DC motor opinion/question
in [Design]
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From: JosephKK on 1 Aug 2010 13:10 On Wed, 28 Jul 2010 14:46:01 -0400, Spehro Pefhany <speffSNIP(a)interlogDOTyou.knowwhat> wrote: >On Tue, 27 Jul 2010 23:05:00 -0500, "mook johnson" <mook(a)mook.net> >wrote: > >> >><hondgm(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.
From: kevin93 on 1 Aug 2010 14:43 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 kevin
From: JosephKK on 1 Aug 2010 22:59 On Sun, 1 Aug 2010 11:43:29 -0700 (PDT), kevin93 <kevin(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.
From: cbarn24050 on 1 Aug 2010 23:47 On Jul 28, 1:23 am, hon...(a)yahoo.com wrote: > 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? no
From: cbarn24050 on 1 Aug 2010 23:49
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 > > Might be better off with a variac and a large conductor extension cord. ;)- Hide quoted text - > > - Show quoted text - |