On Saturday, July 24, 2010 10:35:10 pm Cathrine Hribar did opine: > Hi Gene: > > hay the info u gave on the 220 motor was great, I would like to know, > if u have time, the dif. between a brushed DC motor and a brushed DC > servo motor if the encoder were removed? > > many thanks > With the encoders feedback path removed, there would be only those diffs in the expected rpms per volt applied to the motor from a low impedance source. IOW, no difference if the motor portions were otherwise alike.
That is why you will often see in the surplus catalogs, a reference to something like 6000 rpms for a 5 volts supply, or possibly something like x rpms per volt. That would be the balance point where the back emf from the running motor has risen to a point where only enough voltage differential exists to force the current needed to maintain that 6000 (or whatever) rpms. Also, this generally assumes a PM field motor so that the back emf really is a function of the rpms the motor is turning at. This scales well as the voltage rises, until the motor self destructs from overspeed, but gets a bit off a straight line scale at very low voltages because of brush and bearing friction, but above that point, the scale is a pretty straight line. Mathematically, one could add those friction loses up and come up with a generally more accurate picture that is correct mathwise, but isn't quite a straight line. Hopefully I am making sense, the CET in me occasionally forgets that not everyone is a CET. Or an EE, and we have some of those on this list too I believe. This motor of course is a different cat entirely from the one in your original question. The diff to remember is that this motor IS a generator, there is no functional difference. The motor you asked about first, is an induction motor, and they do not normally function as a generator because there is relatively little field in the armature unless it is being driven by the rotating field in the stator coils. The rotation of the magnetic field is achieved by the choices of winding resistance and series capacitance between the two sets of coils in such a motor. Remove that field, and the currents in the armature die, which is a very low resistance single turn short circuited winding with no connection to the outside world except through the magnetic field generated by the stator when power is applied. So a current must be applied to stop them as rapidly as they can be started. This brings up the singular exception to that, which is brought into play when the motor is equipt with what is commonly called a suicide brake, where a DC current at a low voltage that will cause a stator current similar to the current that exists when it is running, but the dc field doesn't rotate, so the motor is suddenly very lossy, and stops rapidly, but if this current is left on, and you turn the motor shaft by hand, it will feel as if the motor is full of molasses. This is used occasionally in power tools that are driven by induction motors. My 12" mitre saw is a universal motor, and much the same effect is available there by simply shorting the motor when its turned off. For those motors that are 'PM' fields, there are cautions about shorting them as that can cause the field magnets to be over powered and demagnetized, so a resistor is normally used in place to the short, I use a 8 ohm 20 watter for my small mills spindle, and when the relays turn it off, it stops in about 1.5 seconds from wide open. That is close enough for the girls I go with. ;-) FWIW, while this lossyness might sound like it would make a great dynomometer load, it isn't practical because the armature doesn't have a lot of cooling, and that is where all the power goes, so the copper bars can actually be melted and destroyed in just a few seconds. Far more practical to use a generator, where the power it makes can be both measured quite accurately, and dumped into a dummy load that is actively cooled. For hours at a time if need be. > BILL > > > 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) > > All men profess honesty as long as they can. To believe all men > > honest would be folly. To believe none so is something worse. > > > > -- John Quincy Adams > > > > ---------------------------------------------------------------------- > > -------- This SF.net email is sponsored by Sprint > > What will you do first with EVO, the first 4G phone? > > Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first > > _______________________________________________ > > Emc-users mailing list > > Emc-users@lists.sourceforge.net > > https://lists.sourceforge.net/lists/listinfo/emc-users > > ------------------------------------------------------------------------ > ------ This SF.net email is sponsored by Sprint > What will you do first with EVO, the first 4G phone? > Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users -- 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) The linuX Files -- The Source is Out There. -- Sent in by Craig S. Bell, g...@aracnet.com ------------------------------------------------------------------------------ This SF.net email is sponsored by Sprint What will you do first with EVO, the first 4G phone? Visit sprint.com/first -- http://p.sf.net/sfu/sprint-com-first _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
