quote:
wrote in message ...

I've been looking at the brushless motors used for RC planes and the
like. They are amazingly powerful for their size. And they really spin
up fast! One web site has kits you can buy to wind your own armatures.
Anyway, I have been wanting a rotary tool that is a cross between a
Dremel and an air powered tool I have. Something with an OD of about 1
inch. Like a Foredom handpiece. Faster than any of my Dremels but
slower than my small air powered hanpiece that spins at 80,000 rpm. So
I'm thinking one of the "inrunner" type motors might work. At 80,000
rpm the surface speed is about 2600 SFPM. Even though the speed drops
quickly as soon as the rotary tool contacts the work the speed is
still too high for carbide on steel and the burrs and rotary files
dull quickly, which makes them cut slower. On the other hand, the
Dremel and Foredom rotary tools are too slow and this ends up costing
time too. Something that spins about 40,000 rpm and maintains that
speed along with the smaller diameter would be great. In fact, a tool
that was .75 or even .625 diameter would be even better. I have a
dental handpiece that I use for some delicate work and it is just
under .5 diameter and is very easy to hold because of the small
diameter. If I could build an electric rotary tool with the torque of
a Dremel, 40,000 rpm, and 1/2 inch in diameter I would be very happy.
Even one that matched the Dremel torque and speed specs but was 5/8 or
1/2 inch diameter would please me greatly. So, anyone here play with
brushless RC motors?
Thanks,
Eric,
/quote


I wanted to try one of these as a cheap alternative to a conventional
brushless dc motor in a mechanism we were prototyping at work, so I grabbed
a couple with controllers off ebay to explore. Then I learned that the
control input needed to be a variable duty cycle digital signal, something
like (from old memory so I'm sure it's a little off) 10 or 15 ms period and
0-5 msec on time for 0-100% of motor speed, sigh. I used a signal generator
to fake it for the initial tests but it would have been a pain to drive it
with our controller if it had worked out (we were set up to output 0-5V for
0-100% motor speed). After getting the parts it was clear that the ESC and
the motor winding could handle some pretty hefty currents to make the hp
they claimed, but there was no way they could dissipate even 10% of the
resultant heat steady state. These were airplane motors and ESC's so they
were engineered to run for 1-2 minutes max, and then have many, many minutes
of cool down time while the battery pack was recharged or swapped out. The
motor bearings were pretty wimpy, too, so I was afraid that putting a pulley
on the motor shaft and having the lateral load from a belt would kill things
pretty quickly, but I never got that far since it was obvious that the
package wasn't what we needed. Hey, it was a fun experiment, anyway :-).
I'll dig one out Monday or Tuesday and see what rpm it's rated for, maybe
you could play with one.

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Regards,
Carl Ijames