On Wednesday, June 01, 2011 11:14:32 PM Peter Blodow did opine: > Gene, I suppose that your "AC universal motor" is an induction motor > with a field winding and a cage rotor.
Nope. > (If it were a motor with rotor > windings and a brush armature, it would be a AC-DC-universal motor It is, complete with replaceable brushes. > 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. Normally, due to hysteresis in the iron, there will be just enough residual magnetic field to get the generator started. But a dead short has absolutely zero effect on two modern motors. I say modern, that Skil router is at least 10 years old. > 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. What is the line voltage in .de land? > 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. Yes, I have had that happen with my Dewalt 12" chop saw, but it only loosened the nut enough to let the blade spin, which made an odd noise that prompted me to tighten the nut to about 1/16th turn from broke. Dewalt doesn't supply a decent spindle lock, so the best you can do is snap a honking big set of vice-grips on the blade so that you can torque on the nut till happy. They do a spindle lock on the $75 angle grinder, seems remiss to me that a nearly $500 chop saw doesn't have one. Now I am wondering how that chop saw stops itself as well as it does. It did not come with a schematic. :( -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) <http://tinyurl.com/ddg5bz> <http://www.cantrip.org/gatto.html> That woman speaks eight languages and can't say "no" in any of them. -- Dorothy Parker ------------------------------------------------------------------------------ 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
