Electric brake on miter saws

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 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.

Joe Gwinn

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....

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

Yes, that's been mentioned before. I think that's how VFDs stop the motor as well.

Joe Gwinn

PS: By the way, there is something wrong with how quoting works in your newsreader - everything ends up double-spaced. Perhaps a configuration problem? JMG

Six seconds isn't that hard on a small motor. And lots of big lathes powered by three-phase motors do plug reversing, where two wires are abruptly reversed, causing the motor to be driven backwards. Industrial motors are designed to tolerate this. And I assume that miter saw makers like B&D also design their motors to tolerate electric braking.

I have the VFD driving my lathe (Clausing 5914) configured to stop rotation in one second.

This can be done in B&D's design. I've read that there are three wires between the motor assembly and the switch assembly, red, black, and gray, and that if one interrupts the gray wire, the electric brake is disabled.

See plug reversing, above. And, no, the windings aren't any larger than normal.

And, the B&D approach actually does drive in reverse for a moment. This is explained in the patent.

Joe Gwinn

Some of the early "fast stop" saws just used a 2 position switch. One position put both lines to the motor terminals. The other position shorted the motor terminals while disconnected from the line. Basically just shorted the "generator" output.

Again, just works on universal, or brush style motors -( and works better on permanent magnet (DC) motors which are usually used on battery powered motors) It does not bring the motor to a rapid and sure stop, but it really reduces the spin-down time significantly.