On Sun, 06 May 2018 17:38:29 -0700, etpm wrote:

I want to use a hobby BLDC motor to power a compact high speed spindle
to be used for engraving and the like. Mainly because they are so
compact for the power and speed. I cannot find, for a price I can
justify, a 24,000 to 40,000 RPM spindle that will fit in the space I
want to put it.
Looking at some hobby BLDC motors and then buying some to
experiment with leads me to think that one of these motors may work.
I have looked online for answers about these motors and maybe I'm
just dense (good possibility) but I haven't been able to find exact
answers. Maybe someone here can help.
Motors are rated RPM/volt. This means maximum RPM per volt of
course. If the motor is supplied with a higher voltage from a supply
that limits the current to stay below the specs of the motor will higher
RPM be possible?
The ESCs (Electronic Speed Controller) are voltage and current
rated. Could the output an ESC rated for a lower voltage and current be
used to drive control power transistors in order to get around the lower
voltage and/or current rating if a sensored motor is being used?
I don't need to use a hobby ESC but they are plentiful and cheap.
And all assembled. Which is most important. But I would be fine using
some other BLDC motor driver in order to get the desired RPM that I
want.
I can do all the precision machining necessary to make the spindle
that will hold the carbide cutting tools and to interface the motor with
the spindle. I do not yet know enough about balancing in order to
dynamically balance a motor which causes too much vibration from an out
of balance condition but I think I can learn enough to do it. I am good
with mechanical systems and have a pretty good intuitive feel for them.
It's the electronics I'm not so good at.
Thanks,
Eric


They are not so compact when you factor in cooling. Marine ones (often)
use watercooling jackets and they do short bursts at max amps.

The speed goes down as the size goes up. There are differences in windings
- more (thinner) turns means less amps but more rpm.

There are differences in numbers-of-poles and numbers-of-magnets. These
affect the necessary programming of the ESC, and its programming-and-
design will limit the maximum frequency (~rpm) at which it can drive any
motor.

There are inrunner and outrunner types - in and out referring to where
the magnets are in relation to the poles. Inrunners tend to have higher
speeds for a given power, and are probably simpler to mount and cool for
what you want to do.

Motors are limited by heat, not current(directly). If you can keep it
cool it does not matter what the rated limit says. The ESC is limited by
both heat and voltage - if you can keep the power chips cool you can beat
the first. You can't beat the second.

I don't think there's much point to using a "sensored" motor - those are
useful for controlling the motor from zero rpm to some small value. They
have a fixed sensor and cost more and need a more complex ESC. I think
they might also use the sensorless feedback - it's there and they have a
computer on the ESC, why not.

The "sensorless" ones just need the ESC to use feedback to decide which
way the motor is going. What they do is guess, give the motor a tiny
kick and see what happens after that, and if it's wrong they switch
direction. I don't know but it seems reasonable it could decide in less
than 1 rev, maybe a few degrees.

Sensorless ones means you can't use outboard power transistors because
the ESC will be blind. Just get a bigger ESC.

There is likely some combination out there that will work - most motor
vendors will give a max amp@volt or watt figure as well as a kV and it
does not sound like you need much. 40K is on the high end of what I've
seen and they were small. With a good quality motor and ESC and cooling
jacket and water-pump and powersupply you may be on the far side of $200.

I've wondered about that "kV" figure too - I may get an optical tach to
play with.