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Instant Motor Stop

Posted: Sat Feb 16, 2008 9:04 pm
A shot of DC into an AC motor will instantly stop the motor. Plan is to open one of the 240 V Ac lines and insert a NC, Sw in parallel with a diode. Opening the SW will send the DC shot to the motor; the regular motor SW is shut off at the same time. What should the PIV and A ratings of the two diodes be?
1) max draw 8A, operating draw 3-4 A (table saw)
2) " " 5A ' ' 2-3 a (radial arm saw)

I'm aware of the inertial hazard of the blade coming off - turn the saw back on quickly.
What are the heating issues (time) regarding the motor windings?
Advice - please.

Posted: Sun Feb 17, 2008 5:49 am
by Dean Huster
Sounds like you're trying to do something like a SawStop table saw does. The SawStop system is a very violent way of stopping the blade that physically shoves a pawl into the blade teeth and yanks the entire arbor down and out of the way. There has to be blade tooth damage sacrificed for the safety it provides. It does stop fast enough that you only risk a few teeth coming in contact with your thumb vs. a few hundred.

Stopping like you want has the possibility of spinning the blade off or otherwise damaging the motor shaft. I wouldn't make a braking system like this to stop the blade as a routine shut-off procedure. The SawStop system is designed from the beginning to handle all those stresses, but for an emergency situation only, not routine.

Yeah, DC stops the blade quickly, but not instantly. It can't stop the blade much faster than the blade can start and the bigger the blade, the bigger the problem.


Posted: Sun Feb 17, 2008 6:00 am
by SETEC_Astronomy
Dean Huster wrote:Sounds like you're trying to do something like a SawStop table saw does. I don't know how SawStop brakes their blade, whether it's magnetic, a true blade brake or a combination. But it does stop fast enough that you only risk a few teeth coming in contact with your thumb vs. a few hundred.
The sawstop works by throwing a honeycombed aluminum block into the blade which catches it and the force of stopping the blade in that fashion causes the blade to retract below the table surface. After the brake is activated you need to replace the blade and the brake mechanism.

VARISTORS I'm not sure what your goal is but I wouldn't trust anything I built to help in a situation where I could lose a body part. You know you limits and I'm sure you understand the risks. Sorry I only have comments and nothing more to contribute. Good luck with your project.

Posted: Sun Feb 17, 2008 2:09 pm
by Dean Huster
If you'll note in my post, I had edited and changed my "how it works" stance. You must've caught me in the few minutes from original post to the edit! Check out the SawStop site for some interesting videos as to how it works.

After you see the price, you'll see why I stick to my Craftsman table saw and be careful when I use it!


Posted: Wed Feb 20, 2008 3:29 pm
by jollyrgr
I have had a circular saw since I was 12 (no, this was not a toy, it was the real 7.25" type) and have had plenty of experience using "dangerous" tools. I have been extremely careful with all power tools from drills, angle cutters, "rotozip", routers, chain saws, and table saws. Of all the tools I own my table saw is the scariest. The speed at which these things will chew through a board won't give a hand a chance. Of all of the tools I use, the table saw gets the most respect.

I've always thought there needed to be something on these and the plunge cut saws to guard accidentally getting a finger, hand, etc. in the reach of the blade. I've been hearing radio commercials for a new feature on paper shredders that instantly disengage the cutter as soon as anything like fingers touches the input. Possibly something like that is needed for table saws? (Be it urban legend or not, the reason I heard that paper shredders now have the stop sensor was because of a number of instances where dogs started licking the input to shredders and ended up losing the tip of their tongues.)

I like this discussion of stopping the blade. My thought, though, is not to ram a piece of metal into the blade teeth. DC might stop induction motors but I believe table saws use universal motors. If it has brushes I believe it is a universal motor and will run on DC. So I don't think that is a solution. I'm thinking of something akin to brake pads like a disc brake on a car or motor cycle. Considering that without antilock brakes a car wheel can be stopped dead with minimal foot pressure; this on a several ton car traveling 70 miles an hour or more. For a current design table saw the brake could be mounted to grab the blade itself and have a "return" spring to push the pads away when not being stopped. For future designs the motor could have two output shafts and have a "drum" wheel on one side, the blade on the other. The drum would work just like drum brakes on cars. Possibly an "internal" drum or a band brake on the outside of the disc.

Feedback welcome and encouraged on this suggestion.

Posted: Wed Feb 20, 2008 3:46 pm
by Bigglez
Greetings jllyroger,
jollyrgr wrote:I like this discussion of stopping the blade.
.....I'm thinking of something akin to brake pads like a disc brake on a car or motor cycle.
.....Feedback welcome and encouraged on this suggestion.
There are four parts to this problem, all must be satisfied
to solve it:

(1) Detector. A reliable way of differentiating between flesh
and other material.
(2) Brake. A jamming device would have reliable stopping
power, but also it would damage the saw, blade, or other
components. A disc brake is better at stopping and reuseable.
There is still the problem of how to dissipate the energy in
the rotating saw.
(3) Cost. Will consumers pay more for a "safe" saw?
(4) Liability. If a safe saw harms a person, who is liable?

Comments Welcome!

Posted: Wed Feb 20, 2008 4:25 pm
by Robert Reed
I have owned 3 table saws arnd operated many more. Every one I have ever seen uses induction motors, but I believe all hand cicular saws are 'universal'. As I understand it, the blade has some sort of ohmic sensor to engage the stopping device. I think the real problem here is the ability to stop RIGHT NOW and not almost right now. Undoubtedly one will end up with a knicked finger which is a helluva lot better than no finger. I think their method of ruining the blade due to an abrupt and powerful stopping force may be the only way. Consider the difference in your cars stopping distance between standing on the brakes vs. hitting a solid brick wall. The car is a wreck, but it stopped RIGHT NOW.

Posted: Wed Feb 20, 2008 5:21 pm
by Dean Huster
The bigger cabinet table saws at a minimum use a 240v induction motor (usually around 5hp) and if the power is available, they're use a 3-phase induction motor. Smaller cabinet-style table saws will use 120/240v induction motors, usually around 1-2 hp. The familiar and more-affordable contractor saws use 120/240v induction motors, usually around 0.5 - 1 hp, normally wired for 120v operation. The new crops of portable table saws and many radial-arm saws as well all power hand tools (save the battery-operated types) use universal motors. Rather than the saw blades being direct drive or driven via belt through a 1:1 pully system, the portables are usually geared down to increase the torque and reduce the speed. Some circular saws are worm-drive which REALLY increases the torque to the blade.

A 10-inch table saw with an 80-tooth Freud blade driven with a 1:1 pully system from a 3450 rpm motor is a typical contractor's table saw setup. (That's my setup, and man does it ever give smooth cuts!) At that rate, that's 57.5 revolutions per second and 4600 teeth flying past your fingers every second. Give a fast reaction time of 10ms, that means that if you're really pushing a board through recklessly, you'll have a minimum of 46 teeth tear into your meat before you can react and pull away -- and 10ms would be an unreasonable pushing of the reaction time. I'd bet more like 100ms and 460 teeth! That's why the SawStop system has to do such a drastic and damaging halt of the blade. It's the only way you're going to keep the operator from losing a finger -- and 46 teeth, let alone 460 teeth will take off a finger. You're not going to get a dynamic brake (whether DC to the motor or through motor reversal, a common industrial trick to quickly stop a motor) or even a friction brake to stop that blade fast enough.

The deal is that you use a blade brake to stop the blade in a fairly short time, say 1 to 3 seconds just to make things a little safer, much like is done on a power miter saw.

I don't consider a table saw to be the MOST dangerous saw. Yes, it's worse than a bandsaw (with the possible exception of a bandsaw at a sawmill), a saber saw, a jig saw, scroll saw or a reciprocating saw .... but for me, the "little" circular saw is the most dangerous. I had an el cheapo Black & Decker circular saw I bought back in 1972 fail two ways simultaneously: the blade guard jammed in the open position and the trigger switch stuck on. I was doing one cut-off after another and put the saw down on the ground between cuts (it was old 2 x 8 stock), usually letting it drop the last four inches. Never noticed the double-failure after a cut. Talk about someone doing a jig trying to avoid that demon-possessed saw that was hopping all over the place. It finally chopped into its own cord and saved my feet from certain destruction!

MAYBE only a gas-powered chainsaw would be more dangerous than a circular saw.


reason for ac motor stop

Posted: Thu Feb 21, 2008 6:01 pm
The objective is to cut down on lost time waiting for the blade to stop. They are both inductive motors (3/4 and 1 HP). What size should the diodes be?
What is the time issue with the winding heating up? Does putting a 30 ohm wire wound resistor in series with the diode; limiting the current to 8 A make
Yes on all of the above tools and the chain saw wins hands down. When it hits a hidden piece of metal the chain comes out like a bullet and always needs to be aimed on the assumption it will.

Posted: Thu Feb 21, 2008 9:03 pm
by Robert Reed
Do you own/use a random orbital sander. To me this is the most annoying of all my tools in regards to free running after stop swith is actuated. If you come up with a brake system for that in the future, Please post it.

Posted: Thu Feb 21, 2008 9:03 pm
by Robert Reed

Posted: Fri Feb 22, 2008 9:25 pm
by haklesup
So you plan to use a series diode to make DC. you won't get DC, you will get half of a sine wave. This probably won't stop the motor the way you think.

Posted: Thu Feb 28, 2008 2:15 pm
by haklesup
I guess the diode turned out to be more of an instant topic stop.

I was mulling this concept around and thought of another equally harsh way to stop an AC motor. Recognising that common house AC has 240V or 120V x 2 each out of phase by 180deg. The motor is powered by one of these two phases. If you could somehow switch over to the other phase voltage while the motor were running, you would have the rotating magnetic field (field coil) suddenly out of sync with the rotor. The motor would slow and may stop before resynchronizing the rotor with the new field.

Continuous application of the other phase would not stop the motor but application of a few hundred ms of it should have some braking potential. More likly to blow the breakers though.

IN any case, the original question about diode current assumed an AC impedance of the motor. At DC impedance the motor would look more like a short and would take substantially more current. At least as much as at startup probably more.