How many turns for 24V

[alibro]

Hi again guys

This question can probably be best answered by DanB

I'm trying to decide how many turns I need for 24V system for the genny I am building. It is a Volvo brake disk design with 12, 2" dia x 1/2" thick magnets on each rotor same as BanB's 10 foot turbines.

I was playing around with a test winding of 70 turns (that just happened to be the number which matched the roll of wire I was using) and got numbers shown below.

These numbers are approximate as my digital multimeter was jumping about.

60. rpm 1.6Vac
    
120. rpm 3.3Vac
     
     

And 120rpm 3Vdc through a Bridge Rectifier

These figures would give the following results if my maths is correct

3. 9 x 3=11.7 x 1.73=20.24 x 1.4 = 28.3V for Star
   
3. 9 x 3=11.7 ÷ 1.73=6.76 x 1.4 = 9.4V for Delta
   
   

Now given that this is a rough test with a single coil, my air gap is big and the meter is jumping about I recon my results are not too far away from what DanB was getting. He was using 35 turns to get a cutin of approx 120rpm at 12V wired in star. Therefore if I use 70 turns and wire in star I should get cutin for 24V around 120rpm

My confusion is from the earlier 10 footers. They were wound with 65 turns and wired in Delta to give a similar cutin rpm. As the Voltage from Star is three times that of Delta does that not mean that when wired in Star these turbines would give 36V at 120rpm? Perhaps a bit less as they have 65 turns. But as 65 is only about 8% less than 70 it should only come down to around 33V 'ish' or is there more to it?.

Am I right in thinking I can adjust the length of the blades to achieve the ideal TSR and cutin even if the stator is not perfect?

I'm sorry if I seem a bit dim  and my maths is probably all wrong but I don't want to go to the expense and trouble of casting a stator only to find that it doesn't work very well. I get the feeling from reading the diarys that 60 turns would be about right for 24V in Star but I would like one of you more experienced guys to confirm please.

Thanks

Alister

Yes dear I am going out to play with that infernal thing again.

No dear I don't love it more than you.

[Flux]

If Dan used 35 turns for 12V then 70 for 24 is ok.

65 turns delta would be the same as 65/1.73 = 38 turns so that checks out ok.

Ignore your single coil into a bridge rectifier, that tells you nothing.

Using your 70 turns giving 3.3v ac at 120 rpm.

Volts per phase will be 3.3 x 3 = 9.9, Line volts will be 9.9 x 1.7 = 16.9

DC volts will be 16.9 x 1.4 = 23.5

So that checks out for 120 rpm cut in. The only thing I would question is why choose such a low cut in for a 10ft prop.

Yes you could mess about with the blade length but that seems silly, as soon as you shorten the blades you reduce the available power, it would be a last resort if the alternator was too fast.

With your 70 turns you are going to err on the slow side so you can raise the cut in speed by increasing the air gap. Unless you are trying to hold things down with a very long line with high losses I would think that you should increase the air gap to bring cut in up to nearer 150 rpm for a 10 ft prop. If you stall then, add line resistance.

I think I would tend to agree with your 60 turns, but it probably won't let you go up a wire size.

It's nice to err on the slow side, you can always try the lower cut in, but unless you are in a very poor wind are I think you will reduce the total energy capture with the 120 rpm cut in. Partly depends on how much you value an amp for a few hours a day compared with reduce energy production in the 5 to 10A region on better days.

Flux

[sdscott]

"I think I would tend to agree with your 60 turns, but it probably won't let you go up a wire size."

Just curious. What gauge wire?  How many parellel (in-hand) wires?

[alibro]

Hi Guys

Thanks for the comments

Wire size used for the test is 1.6mm or 16swg only one wire in hand. Not sure what that is in awg but I think it is pretty close to 14awg. From reading on this board and Hugh's book I get the feeling 150rpm is a better cutin for a 10 foot blade. Do you think 60 turns would achieve this given?

Cheers

Alister

[Flux]

Unless Dan can think of some reason not to, I would not use more than 60 turns.

You can increase cut in speed with a bigger gap, but given the choice it seems better to reduce turns and resistance even if it needs that resistance added back as line resistance, at least the heat is outside the stator.

You need the prop running as fast as it will perform at, at cut in, otherwise you start to come into stall quickly in the normal operating range where you need the best results. You are relying on the fact that a lightly loaded prop is happy to run fast as part of your matching.

With a converter tracking the wind speed you can cut in slower, but even then you will get very little in those low winds,but at least it doesn't come at a price in the mid wind speed range.

Flux

[alibro]

Thanks Flux

I will get winding right away with 60 turns. I think Dan mentioned that 35 turns for 12V was a bit much and it might be better with 30.

Alister

[DanB]

Hi Allister -

For a 10' blade I like to use 70 turns and there's plenty of room for that if you use 14 gage wire.  In low winds we get noticably more power from 10' machines if they cutin around 140 rpm  - a few machines we made cutin around 150 and they seemed less than ideal.  That's with the blades we use anyhow - faster blades would prefer a higher cutin speed.

Normally now on 12V machines we wind with 37 or 38 turns to make up for rectifier losses.

[alibro]

Hi Dan

Is that 70 turns for a genny using 2" x 1/2" round mags like I have or are you talking about the newer ones with rectangular mags. This is a rebuild of an old Volvo brake disk genny I built a couple of years ago.

Alister

[DanB]

Sorry - glad you brought that up.

In that case, 60 turns would probably be better.

If you want to remove some resistance from teh stator you could probably fit 13 gage wire in there.  It may stall the blades in higher winds so you might have to add some resistance downstream...  

[alibro]

Thanks Dan

You had me going for a while there. 60 it is and I will experiment with wire sizes to see what fits.

Alister

[nosbarry]

well i am Nosa a student of Enugu State University of Science andm Technology

in the of Electrical Electronics Engineering overhere in Nigeria.

There are 48 turns