On Saturday 20 December 2008, John Thornton wrote: >DC applied to an AC motor will slow the motor down and stop it... > Generally speaking it won't stop it dead because it has to turn to generate the braking force currents in the always shorted armature of an AC induction motor. This force can be quite violent if the armature is turning at a good rate. A short circuit applied to a spinning universal motor also works well. I have a couple chop saws, both with universal (brushed) motors that short the motor to stop them quicker when the trigger is released. The 8.5 incher has a habit of unscrewing the arbor bolt with the stopping torque unless its no looser than 1/8 turn from broke. Cheap arbor, no d-flats... The stopping torque of the 12" Dewalt slams the blade the rest of the way to the lower stop if I don't have a good grip on it.
I'm told one should not use a dead short on PM field motors though as the currents can be great enough to damage the PM field pieces magnetically and permanently. So when I converted the spindle on my micromill to computer (EMC) control, the short I use to stop it isn't, I used a 10 ohm 20 watt R, so the stop, while still fairly quick, isn't nearly as violent. Perhaps 3/4ths second from full speed. If I were designing a stopper for a multihorse AC spindle motor using the DC method, I would do so with electronics so as to apply a small starting current and ramp it up for a second, then turn it off to save the semiconductors, ramping it high enough to get it mostly stopped in that second. The stop would be just as good, and a lot less violent to the machinery. >dunno if this helps but it is an old technique for braking. I think Otis was letting elevators down that way in the at about the turn of the last century (1900?), so yes, its an old idea. Now if talking about a road simulator, and automotive horsepower levels, then I think I'd design with DC motors purely because the power they generate as a generator when loading with them can be largely dissipated with an externally cooled resistor bank, or better yet used to recharge the batteries that run it. Using an AC motor implies that all the heat will need to be absorbed by the armature of the motor, and its mass vs Curie point then becomes the time limiting factor. If the iron is heated above its "Curie point" by the power input, it likely will be permanently toasted. I've seen that happen in ferrites. No sign of damage, but they are magnetically gone forever. Regular silicon steel as used in those motors has a much higher Curie point than most ferrites (some are below 100C!) though so the damage will probably be to the stator winding varnishes from the heat radiated by the hot armature. Curie point, for those who haven't heard the term before, when talking about ferrous materials, is defined as that temperature where, as its warmed up, the material suddenly becomes non-magnetic. Quenching rapidly can freeze this state into the steel and it is then called austenitic because while its steel, it can't be picked up with a magnet. It has advantages and automotive engines have used it for exhaust valves since the 50's cuz its much harder to burn than regular steel. Its main downfall is a horrible thermal expansion which caused those engines without hydraulic lifters to require from .019" to .027" as tappet clearances just to make sure the valve did close all the way as the engine warmed up. More than you wanted to know probably. :) I'll find the door thank you. -- 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) Death before dishonor. But neither before breakfast. ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
