DC motor opinion/question
in [Design]
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From: Greegor on 2 Aug 2010 13:49 G > Didn't they used to use smaller capacitors G > to reduce arcing at brushes on DC motors? TW > Not to reduce arcing -- those are more to TW > suppress the EMI generated by the motor, TW > by preventing the motor leads from TW > becoming nice antennae. Thanks!
From: JosephKK on 3 Aug 2010 06:47 On Mon, 2 Aug 2010 10:00:40 -0700 (PDT), kevin93 <kevin(a)whitedigs.com> wrote: >On Aug 1, 7:59�pm, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> On Sun, 1 Aug 2010 11:43:29 -0700 (PDT), kevin93 <ke...(a)whitedigs.com> >> wrote: >> >> >> >> >> >> >On Aug 1, 10:10�am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> >> On Wed, 28 Jul 2010 14:46:01 -0400, Spehro Pefhany >> >> >> <speffS...(a)interlogDOTyou.knowwhat> wrote: >> >> >On Tue, 27 Jul 2010 23:05:00 -0500, "mook johnson" <m...(a)mook.net> >> >> >wrote: >> >> >> >><hon...(a)yahoo.com> wrote in message >> >> >>news:0458f9ee-02f4-41fe-bc54-b163fadf8e79(a)x21g2000yqa.googlegroups.com... >> >> >>>I have an electric lawn mower that, upon taking the cover off to clean >> >> >>> all the grass junk out, discovered it uses a permanent magnet DC motor >> >> >>> and a bridge rectifier. �I started thinking, in a Tim Taylor sort of >> >> >>> way (you know, the sitcom Home Improvement) that maybe putting a >> >> >>> capacitor across the motor would give it slightly more power. >> >> >> >>> Now I know that even if this does improve how it runs, the improvement >> >> >>> won't be very much, if even at all. �I used to have two 2900uF/200V >> >> >>> capacitors around, but I'm afraid they may have been thrown out. �So >> >> >>> my question is, do you think this is even worth doing, or will it be a >> >> >>> waste of time? �I don't know much about electric machines, but can >> >> >>> readily design a power supply or embedded system. �I also am well >> >> >>> aware of the safety issues with regard to high voltages and exploding >> >> >>> capacitors, and possibly putting a lot of stress on the bridge at >> >> >>> startup. �I can deal with those. >> >> >> >>> What I want to know is, would doing this help anything? >> >> >> >>You'll get about 15% more RPM from the motor. �Not worth fooling with if you >> >> >>ask me. >> >> >> >A very large capacitor would give almost pi/2 => 57% more voltage. >> >> >> >The motor could overheat. >> >> >> pi/2? no way. �Max is sqrt(2) �=> about 40% more voltage, if that. >> >> Probably blow the rectifier though. �The rectifier conduction angle >> >> would drop precipitously (say from 130 degrees to about 10 degrees) >> >> with a concomittant increase in forward current. >> >> >No - sqrt(2) is the ratio of peak to RMS. �The ratio of peak to >> >average from a full wave rectifier is indeed pi/2/ >> >> >http://en.wikipedia.org/wiki/Rectifier >> >> >I'll agree that because of limited conduction angle you won't get that >> >full peak voltage. >> >> >kevin >> >> And the motor like most reasonable loads run to RMS voltage and power, >> thus RMS voltage not average voltage. �Or more simply put; average >> voltage, current, and power are not in play here. > >A motor will tend to respond to the average voltage. Only heating >loads have a response proportional to the RMS value. > >Actually because of the diodes it is probable that the inertia of the >motor will act somewhat like a capacitor anyway so if the voltage >across the motor is measured it will be larger than the average >voltage expected and so the conduction angle of the diodes will be >less than 180 deg. This will result in less of an increase in voltage >when adding a capacitor. > >kevin You need to do your physics better.
From: JosephKK on 3 Aug 2010 06:49 On Sun, 1 Aug 2010 20:49:41 -0700 (PDT), "cbarn24050(a)aol.com" <cbarn24050(a)aol.com> wrote: >On Jul 28, 5:05�am, "mook johnson" <m...(a)mook.net> wrote: >> <hon...(a)yahoo.com> wrote in message >> >> news:0458f9ee-02f4-41fe-bc54-b163fadf8e79(a)x21g2000yqa.googlegroups.com... >> >> >> >> >> >> >I have an electric lawn mower that, upon taking the cover off to clean >> > all the grass junk out, discovered it uses a permanent magnet DC motor >> > and a bridge rectifier. �I started thinking, in a Tim Taylor sort of >> > way (you know, the sitcom Home Improvement) that maybe putting a >> > capacitor across the motor would give it slightly more power. >> >> > Now I know that even if this does improve how it runs, the improvement >> > won't be very much, if even at all. �I used to have two 2900uF/200V >> > capacitors around, but I'm afraid they may have been thrown out. �So >> > my question is, do you think this is even worth doing, or will it be a >> > waste of time? �I don't know much about electric machines, but can >> > readily design a power supply or embedded system. �I also am well >> > aware of the safety issues with regard to high voltages and exploding >> > capacitors, and possibly putting a lot of stress on the bridge at >> > startup. �I can deal with those. >> >> > What I want to know is, would doing this help anything? >> >> You'll get about 15% more RPM from the motor. �Not worth fooling with if you >> ask me. > >depends on the load, no load = same speed > Not if there is a different voltage applied. > >> >> Might be better off with a variac and a large conductor extension cord. �;)- Hide quoted text - >> >> - Show quoted text -
From: JosephKK on 3 Aug 2010 06:51 On Mon, 2 Aug 2010 10:35:05 -0700 (PDT), Greegor <greegor47(a)gmail.com> wrote: >Didn't they used to use smaller capacitors >to reduce arcing at brushes on DC motors? It was done to reduce EMI, it changed commutator arcing to lower voltages and currents in some cases.
From: Jamie on 4 Aug 2010 20:05
JosephKK wrote: > On Mon, 2 Aug 2010 10:35:05 -0700 (PDT), Greegor <greegor47(a)gmail.com> > wrote: > > >>Didn't they used to use smaller capacitors >>to reduce arcing at brushes on DC motors? > > > It was done to reduce EMI, it changed commutator arcing to lower > voltages and currents in some cases. Interpole type DC motors bring the arching down on the brushes with out losing too much efficiency.. |