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calculating voltage of test coil
By defed, Section Wind Posted on Mon Jun 29, 2009 at 09:26:32 PM MST
not sure if i calculated properly.

i'm making a Piggott 4' at 24v. 155 turns of 19 awg per coil. it calls for a cut-in at 300rpm. i made a (rough) test rig where i set the coil on a thin piece of plexi just above the magnets. i'm sure i need to find a way to suspend the coil w/ nothing in between for an accurate account, but for now, i just want to see if i calculated everything correctly.
i spin the rotor to 100 rpm and get 1.3v and 3.9v at 300rpm. according to the homebrew book, i multiply that by 1.4 for peak AC when converting to DC. gives me 5.46v. next multiply by 2 (number of coils per phase), 10.92v per phase. next is the part i'm not sure about. it says multiply by 1.73 (square root of 3, 3 phases). gives me 18.891. now this is the TOTAL output of all 3 phases (6 coils)? after i subtract rectifier loss of 1.4v, gives me 17.491v at 300 rpm. not enough for charging a 24v battery.
so, either i misunderstand the calcs, my coils are not right, or (most likely) i am seeing a hefty loss in voltage since there is a barrier between the coil and the magnet.
just wanted to make sure i was doing the math right (and understanding everything) before i worked on a better way to suspend the coil.
thanks!
Dave

calculating voltage of test coil | 3 comments (3 topical)

Re: calculating voltage of test coil (3.00 / 0) (#1)
by Flux on Tue Jun 30, 2009 at 12:46:04 AM MST

That design is fairly critical about spacing between magnet rotor and the coils. You will find the voltage comes up when you reduce that spacing. Anyway it doesn't matter much if the cut in is over 300rpm. At present you are a bit high at 400 rpm but I suspect with the coils closer to the magnets it will be fine. If you get cut in at 350rpm you will be ok. In very low wind areas there may be some advantage at 300 rpm but for most sites 350 may be better.

Your calculations are ok.

Flux

Re: calculating voltage of test coil (3.00 / 0) (#2)
by defed on Tue Jun 30, 2009 at 05:44:43 PM MST

thanks Flux!

the only thing i guess i don't actually understand is why you multiply by the square root of the number of phases (1.73). so if you get 1v from one phase, you only get 1.73 when all 3 phases are connected in star? seems like you should get 3v...but then again, i don't understand the details of 3 phase.

time to do some research to learn about it.

Re: calculating voltage of test coil (3.00 / 0) (#3)
by defed on Wed Jul 01, 2009 at 03:24:16 PM MST

got the air gap smaller - quite small actually, probably too small for the finished stator! but, up to 4.5v at 300rpm.

i flipped the magnet rotor over so the magnets point towards the floor, so i could set the coil on some wood blocks to position it. does anything behind the coil besides air affect anything? flux return etc? if so, i wouldn't think it would be too drastic.

calculating voltage of test coil | 3 comments (3 topical)

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