DC motor opinion/question
in [Design]
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From: hondgm on 27 Jul 2010 20:23 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?
From: mook johnson on 28 Jul 2010 00:05 <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. Might be better off with a variac and a large conductor extension cord. ;)
From: whit3rd on 28 Jul 2010 11:23 On Jul 27, 5:23 pm, 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. Probably not. The rotor/blade mass acts as a flywheel, is pretty effective. You'd have more effect on the motor noise than on the speed or power (it'd hum softer).
From: Spehro Pefhany on 28 Jul 2010 14:46 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.
From: Grant on 28 Jul 2010 20:58
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. > You need to allow for higher than normal line resistance on the peak current charging the cap would cause? Grant. |