The Box Fan- Again! Just motor anyway.

[coldspot]

Box Fan-

I know this have been done and covered, but.

After reading every post the search turns up I did't see that I was looking for.

My box fan has 6 coils, each coil has three different sized wires. 3-speed fan.

What I'm wondering is if I should hook them together, at each coil or at the end of the coils?

Also about the lam T's sticking out of the coils, should I hack them off or leave them?

I can't really see what the coil inside size is with the T's in the way.

I've read about using 4 mag's with Box Fan's. I was wondering about the 3-4 rule,

3 coils per 4 mag's. That would be 8 mag's then for the 6 coils. cogging is a known issue I understand. But I didn't get the stock fan blade so I will have to build something anyway.

 What I'd like to do maybe, is to use the Neo's I have, (1/2" X 1/16" Disc's), by drilling down into the arm at the right places and stacking them up in these holes.  

 If I cut the T's off I would then have clearance for some mags. If I didn't then I'd have to use the hole's.

Thanks

[johnlm]

The answer to your first set of questions is not simple.  Usually the number of turns on these various sized wire coils is not the same.  Meaning one winding may have 150 turns per pole of say awg 24 wire and maybe 200 turns on the same pole of say awg 26 wire.  And the third winding something different than those two.  Because of this it presents difficulties in tieing all the windings in parallel as one winding will be trying to generate a different voltage than another winding.  You can tie all the windings in series but you end up with a a very high resistance (and high inductance) winding that will limit you to pretty low current - maybe in the order of 100 mA.  However your cutin voltage will be pretty low rpm.  If you want to generate 40 or 50 Volts at 500 to 600 RPM and maybe 100 to 200 MA then tieing all the original windings in series will work ok.

I have converted several of these box fan motors and Ive found that if you want more than 5 to 10 watts out of them at 12V you need to take out the old windings and rewind it with larger guage wire (AWG 18 to 22 works pretty well.)  Do not cut off the laminates (the top of the T if I understand you correctly).  You can cut (mill)down the armature if you want maybe 1/8 of an inch so that your 1/16 in thick neos will fit.  Since its only 1/8 of an inch you might be able to just grind down 6 flatted out areas to place the magnets.  You likely will have a fair amount of cogging depending on how much of the face of the laminate poles lines up with the size of the magnet diameters. You really have to experiment with magnet placement and orientation (although you dont have much latitude on orientation with round magnets) to reduce the cogging.  I have used the domino shaped (1-7/8 X 7/8 X 3/8 inch) Lowes ceramic magnets cut in half to give ~ 7/8 X 7/8 X 3/8 magnets to use in these motors with some success but I had to press off the original rotor from the shaft and build a new rotor by cutting out a circular piece of wood out of plywood (thickness to match the thickness of the original rotor) using a drill type hole saw and then slipping it into a piece of pipe the right diameter to allow for the magnets to be glued around the outside of the pipe so that the magnets have a couple millimeters clearance to the laminate poles.  Ive used this same approach and used 1 X 1/2 X 1/8 inch neos glued to the pipe.  On one motor rewound with 100 turns of AWG 18 per pole (X 6 poles) and using 2 ea of the neos mentioned per pole (12 total) I ended up with a conversion that has a cutin (12V) RPM of 300 and can produce around 70 Watts at 1000 RPM (single phase).  Works well with a 3 to 4 ft prop.  I was able to arrange the neos at a bit of an angle so that the cogging is pretty low (1 oz-ft).  I have tried using 8 magnets across the 6 poles for 3 phase on some of these motors with some amount of success.  Getting around 20 Watts out of these 20 inch box fan motors is not too difficult using ceramic magnets and either rewinding or carefully determining how many turns of each wire size is on each pole and trying to connect up some kind of parallel/series arrangement of the original windings.  But rewinding makes it much easier to get the winding resistance down to something that will allow a respectable amount of current.

Johnlm

[coldspot]

THANKS Johnlm

I've read all your posts

I could just chuck up the arm in a drill and hold a file agaist to reduce the size but

drilling 1/2" holes to place the little neo's seemed simpler. And having 300 to play with, I could put a few in each hole.

 I only have old wire, no new yet so I didn't want to rewind.

 I guess I should look at my collection of induction motors closer.

 I also need to do a search for a wire size cross refferance, (gauge to 0.000)

Anyway thanks and I'll put my newest find, (box fan motor) away for now.

Have a great Day/night!!!

[ghurd]

I don't believe placing magnets in a hole is a good idea. It 'shorts out' much of the flux.

With 8 magnets in one of those, the flux path may go just across the top of the Ts, not really linking the coils.

Did you see my box fan post about using 4 magnets and the factory wire?

It got around single phase cogging, has 3 phases, and didn't take any fancy tools. I'll get back to it one of these days... probably 150 turns of #20 per tooth.

G-

[johnlm]

Coldspot,

Ghurd has alot of experience in modifying these little motors so I would trust what he said about drilling a hole and burying the magnets, especially if you are going to be stacking them.  Alot of the flux will just bleed into the surrounding metal.

As you suggest you could chuck it up and rotate it to file off 1/8 inch, but I think grinding 6 flatted areas with a bench grinder would be faster.  I think Ghurd's 4 magnet 6 pole approach results in less cogging but does not allow for as much output power as one would get using all the poles, wire and  6 magnets.  It is pretty tricky trying to keep the cogging down to an acceptable level (especially if you use neos) that you can get the unit rotating with a small 2.3 to 3 ft prop - which is about the size that works best with these box fan motors.

There are several references to wire guage tables on this site that give the guage, diameter, circular mils, and resistance/1000 ft.  You will need this table, and a fairly accurate method (calipers) to measure the wire size if you are going to try to determine the number of turns of the original wire.  The method I use is to measure the wire diameter and go to a table to determine what guage it is.  Then make a best estimate of the average diameter of the windings on the stator poles.  Then measure the resistance of each winding - meaning the total end to end resistance of one of the windings (usually the separate windings are a different color of enamel coating).  From these measurements you can calculate the number of turns of each set of windings.  Its not 100% accurate as it is difficult to exactly determine the number of feet of each winding because you are estimating the average diameter of the windings (diameter X pi = circumference); but it gets you a rough idea.  If you find you have the same number, or very close to the same number of windings in 2 sets of windings then you can parallel those windings to get a lower impedance winding which will result in less IR loss as the current increases on the output.  On many of these motors there are so many turns on each pole that you get a fairly low V/RPM factor so you can even cut the wires at the mid point (3 poles included in each set) and wire the 3pole sets in parallel to further reduce the impedance.  For these small motors if you can get around 25 to 30 RPM/V by playing around with the windings and a DC resistance in the neighborhood of 5 to 10 ohms (your actual output impedance will be higher than this because there are so many turns wound around the laminates that you get quite a bit of inductance) you can get at least 10 to 15 watts out of them with a 2.5 to 3 ft diameter prop in a 15 mph wind.

Johnlm

[coldspot]

Thanks-

But,  "wire guage tables" shows 0 results.

Even after fixing the spelling to "wire gauge tables", 0 results

On-line search shows pletty.

I do have a caliper or two. but no bench grinder. also no real shop.

just doing what I can with what I got, but have to clean-up and drag out every thing and time I do.

 Scored a garage door opener the other day, don't know if it works or not. But good parts at minimum. If works, I'll be building a "Mount to tree and raise a pole" this spring/summer at girlfriends cabin.

As far as the box fan, I'll look thru my induction motors to see if any would make a better conversion. Lately I've been getting a lot of little motors with only two outside coils and a wrapped arm. Some are very new and maybe the wire could be recycled.

 Back to on-line search for wire gauge to 0.000.

Thanks  

[DaveW]

try powerstream.com/wire_size.htm

[coldspot]

After I wrote a page on wire gauge size, (copy from on-line search)

I remembered my N.E.C. handbook. Darn age sucks! lol.