"Joseph Gwinn" wrote in message
...
In article ,
"Existential Angst" wrote:

"Joseph Gwinn" wrote in message
...
In the thread on SawStop, the issue of making saws stop rotating
shortly
after the user turned the power off came up, with the central example
being miter saws, which stop in a few seconds after the trigger is
released.

But it was not clear how this worked, and if it was more widely
applicable.

Anyway, I did a little research, and found the governing patent. The
electric brake was invented by Black and Decker, and apparently
licensed
widely.

The patent is 6,037,729. To get a copy, go to http://www.pat2pdf.org
and follow the directions.

This approach is intended for universal motors, as used on small saws
and drills and routers, and is not directly applicable to induction
motors, as used in table saws.

Yeah, but to get SawStop-type decelerations, you'd proly need motor
windings
as thick as yer pinky.
iiuc, electric braking is wear'n'tear on the motor windings.

The brake winding is no heavier than the run winding, according to the
patent.

The guarantee is to stop the blade in no more than six seconds, which
implies that the typical time is about three seconds.

The mechanism described in the patent requires that the brushes and
commutator be in good condition, as well as the switches.

As for induction motors, rapid stop is implemented in almost all VFDs,
by a related but different mechanism.


The discussion continued under the thread Ping Leon: Design Q on
sawstop,
in rec.woodworking, which turned into a bunch of silly backbiting with
those
silly territorial rw ninnies, but still some inneresting points,
alternative
strategies were discussed -- namely, that you don't really need to STOP
the
blade so suddenly, as much as just *get it out of the way in a hurry*,
which
can be done in a mouse-trap trigger like setup with standard springs,
with
large but realistic forces. Then a non-destructive rapid deceleration is
possible, with regular ole piston disk-brake calipers.
Some calcs appear in that thread.

The subject of the present thread is how miter saw electric brakes work,
and more generally how to stop induction motors, and is not a
continuation of the SawStop thread, which has degenerated, as you
mention.

I was just highlighting the point that there is rapid stop, and there are
..001 sec stops.

And also that even 6 sec braking can be tough on motor windings, if what I
read was correck.
Would be nice, in electric brake systems, to be able to switch it off when
it isn't nec, and it is not always nec -- like the manual brake on lathes.
Unless yer switching parts in a snap-handle collet, no point in wearing out
the brake if coasting to a stop is ok.

Also, there are at least three types of electric brakes: two types of
regenerative: the stuff you are talking about is proly an "internal" type,
thru winding switching. There is also regenerative braking back into the
"grid", or thru high-power resistors.
Or, you just power the motor backwards! Which would require pinky-sized
wiring....
--
EA



Joe Gwinn