Charging door

[cruiter]

I think you have a "magnetic" fetish

Originally Posted by drmarty

OK I got some photos uploaded to my gallery cleverly named drmarty. It shows some of the many magnets I have. I do not want to modify mine because it isn't broken. So I will be glad to send you magnets to try and if you need weaker or stronger ones then we can do that. I have attached some photos. There are a couple more at my gallery.

Segsby,
I put two litlle flat magnets where the original two are and it makes the cover harder to pull off. It holds the cover up ever so slightly. You wouldn't notice it if you didn't check it. I didn't glue these in or anything. I just put them on top of the old ones. I have attached two photos. The second one shows the magnet pushed off to the side to show it's thickness and that it is not hooked on any way. It works just set in place.

marty

[bentbiker]

Originally Posted by cruiter

I think you have a "magnetic" fetish

What a nice way to say they have such magnetic personalities.

[SEGsby]

That's a lot of magnets...

SEGsby

[drmarty]

Hmm

Might be a few more magnets too. Somebody said if I put magnets in my shoes or under my mattress I would be cured.

I didn't have anything yet but why wait.

But they did make my feet hurt like there were rocks in my shoes and my mattress sure was lumpy. I may be doing this wrong.





OK I do like magnets and the new rare earth ones are very very strong (fun) but most of those I bought to make motors. We used to buy brushless motors for our RC airplanes for over $100. Then we found that we could take plain old broken CD ROMs out of computers and tear them apart. The motor that spins the CD is a three phase brushless motor but not quite right. We take them apart and rewind them with fewer turns of bigger wire and then we remove the weak magnet ring from the part that spins and glue in twelve new rare earth magnets (the ones in the photos in the little white holes) and voila! for less than $5 a beautiful motor, lot's of understanding of how it works, and lot's of satisfaction.

They flew me to conventions in Texas and Washington (state) to teach classes on making the motors. Too much fun.

So I got lot's of magnets with no homes. You need 'em or you want 'em, you got 'em.

marty

[bentbiker]

Originally Posted by drmarty

We take them apart and rewind them with fewer turns of bigger wire and then we remove the weak magnet ring from the part that spins and glue in twelve new rare earth magnets (the ones in the photos in the little white holes) and voila! for less than $5 a beautiful motor, lot's of understanding of how it works, and lot's of satisfaction.
marty

Marty,
I have to ask -- the changes result in a motor that has greater torque? Quicker response? What? I love the idea of recycling legacy hardware. I recently found a rare earth magnet not much bigger than a hearing aid battery and it was almost impossible to pull it off of the steel bar to which it was stuck -- amazing.

John

[ryan_walters]

With self-wound brushless RC motors, we usually talk "kv", which is an abreviation for "rpm per volt". Higher kv means more rpm, but less torque. (smaller prop turning faster). Lower kv means more torque, but less rpm. (big prop turning slower).

The less turns on a motor, the higher the kv it will have (hot winding). Consequently, the more turns on a motor, the less kv (cool winding). The right number of turns on any given motor depends on a lot of things, prop size, how much current you want to draw staying within limits of batteries, speed control, the motor itself, etc. And also the kind of flying you intend to do with the motor. Faster high speed flying you want a small prop turning high rpm, so a high kv motor. For slow 3d type flying you want a big prop turning low rpm for max static thrust. Regardless of the number of turns on each motor pole, you want as thick a wire as you can use while still getting the number of turns you want. (thicker wire means less resistance, and less heat generated, therefore more efficient)

Marty,
I have to ask -- the changes result in a motor that has greater torque?

I imagine a stock cdrom motor is probably wound for extemely low rpm, but high torque, in order to spin the cd. If Marty's re-winding with thicker wire, and fewer turns, he's creating a motor with less torque, but more rpm, more suited for turning a prop. Of course changing the magnets out for stronger ones gains efficiency all round, and may even outweigh the 'hotter' wind having less torque.

We probably don't even want to get into stator size changing the kv / torque output, or chaning the number of magnets on the bell (can), or how to wire up the three phases, etc...


In order to make this post somewhat segway related, I will say that the segways motors are 'inrunners' instead of 'outrunners' that we're talking about here. Inrunners typically have a VERY high KV, and therefore not alot of torque. Insert a 24:1 gearbox, and you get huge torque, and lower rpm. I think I calculated once that a segway right on the speed limiter (20km/h, 12.5mph) has it's motors turning at 5 - 6,000 rpm.

[drmarty]

There are three motor constants that completely characterize any electric motor from the little motors that make your pager vibrate (yes that is a motor with an eccentric on it so it shakes when it spins) to the largest motors in industry. The constants are No Load or idle current - Io, the voltage constant - Kv(as mentioned before it is RPM per volt) and the Rm or motor resistance (some people say Ra - armature resistance. I am not an engineer so I will leave it to them.) You can characterize or describe any motor with these three numbers or so they say.

CD ROM's have a fairly fixed speed of about 500 RPM for a 1x drive. A 12X drive is 6000 RPM then.

And many things affect the performance of the motors. First of all, they are outrunners, that is the outer part spins, not the inner part. That gives much greater leaverage and more torque to start with. There are two ways to connect them to the speed control called delta and wye but they are essentially series and parallel. One is 1.73 times faster but the other has 1.73 times more torque.

Stronger magnets give more torque and more efficiency.

Mainly winding is based on the number of turns and is direct. Half as many windings gives twice the speed. So you decide how many windings you want and then use the biggest wire you can to allow those turns.

And so on and so forth. There is lot's of trial and error but that is the beauty of it. No body would tear apart a $100 motor and rewind it (almost no-one) but with these you can make a good rotor (the part with the magnets that spins) and then try any number of stators and spend mere cents on wire, etc.

Marty

[drmarty]

How about a couple of photos? These are just a few quick ones off the internet.

thumb-08_-_Side_closeup.JPG thumb-09_-_Bell_closeup.JPG thumb-12magnets_x_10x3x2_(1).jpg

thumb-BluebusterMotorCore.jpg thumb-BluebusterMotor.jpg

You can see in the top one they stacked two motors together for a "Double Whopper" and are using a custom made rotor. The bottom motor is more "stock" using the rotor off the CD ROM and just replacing the magnets.

Marty