Very good point Peter. I've had the same thing happen to me. A lot of angular momentum in a spinning blade.
________________________________ From: Peter Blodow <p.blo...@dreki.de> To: Enhanced Machine Controller (EMC) <emc-users@lists.sourceforge.net> Sent: Wed, June 1, 2011 1:45:20 PM Subject: Re: [Emc-users] Suicide brakes on an AC universal motor? Gene, I suppose that your "AC universal motor" is an induction motor with a field winding and a cage rotor. (If it were a motor with rotor windings and a brush armature, it would be a AC-DC-universal motor and the following recipe would not apply). When the power supply is cut off, there is no magnetic field any more to exert a braking momentum. Therefore with that sort of a motor you need an additional field supply, say 24 V DC or even better 117 V one phase rectified. After power off, this voltage must be applied for a few seconds (time relay), and it will bring the rotor to a stop very quickly. Machines without this feature are not allowed to be sold or applied in Germany. The only other way would be a mechanical spring loaded brake right on the saw shaft, held open by the supply voltage as long as the motor is running. In any case, be sure that your saw blade is secured with one or two dowel pins protruding from the flange through the saw blade, preventing the left hand thread fastening nut from turning lose by the braking momentum or else you might catch the blade hopping through the shop with your hands..... I have seen a what a 45 cm blade coming lose at 3000 rpm can do, and I almost experienced it myself when a (smaller) blade came lose that I used with a shaft locked in my lathe chuck when I stopped the lathe. Peter gene heskett schrieb: > Greetings all; > > Is anyone aware of a quick & dirty retrofit kit to brake an AC universal > motor when the power is removed? > > I have a Ryobi BT-3000 table saw, which when kept properly adjusted, does a > fine job. Its motor package is a universal motor that drives the blade > with a gilmer belt. > > I nicked a finger on my table saw blade yesterday because it was still > spinning 2 or 3 seconds after the kill switch was hit. Not seriously, just > a slight nick that bled for 2 minutes, and is a red spot about 1/8" in > diameter this morning. > > If practical, I would like to replace the existing power switch with one > that is both handier because its not such a long reach under the table to > hit it, and which shorts the motor in the off position, or at least applies > a power resistor dummy load to it, commonly referred to as suicide braking. > > So, before I reinvent this wheel, is anyone aware of such an aftermarket > device? > > ------------------------------------------------------------------------------ Simplify data backup and recovery for your virtual environment with vRanger. Installation's a snap, and flexible recovery options mean your data is safe, secure and there when you need it. Data protection magic? Nope - It's vRanger. Get your free trial download today. http://p.sf.net/sfu/quest-sfdev2dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------------ Simplify data backup and recovery for your virtual environment with vRanger. Installation's a snap, and flexible recovery options mean your data is safe, secure and there when you need it. Data protection magic? Nope - It's vRanger. Get your free trial download today. http://p.sf.net/sfu/quest-sfdev2dev _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
