go but it seems from your results that you may be intending to make it single rotor with no flux return path.

Forget single phase no point in going there. You need a flux return path  to make better use of the magnets than 8 pole dual rotor. Fixed laminates work at the cost of lots of start up drag and low wind loss. They also add complicated axial loadings. These snags are not balanced by the slightly easier construction.

Use a spinning disc with no magnets as a return path. Keep your coils no more than 8mm thick and try to keep the air gap below 12mm.

I guess you can get something near 450mT in the gap ( can't be bothered to convert to those gauss things). I suspect you will need less than half your predicted turns for star connection with 4 coils/phase.

Should be able to make a perfectly workable alternator this way even though you don't have lots of magnet to play with.

Flux

[Flux]

Probably should have added that if you want to try the overlapped coil method then keep the coils no more than 4mm thick, keep your air gap below 8mm.Be prepared for lots of extra work with no likelyhood of much better results with round magnets. You will add so much coil turn length that I seriously doubt that you will better the much simpler single layer winding.

Flux

[mab]

Great feedback guys!

The only reason I was thinking of a stationary laminated flux path was 'cos it wouldn't need clearance - but it looks like the experts know better  .

I'll go with the experts and make it 12 coil Y 3 ph. with 2nd rotor.

If I might ask a second question:-

if the magnets are 20mm diameter, I assume the coil should have an inner diameter of 20mm; what is the optimum outer diameter?  60mm so the 'width' of the coil = magnet diameter? (width as opposed to thickness, thickness being defined as the direction that needs to fit between the rotors).

As I haven't fixed the magnets I can still choose the spacing between them.

[Flux]

I doubt that there is any optimum, it all depends on what you want to trade for what. If your discs are big enough then using 60mm outside diameter is not a bad way to go. If you are restricted by disc size then you can make it less. Also there is no reason why you can't go under the 20mm inside diameter, not all turns will link all the flux but you still gain as the turn length is very short and adds little resistance ( don't go silly).

You can also use Dan's trick of squashing the coils elliptical to get more wire in the same space, this trick works best if you have to crowd the magnets on a small disc. If you have good spacing between magnets then squashing is less beneficial but you still gain a bit with more area for flux linking the centre ( again don't go silly or you will loose out with increased Resistance from increased wire length).

Some will tel you to use triangular or trapezoidal coils to keep the sides radial but I think you will increase Resistance and gain nothing. The circle has maximum area for a given perimeter. In the end it comes down to flux linkage and circuit resistance for round magnets that means circular or a bit elliptical depending on magnet spacing.

Flux

[mab]

Thanks Flux,

I've actually just spent a few hours reading old posts on this very subject as it's been asked before.

Having distilled the general consensus it seems to match your advice quite closely (I could've just read your post and saved myself a few hours  ).

I've drawn a template for my magnets, disks & coils with the gap = mag diameter, the max coil diameter is about 52mm (for 12 round coils just touching). That does mean that the centers of the legs don't line up with the centers of the poles unless the inner diameter of the coil is 28mm.

Having such a large hole seems daft, so, plucking a number out of the air, I think the inner diameter of the coil might be OK at 18mm, so I think I'll use those dimensions for the test coil and see what it does. If I understand the way it works, that should 'broaden' the voltage peak of the coil output for a small increase in resistance.

Thanks again everyone - time to start constructing a coil former methinks...