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can a 3 Ufd ac start capacitor be replaced with a 3 Ufd 600 volt dc capacitor

Started by BEN June 7, 2014
I have a small fan motor that has uses a 3 ufd start capacitor  This capacitor
checks bad. It is also very old (60 years)  and in a fan. Can I substitute a 3
ufd capacitor rated at 600 vdc to check  the fan motor. 
  I checked the voltage to the fan and it is 115 volts ac with the switch set
at  high speed. Your advice would be very much appreciated. 
Regards.
Ben 

-- 


In article <97299$53938f9d$43de0cc0$7371@news.flashnewsgroups.com>, BEN
<f6ceedb9c75b52f7fcc0a55cf0cfbf5d_984@example.com> wrote:

> I have a small fan motor that has uses a 3 ufd start capacitor This capacitor > checks bad. It is also very old (60 years) and in a fan. Can I substitute a 3 > ufd capacitor rated at 600 vdc to check the fan motor. > I checked the voltage to the fan and it is 115 volts ac with the switch set > at high speed. Your advice would be very much appreciated.
No. A DC capacitor will explode on AC. Look for film capacitors intended for use as motor start or in this case run capacitors. These are easily obtained. Look in places like Automation Direct and DigiKey. Joe Gwinn
On Saturday, June 7, 2014 11:18:05 PM UTC+1, BEN wrote:

> I have a small fan motor that has uses a 3 ufd start capacitor This capacitor > checks bad. It is also very old (60 years) and in a fan. Can I substitute a 3 > ufd capacitor rated at 600 vdc to check the fan motor. > I checked the voltage to the fan and it is 115 volts ac with the switch set > at high speed. Your advice would be very much appreciated.
If the new cap is electrolytic, no. If its a film type, yes certainly. If you're thinking of putting the new cap in the old can, be aware that oiled paper caps were still in use then, and the 'oil' was nasty toxic stuff. Keep it off your skin. NT
On Saturday, June 7, 2014 6:49:43 PM UTC-4, Joe Gwinn wrote:
> In article <97299$53938f9d$43de0cc0$7371@news.flashnewsgroups.com>, BEN > > <f6ceedb9c75b52f7fcc0a55cf0cfbf5d_984@example.com> wrote: > > > > > I have a small fan motor that has uses a 3 ufd start capacitor Can I substitute a 3 > > > ufd capacitor rated at 600 vdc to check the fan motor. > > > No. A DC capacitor will explode on AC. > > > > Look for film capacitors intended for use as motor start or in this > > case run capacitors. These are easily obtained. > > > > Look in places like Automation Direct and DigiKey. > > > > Joe Gwinn
Other places to look are W.W. Grainger and Herbach and Rademan. 3 ufd is pretty small to be a start cap. Are you sure it is a start cap and not a run cap. Dan
"Joe Gwinn"
>BENf6ceedb9c75b52f7fcc0a55cf0cfbf5d_984@example.com> > >> I have a small fan motor that has uses a 3 ufd start capacitor This >> capacitor >> checks bad. It is also very old (60 years) and in a fan. Can I >> substitute a 3 >> ufd capacitor rated at 600 vdc to check the fan motor. >> I checked the voltage to the fan and it is 115 volts ac with the switch >> set >> at high speed. Your advice would be very much appreciated. > > No. A DC capacitor will explode on AC.
** Correct for normal ( not bipolar) electros and most plastic film types.
> Look for film capacitors intended for use as motor start or in this > case run capacitors. These are easily obtained.
** Look for caps marked as being AC rated or with "class X1" or "class X2". .... Phil
responding to
http://www.electrondepot.com/electrodesign/can-a-3-ufd-ac-start-capacitor-be-replaced-with-a-3-ufd-6-725094-.htm
, ben wrote:
> meow2222 wrote: > > On Saturday, June 7, 2014 11:18:05 PM UTC+1, BEN wrote: > > > I have a small fan motor that has uses a 3 ufd start capacitor > This capacitor > > checks bad. It is also very old (60 years) and in a fan. Can I > substitute a 3 > > ufd capacitor rated at 600 vdc to check the fan motor. > > I checked the voltage to the fan and it is 115 volts ac with the > switch set > > at high speed. Your advice would be very much appreciated. > > If the new cap is electrolytic, no. If its a film type, yes certainly. > > If you're thinking of putting the new cap in the old can, be aware that > oiled paper caps were still in use then, and the 'oil' was nasty toxic > stuff. Keep it off your skin. > > > NT >
Thanks everyone for the fast reply I found this site by accident looking for info on capacitors The cap is a run cap in an old emerson fan. It is marked 2.9 uf , 250 volts and in a can that looks looks like a sardine can. I subbed a 3 uf /600 vdc and the fan runs. This was just for test. I will get the proper type cap for permanent placement. It is an old fan that was in the attic approx 60 years old. Thanks everybody. I will be monitoring this site and inputing where I can. Have 60 years in various electronics fields. Retired now but still into electronics/ Again Best Regards to you all. Ben
On Saturday, June 7, 2014 6:18:05 PM UTC-4, BEN wrote:
> I have a small fan motor that has uses a 3 ufd start capacitor This capacitor > > checks bad. It is also very old (60 years) and in a fan. Can I substitute a 3 > > ufd capacitor rated at 600 vdc to check the fan motor. > > I checked the voltage to the fan and it is 115 volts ac with the switch set > > at high speed. Your advice would be very much appreciated. > > Regards. > > Ben > > > > --
Lots of film/polystyrene AC capacitors are available, cheap and very reliable in our experience. DC capacitors, especially polarized ones, would lead to an electrical fire. Often the AC polystyrene capacitors come in small capacitance values, but connecting several in parallel, works fine.
<dakupoto@gmail.com>
> > Lots of film/polystyrene AC capacitors ....
** Not polystyrene. Either polypropylene ( PP) or polyester (PET) films are used. AC rated types use a special winding technique that results in two or more caps in series. DC rated, single wound types fail prematurely when subjected to AC voltages of more than 100V rms. .... Phil
On Monday, June 9, 2014 5:24:51 AM UTC+1, Phil Allison wrote:
> <dakupoto>
> > Lots of film/polystyrene AC capacitors ....
> ** Not polystyrene. > Either polypropylene ( PP) or polyester (PET) films are used. > AC rated types use a special winding technique that results in > two or more caps in series. > DC rated, single wound types fail prematurely when subjected > to AC voltages of more than 100V rms.
The latter is a puzzling thing to say. I've used plenty of dc rated film caps at >100v ac with an excellent reliability record. NT
On Mon, 09 Jun 2014 16:41:04 +1000, <meow2222@care2.com> wrote:

> On Monday, June 9, 2014 5:24:51 AM UTC+1, Phil Allison wrote: >> <dakupoto> > >> > Lots of film/polystyrene AC capacitors .... > >> ** Not polystyrene. >> Either polypropylene ( PP) or polyester (PET) films are used. >> AC rated types use a special winding technique that results in >> two or more caps in series. >> DC rated, single wound types fail prematurely when subjected >> to AC voltages of more than 100V rms. > > The latter is a puzzling thing to say. I've used plenty of dc rated film > caps at >100v ac with an excellent reliability record. > > > NT
Phil already mentioned the importance of mains rated caps (marked as X1 or X2). Smart people listen to Phil. *When* that DC cap fails it will do so catastrophically with smoke, noise, fire and possibly attorneys involved.