Author Topic: coil size vs # of turns  (Read 7998 times)

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defed

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coil size vs # of turns
« on: July 02, 2009, 11:38:36 PM »
i wanted to add this question to a previous posting, but it seems that if you reply to an old post, it doesn't move it up, so noone would ever see it...


if one winds a coil, and it is a little smaller in overall dimensions than it should be (ie wound too tight), how much difference does this actually make in voltage generated?


let's say the magnets are 2x1, and the coil ends up 1/8" too small all the way around..155 turns.  would there be enough difference in the amount of wire used to make any difference in output?  does the actual amount of wire in a coil make much difference or is the # of turns the most important thing?  i can't see there being very many feet of wire to make the coil slightly bigger, but then again, i never calculated how much wire would be in a coil of the smaller size.  i suppose if the small coil had 100' and 10' was used to make it the 'right' size, that's a 10% increase.


i did a brief google search for this info, but didn't find anything good about it.


thanks.

« Last Edit: July 02, 2009, 11:38:36 PM by (unknown) »

wdyasq

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Re: coil size vs # of turns
« Reply #1 on: July 02, 2009, 08:21:24 PM »
defed,


There are three elements that determine how a generator performs. They are:


Density of magnetic field.


Rate of change of magnetic flux.


Number of wires in the changing field.


The "Science" of how to calculate these is a major undertaking. In actual practice, engineers make a SWAG and them refine the data for a finished product.


To answer your question, IF you keep all other parameters the same, the NUMBER of turns will will change the voltage at a particular RPM. more turns, more voltage, less turns less voltage.


Ron

« Last Edit: July 02, 2009, 08:21:24 PM by wdyasq »
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brokengun

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Re: coil size vs # of turns
« Reply #2 on: July 02, 2009, 10:46:16 PM »
I've heard as a rule of thumb that the coil should have a hole in the middle roughly the size of one of the magnets you're using. Not entirely sure why this is as far as the science is concerned it might conform to the magnetic field better or something. I think you will see a difference if this hole is too small but probably not that much.
« Last Edit: July 02, 2009, 10:46:16 PM by brokengun »

Flux

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Re: coil size vs # of turns
« Reply #3 on: July 03, 2009, 12:09:01 AM »
There is no simple answer to your question but if the hole size is the same for both cases then the tight coil will produce virtually an identical voltage to the loose one as long as the thickness doesn't make you change the air gap.


The voltage depends on the flux per pole of the magnet, rotational speed and number of turns. If you kept the centre hole size the same and wound with very thin wire in theory the volts would be the same. In real life the voltage is a distributed thing and not exactly the sum of all the voltages of all the turns so the coil size does affect volts to some extent.


The loosely wound coil will have a longer wire length and will have more resistance than the tight coil and so in theory the tight coil will give a bit more current for the same voltage. With the correct wire size and number of turns the difference will be very little in practice and as long as the loosely wound coil fits it will be good enough. There are going to be variations between individual winders but unless you are so sloppy in your winding that the coils don't fit or you have to change the air gap then you will see no difference and you would be hard pushed to measure the difference on a careful test.


Flux

« Last Edit: July 03, 2009, 12:09:01 AM by Flux »

David HK

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Re: coil size vs # of turns
« Reply #4 on: July 03, 2009, 03:47:51 AM »
Flux is always a ggod guide on this.


But from experience may I add:-


Wind all coils from exactly the same jig,


Include a trip counter so that it counts and follows the same set up routine.


After winding the coils weigh them on an accurate (digital) set of kitchen scales (for example) and then note down all weights.


The odd extra coil turn or two will make little or no difference.


David HK

« Last Edit: July 03, 2009, 03:47:51 AM by David HK »

defed

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Re: coil size vs # of turns
« Reply #5 on: July 03, 2009, 05:35:59 AM »
thanks for all the explanations.


this is a Piggott 4', sideways magnets and coils.


i wound my 6 coils.  the 1st one came out good, but i was not aware of how to check the 'size' of it at that time.  so i did the other 5 the same since it looked good.  when i set them in my layout, i noticed they were a little too small (SHOULD HAVE CHECKED THAT W/ THE 1ST ONE!!!).  all are the same size and thickness.


now i am unsure if i should use the original magnet spacing as a guide and leave a little space between coils, or push the coils in a little and then the magnets would be a little closer too.  again, maybe none of this makes a difference, or there is no easy explanation!


i would rather not have to make 6 new coils since i probably would be a little short if i tried to rewind my current ones looser...but, if it will make a big difference, i will do it!


thanks again and for the patience of a rookie.


Dave

« Last Edit: July 03, 2009, 05:35:59 AM by defed »

Madscientist267

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Re: coil size vs # of turns
« Reply #6 on: July 03, 2009, 06:33:35 AM »
The flux needs to pass through the same 'side' of every turn in the coil. If not, the effect is that the field riding both sides cancels out, reducing the efficiency.


When the hole is the same size as the magnet that passes over it, you get the highest flux concentration possible for the entire coil, with minimal cancelling.

« Last Edit: July 03, 2009, 06:33:35 AM by Madscientist267 »
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Flux

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Re: coil size vs # of turns
« Reply #7 on: July 03, 2009, 08:01:05 AM »
Just leave a bit of space between the coils. As long as the centres of the coils line up with the magnets it will be fine. A tiny bit of change in the distance from the magnets to the coils will make more difference than anything you can alter with the coils themselves.


Flux

« Last Edit: July 03, 2009, 08:01:05 AM by Flux »