Coil testing for a 20' wind turbine

[DanB]

Geez... it's only 3PM and we've allready had so much fun.  It's been very windy this winter.  Toms wind turbine died on Sunday... quite turning, like it was shorted out.  We were almost certain that the wires outside the stator had shorted together.  We made a few machines a couple years ago where all the connections for the coils were on the outside of the stator.  We tried to epoxy the wires securely so they woundnt vibrate but this is the 3rd failure in these machines where the sun had degraded the insulation - between that and vibration wires outside the stator shorted together.  (glad we do things differently now).  We fix it by seperating them, and caulking all around them in hopes that it'll last a while.  Took about a half an hour and Toms machine (one of the 'triplets') is running fine again.

Next fun activity was designing a coil for the new machine.  I've decided to shoot for a 20' machine here on my latest project.  The inside hole of the coil is 3" tall (same as the magnets), 1.5" wide at the top (same as the magnets) and 1" at the bottom (a bit smaller than the magnets.  The legs of the coils are about 1.25" wide.  First I draw the coil shape on the mold itself.

Then I design it on paper and cut it out so I can look at it on the magnet rotors.

I used the same coil winder we did for my 17' machine, but since the magnets are further apart from one another the inside of the coil is bigger.  We just drilled 9/64" holes into the coil winder and cutoff 16D nails for pins and stuck them in to wind the coils around.

We wound two test coils, one fit 108 windings, the other fit 110 - of #14 gage wire.  Doesn't matter what we wind the test coil from or how many windings it fits - it just tells us volts/winding, and how much copper fits into the right shape (or rather... our designed shape, be it right or wrong or somewhere inbetween)

We glued the coil to a piece of plywood which is mounted to the stator brackets.  After gluing it I gave her a spin and shorted it just for fun.. of course the coil went flying.

If we short the coil and then turn the alternator, it's very obvious that maximum current is flowing in this position (it gets quite stiff to turn even with only one coils especially when we have opposite poles over opposite legs of the coil).

It came out about like I figured.  Looking at my 17' machine, I figured I'd want about 45 windings in here for a 20' blade.  As it turned out, our test coil (with 110 windings) gave up 10 volts @ 60 rpm.  (actually somewhere between 9.4 and 9.  We're measuring AC volts here.  Peak AC volts (that which is important to know when we're rectifying things to DC) is very close to .7 x RMS AC volts (that which our meters show us).  So - the goal at 60 rpm for 1 phase is about 33.6 Volts.  Weve got 10 coils in each phase, and we know that the sum of the output of 5 coils x 1.73 must equal 33.6 Volts @ 60 rmp.  33.6/1.73 = 19.42 Volts, so the sum of 5 coils in phase with one another must equal about 19.42 Volts.  19.42/5=about 4 Volts.  So we want about 4 volts AC from each coil @ 60 rpm.  Turns out about 44 windings is appropriate here.  I was hoping we could wind with 3 strands of #14 in hand, but it's looking like the best we can do here is 3 strands of #15 to get 45 windings per coil (throwing in that extra winding for rectifier loss).  Not very scientific but lots of fun and it should come out close.  If it doesn't we'll either open or close the airgap (we have plenty of room to do either I think).

Monday we'll hopefully start winding up the stator if Im lucky enough to have 6 rolls of #15 gage wire available.  I figure there will be around 22 - 25 pounds of copper in this stator.

[tecker]

 I like the sharp turns the nail gives there .I noticed a 10 or 16d nail scuffs off the insulation I now run a meter lead over the coil after to test . I may have been  little ruff or the first row trying to get a tight base on that row .Anyhow this may not be a factor considering the resin mold in there but I went to a 12/ 24 nylon screw with slotted holes to release the screw toward the center . The threads make a firm first row.Also interesting the repulsion your getting .

[willib]

Dan , What is the air gap , as it is  or was for the test?

Question? if you were to use 3"x1.5"x1" magnets , would you get more voltage ?

everything else being equal ...

This was a treat , and it's not even Monday!

[force9BOAT]

The sharp turns around the nails is something I was wondering about.  I've been under the impression that electrons like to travel in a straight line.  Sharp turns increase resistance.  I'm I wrong about that?

Rob

[RP]

At these frequencies you can ignore tight turns as a factor.  If the bend radius is "really" tight (like less than 2 wire diameters) I'd worry about cracking the copper but other than that don't worry about it.

[Flux]

Dan, nice write up as usual, it's going to be a fine machine.

Just a comment on one bit of your explanation, I know what you mean but it is difficult for others to follow.

."  Peak AC volts (that which is important to know when we're rectifying things to DC) is very close to .7 x RMS AC volts (that which our meters show us).  So - the goal at 60 rpm for 1 phase is about 33.6 Volts. "

Peak ac volts are 1.4 x rms volts. Rms is .7 x peak volts ( dc volts)

You are right that for 48v dc you need an ac line voltage of 33.6v rms ( ac meter reading.

The rest of the arguement follows logically.

Flux

[tecker]

That's a valid point there at what point would the bending radius mess up tha max current available. 14 can take a sharp bend the paint can't maybe a dab of varnish on the first and second row.

[hvirtane]

I'm in agreement with Flux.
It is maybe a bit difficult
to follow the explanation.

I'm thinking it this way:

1)
To calculate the needed 3-phase
RMS output:

For 48 dc volts you need
48/squareroot(2) RMS =
48/1,4142136 = 33,941125 RMS

2)
To calculate one phase voltage:

For one phase you need
(using 'star' connection)
33,941125/squareroot(3) (RMS) =
33,941125/1,7320508 (RMS)
= 19,595918 (RMS).

3)
One coil voltage, if you
will have 5 coils in a phase:    

19,595918/5 RMS = 3,9191836 RMS.

You have used in the calculation
1) an approximation 1/squareroot(2) ~ 0,7
and in the calculation 2) an approximation
squareroot(3) ~ 1,73. And you got
an approximation 4 volts for one coil,
which is very near to the above calculated
3,9191836 RMS.

- Hannu

[paradigmdesign]

2. questions DanB
   
1. How much did you get thoes rotors for? (I think I am getting ripped off) And what is there diameter?
   
2. How much power are you going for? 5-6K?
   

[willib]

They were around $70, each i think?

and 22" in dia i believe.

[DanB]

Right now it's .9" Willi.  I could probably run it closer, but I prefer to keep the airgap safe.  Yes, I expect 1" magnets would increase voltage a bit - it would be a more powerful machine that way.  This is all I really want and probably more than I need though.  The idea here is to build a slightly bigger machine than my current 17' one, and hopefully make it much stronger.

[DanB]

Thankyou for correcting my mistake there Flux - that was a bit confusing the way I put it down.

[DanB]

Actually they were a bit over twice that cost.  Right at $300 for both rotors.

[DanB]

Thankyou Hannu - yes, a much clearer description of whats going on.  I round the numbers off.  I Figure our sine wave isn't perfect so an approximation is close enough and there'sa  bit of room for variation between the coils etc.  It should get us in the  ballpark, then we can play with the airgap and plan the blade according to the alternator.  Should be a fun one!  With luck perhaps we can cast a stator on Monday.

[amiklic1]

A bit more clear to me, now.

I can figure out this.

When I measure AC volts with voltmeter (test coil at some RPM), I can use the formula Vac x 1.4 = Vdc.

If I have 4 coils in one phase (all in series connection), my phase voltage would be 4x V(1coil).

If I get, let's say 2.14V with one coil voltage with 60 RPM in a system with 4 coils per phase, I get 4x2.14=8.57, and than 8.57x1.4= 12V on 60 RPM in total system. Is that so?

I think I need to turn the generator to the speed (RPM) of desired cut-in speed to measure it correctly. Only, I don't know how danB measured all that with 110 turns on a coil, and how he find out that he'll need 44 turns (45 for avoid rectifier losses)?

I still don't understand it.

[willib]

Dan you gotta get us a current reading ,when you wind the new coil (44 turns , three in hand #15 wire),i think it was..

i wouldnt want the coil to go flying again , but would really enjoy reading about those tests

:-)

[Gary D]

Dan, I'm wondering what amp twist lock plugs you and the others up there are using. At Lowes, the highest rating I can get is 50 amp... Sure I can get much heavier at a local electrical supply house. Not sure if you can overrate(proper term?) or over current them like you can the wire sizes?

 Would be great if you gave an update on the winco 11' machine when it's up and flying! A few of us have flown them, mostly smaller diameter tho. It would be interesting to hear how you feel the sound differs from your 3 blade furling units, especially in higher winds(air brake engaged)...

 Love your posts, the ear protection on your "guest" last week was absolutely priceless!     Gary D.

[DanB]

Hi -

"When I measure AC volts with voltmeter (test coil at some RPM), I can use the formula Vac x 1.4 = Vdc."

Yes - although in the end we'll need to subtract for rectifier losses but that comes off the competed stator so we don't worry too much about that with a single test coil.

"If I have 4 coils in one phase (all in series connection), my phase voltage would be 4x V(1coil)."

Yes - but remember that if we wire teh stator in Star, then we have to consider the other coils in series that are out of phase, so we have 4 coils in series, and then the 2nd 4 coils in series will offer 1.73x the voltage of a phase.  So if we're building a 3 phase machine with 4 coils per phase (a total of 12 coils in the machine) then the final output will be from 8 coils, so the actual output will be 1.73 x the sum of your 4 coils.

"If I get, let's say 2.14V with one coil voltage with 60 RPM in a system with 4 coils per phase, I get 4x2.14=8.57, and than 8.57x1.4= 12V on 60 RPM in total system. Is that so?"

That would be true in a single phase machine wired in Delta.  In star though you've actually got 8 coils in series to think about, and 4 of them are out of phase. So the final output is 1.73x the sum of 4 coils.

"I think I need to turn the generator to the speed (RPM) of desired cut-in speed to measure it correctly."

Not really - you can turn it at any speed so long as you know how fast its turning since voltage will increase directly with rpm.  (if you have 10V @ 60 rpm then you'll surely have 20 volts at 120 rpm).  So you just need to know the ratio of volts/rpm.

 "Only, I don't know how danB measured all that with 110 turns on a coil, and how he find out that he'll need 44 turns (45 for avoid rectifier losses)?"

Because voltage is also directly related to the number of windings.  So long as I know the wire gage, number of windings and the rpm at which we're testing, I can extrapolate and figure out how many windings I really want and I can know what size of wire will fit to make the proper sized coil.

If my desired cutin speed is 60 rpm, and I know I want 4 Volts but my test coil is of 110 windings is giving meme 10V, then I know I need 44 windings to get the right voltage.  I also want my actual coils to be the same size, so I have to figure out what wire gage will achieve that.  3 strands of 14 with 44 windings would be the same size as a coil wound with a single strand of 14 and 132 windings.  In this case though my test coil has only 110 windings, and I believe 3 strands of #15, 45 windings will be very close to the same size.

[DanB]

Yes, I think 50 amp is about the biggest I've seen too.  We definitely push more through them at times.  Perhaps for machines producing lots more current than 50 amps we could use 3 plugs - one for each wire and parallel all the leads to be safe.

[hvirtane]

I think I need to turn the generator to the speed (RPM) of desired cut-in speed to measure it correctly. Only, I don't know how danB measured all that with 110 turns on a coil, and how he find out that he'll need 44 turns (45 for avoid rectifier losses)?

Let's put it this way.

a)

We already made the calculation

telling that we will need 4 volts

per coil.

b)

By the test we know now that

110 windings will give 10 volts.

c)

We know that the voltage climbs up

linearly with the number of turns

on the way of the magnetic flux.

So we can calculate.  

For one winding the voltage is:

10/110 = 0,090909091 (volts per winding)

So to get 4 volts we'll need:

4/0,090909091 = 44 (windings)

- Hannu

[hvirtane]

I round the numbers off.

Yes. It is certainly perfectly OK

to use 0,7 for 1/squareroot(2)

and 1,7 for squareroot(3).

I just wanted to clarify a bit,

where the numbers are coming from.

- Hannu

[apie]

This is a great-looking device!  I noticed that the legs of your coils are very wide (almost magnet width?).  I thought that the best situation (thread-off) was a coil with legs as wide as half the magnet width.  So total coil size would be twice magnet size.

Why did you choose this size?

I was also thinking that a coil with wide legs doesn't generate a high peak of voltage since the magnet moves gradually over the legs.  So can you extrapolate from that just linearly?

[theTinker]

ive been researching this morning myself. I found it is a good target to aim for 'half the magnet width per leg for rectangular magnets, but if u cant fit this in for just say 12 coils(16 poles) then u have to drop to 9 coils(12 poles)(assuming  fixed rotor size). then the problem becomes 9 coils with half the magnet width for leg leaves rooms free which is a waste , so just fill up that room with more leg(though this means resizing the wire used to maintain the right voltage. using it as a guideline seems more reasonable that using it as a rule,

[apie]

Interesting Tinker!  I did some calculations and I'm confused.  The diameter of the rotor is 22".  With magnets of 3" long, the inner diameter is 16".  The circumflex of this circle can fit 16 coils of double magnet width.  So your reasoning fits perfectly.

BUT!  I thought there were 20 magnets (as there are 5 coils in each phase)...

So please DanB, how many magnets are there?  And how big is the rotor? (just trying to understand it all)

[apie]

Whoops!  I made a mistake (I must go to bed now!).  It makes perfect sense: there are 15 coils, a bit thicker than usual (there is room for 16) and 20 magnets.

So the bottom line is: use a bigger rotor and then you can make coils bigger.

Simple!  Thanks for the help!

[theTinker]

no problem. it was my first educated explaination

[theTinker]

em...explanation