On Thursday, December 13, 2012 10:53:55 AM UTC-6, Existential Angst wrote:
"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

Don't forget DC injection for stopping motors..
It is not regenerative, and it is not running it backwards..