some coil testing

[willib]

this is the second new coil that i've wound for the Improved mini gen

i guess you just have to believe in yourself rather than give up on the idea of a small hole coil , i decided to wind one more , that one turned out so good that i wound another

the coil is 1.7" dia. with a 0.460 hole

approx 84 turns of 0.045 dia wire.

as you can see the coil is very thin (0.27")

the measured/calculated resistance is 0.127 ohms (1.45A into coil and 0.184V across the coil)

i think i sort of overtested the resistance a bit , at 5.8A the voltage across it was 0.8V

it was starting to get warm

so stupidly i tested the temp of the Fet with my finger WOW was it hot!

i've since replaced the heatsink on the Fet (with a larger one), and i dont think i'll ever test the temp of that again with my finger.

besides the impressivly low resistance the voltage was 3.5V peak at 285 RPM  

i recently got a new meter FINALLY !

it reads RMS values of AC current , voltage , and gives the frequency in HZ

to get RPM from frequency , multiply frequency by 60 and divide by half the number of poles per rotor

for example my machine , 40Hz * 60 / 8 = 300 RPM

at 300RPM the meter reads 2.55V rms

and 2.55* 4 coils per phase=10.2V

 10.2* root2= 14.4V ala flux...

so depending on the state of the battery onder charge it should be charging by 300 rpm

a shot of my long overdue meter

[WXYZCIENCE]

Willib, I got branded from an old ceramic cmos that I was temperature testing with the finger. Wonder why that learning curve is flat.
Joseph E-mail( minswxyz at telus dot net )

[motoman465]

Nice coil!  I always thought that the center of the coil had to be approx the same size as the magnets.  I guess you proved that wrong...LOL!  Also, I am curious about the thickness of the coil.  Do thicker coils provide more power?  Or do they provide more power to a point?  Still so much for me to learn.

Todd

[RP]

Willib,

What size are your magnets?

[willib]

i have had it in my mind for some time that if you could use the space in the center a little more effectively you could wind a coil with more turns and use less resistance than winding turns at the perimeter.

Thicker coils add resistance

thicker coils increase the number of turns , its ok if you need more voltage.( see paragraph three)

to answer your question , i would have to say no ,(see next paragraph) thicker coils do not give more power ,because if you have the voltage you need anything more is just added resistance.

the trick is getting the needed voltage with the smallest possible coil

of course if i wanted to make a more powerfull alternator (alt) i could have used two in hand of the 0.045"dia wire and doubled the thickness of the coil , giving the same effective number of turns (hmmm)...

that would only be 0.54"thick , still within reason with 1/2" thick mags...

ok let me rephrese my answer.If by thicker you mean adding another strand of wire yes it will be a more powerfull alt otherwise no . lol

your magnets themselves have to be thick enough to penatrate a thicker coil, i could have made my coils a lot thicker than six turns thick,but i didnt need to since i got the voltage i needed with just six turns

they are 6 turns thick by 14 turns wide (dia).

this new coil has a rest resistance of .128 ohms 0.028V/0.218A

 the hotter copper gets the more resistance it has,i stopped at 3.202A and 0.52V ,( 0.162 )Ohms

the center of this coil is the distance between magnets

Voltage is not the only thing that makes power , but without it (and without the added complexity of boost converters) you cant charge a battery

current is also needed ,

to get current you need a) powerfull enough magnets to start with

b) thicker wire can carry more current ( without overheating).. (hmmm)

i could double the power handeling capability of my minigen by using two in hand of 0.045"dia wire and the stator would still only be 0.54" thick , something to think about for sure , thanks!

[willib]

0.875 dia x 1/2" thick

there are two other diaries on it in my diaries

[motoman465]

Thanks, Willib.  Sure are a lot of variables when it comes to coils.  I guess that the best way is to come up with something and then just test it.

Todd

[Flux]

This is a difficult issue and there is probably an optimum for each magnet size and spacing. If things were decided on volts alone it would be fairly easy.

Any turns smaller than the coil are not cutting all the lines of flux and are not contributing full volts, but they still contribute something and they add relatively little resistance. I suspect you are justified with a round magnet to reduce the width of the hole considerably, I am not so inclined to agree about reducing the height, you may be better off with oval coils with the long dimension equal to the magnet diameter.

With rectangular magnets I would keep the height of the coil hole equal to the magnet. I would also keep the outer width about magnet width or not much less. As the magnets are close on the inner diameter you may possibly bring the width down to half magnet width where there is more leakage flux. Dan seems to get good results this way.

With larger magnets I would keep more nearly to the hole being the size of the magnets. Small magnets have quite an area round the edge where flux is well down on the centre value, that may be why the small round hole comes out well in this case.

," i would have to say no ,(see next paragraph) thicker coils do not give more power ,because if you have the voltage you need anything more is just added resistance."

Now this is where I am beginning to disagree. If you have got the volts you need and have space to spare, you use thicker wire. Maybe I misunderstood and you are trying to optimise for best output with a given wire size.

The way I see it is that you need 2 mechanical gaps and with small air gaps these waste a significant percentage of the winding area. With a 2mm thick coil you would waste about half the space.

By using a larger air gap you have less flux density so you need more turns, but you have increased winding area to use more turns of thicker wire and still get the volts with less resistance. Neglecting leakage flux, the magnet works at its best point when the air gap is equal to total magnet length (about .6T)

In real life you reach the .6T point with the gap less than total magnet length.

For a dual rotor with 2 magnets in series and with average spacing and leakage this generally means that the best coil thickness is about the same as one magnet. That leaves room for fibreglass and air gaps and will work in an air gap about 1.4 times one magnet length.

Still room for experiment.

Flux

[Flux]

Second paragraph should have said  " any turns smaller than the magnet"

[BigBreaker]

Magnetic flux is proportional to area, while resistance is proportional to a linear measurement (wire length).  At some point the area enclosed by the inner windings will not supply enough extra voltage to offset the ohmic losses of the wire.

It's obvious that the windings should close to at least the outline of the magnets.  Any coils outside the outline capture all the available flux.  Inside of the magnet's outline there is a trade off between flux captured and resistance added.  I think the cross over point occurs with much smaller inner windings than most people are using.

There are secondary considerations as well, such as heat and wire cost.

[SamoaPower]

I think you miscalculated your line volts and cut-in speed.

Line volts = phase volts x root3 x root2 = 10.2 x 1.73 x 1.41 = ~25 Vdc

You should cut in about 160-170 rpm on a 12 V system.

I agree with Flux about coil shape. Don't think thin circular makes the best use of available copper volume space.

[willib]

"If you have got the volts you need and have space to spare, you use thicker wire. Maybe I misunderstood and you are trying to optimise for best output with a given wire size."

i am sort of constrained by wiresize , i have lots of 0.034"dia and enough of 0.045"dia wire

i'm going to try a coil twice the 0.270" thickness with two strands of 0.045" wire,i should get same number of effective turns (84),and get half the resistance (0.069 ohms ), like you said i have the room , so why not?

[willib]

i agree so i'm gonna try doubling the thickness to 0.54" , and use two in hand , keeping the ID & OD the same

did i tell you that i clocked my smallest blades at 900 RPM the other day?

27" dia , i forget the wind speed ,so i'm not going to guess , but i do remember seeing 900 rpm several times

soon , i hope to have a setup where my laptop will record all the data

another problem with the old minigen is its low coilcount (9) and low magnet mass 24 (1/2"dia x 1/2"), consequently low voltage , but plenty of current to raise the voltage of two parallel NiMH batts pretty quickly

maybe the next time i take them out i'll put more in parallel

[SamoaPower]

I still think you're making a mistake with round coils. The problem is that so little of the wire length lies perpendicular to the magnet rotational path where it would have the greatest induced voltage. I don't think the fact that you're using cylindrical magnets makes a difference. It's the described rotational path that counts.

Yes, a circular coil will use the least wire, thus the lowest resistance, but if most of that wire is less effective in producing induced voltage, are you really ahead in the game?

If you want to report something interesting, give us wind speed and electrical power output. Unloaded RPM doesn't mean much.