DC motor opinion/question
in [Design]
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From: cbarn24050 on 1 Aug 2010 23:51 On Jul 28, 7:46 pm, 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. no it wouldnt > > The motor could overheat.- Hide quoted text - the rectifier is in more danger > - Show quoted text -
From: cbarn24050 on 1 Aug 2010 23:58 On Aug 2, 3:59 am, "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 A dc motor is not a reasonable load. run to RMS voltage and power, Nothing like. > thus RMS voltage not average voltage. Again not even close Or more simply put; average > voltage, current, and power are not in play here.- Hide quoted text - That at least is true > > - Show quoted text -
From: kevin93 on 2 Aug 2010 13:00 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
From: Greegor on 2 Aug 2010 13:35 Didn't they used to use smaller capacitors to reduce arcing at brushes on DC motors?
From: Tim Wescott on 2 Aug 2010 13:39
On 08/02/2010 10:35 AM, Greegor wrote: > Didn't they used to use smaller capacitors > to reduce arcing at brushes on DC motors? Not to reduce arcing -- those are more to suppress the EMI generated by the motor, by preventing the motor leads from becoming nice antennae. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" was written for you. See details at http://www.wescottdesign.com/actfes/actfes.html |