How to size capacitor for capacitor run fan motor
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From: Andy on 31 Dec 2009 07:39 I have a small single phase blower motor which requires a capacitor for running (not starting). The motor nameplate says that I need 7.5 microfarads. I have a pair of 5 microfarad caps in the shop. I hooked up the motor with just a single 5 uF and it didn't start. Then I put the pair in parallel and the motor did start. If I use 10 uF I will draw more current into the directional winding of the motor. So my question is, how closely are these caps sized? Is the difference between 7.5 and 10 uF enough to do damage? thanks
From: William R. Walsh on 31 Dec 2009 10:01 Hi! When I needed a start capacitor for an old furnace fan motor, I had to go to an electric motor repair shop to get one. (You'd really think that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store would have these sorts of things, but...) I had to get a capacitor slightly larger than the one I was replacing. However, I was told that the tolerance is 10% either way. A run capacitor might be a little tigher in tolerance, but I don't think you've got enough of a difference there to cause a problem. William
From: JW on 31 Dec 2009 10:41 On Thu, 31 Dec 2009 04:39:47 -0800 (PST) Andy <andrewkgentile(a)gmail.com> wrote in Message id: <a13a138e-e049-43d1-94d5-d8d221c40054(a)z41g2000yqz.googlegroups.com>: >I have a small single phase blower motor which requires a capacitor >for running (not starting). The motor nameplate says that I need 7.5 >microfarads. I have a pair of 5 microfarad caps in the shop. I >hooked up the motor with just a single 5 uF and it didn't start. Then >I put the pair in parallel and the motor did start. If I use 10 uF I >will draw more current into the directional winding of the motor. So >my question is, how closely are these caps sized? Is the difference >between 7.5 and 10 uF enough to do damage? Can't answer your question, but Digikey carries 7.5uF AC run caps. For $10 or so, better safe than sorry I always say.
From: hr(bob) hofmann on 31 Dec 2009 11:32 On Dec 31, 9:01 am, "William R. Walsh" <wm_wa...(a)hotmail.com> wrote: > Hi! > > When I needed a start capacitor for an old furnace fan motor, I had to > go to an electric motor repair shop to get one. (You'd really think > that Lowe's, Home Depot, Farm and Fleet, or a decent hardware store > would have these sorts of things, but...) > > I had to get a capacitor slightly larger than the one I was replacing. > However, I was told that the tolerance is 10% either way. > > A run capacitor might be a little tigher in tolerance, but I don't > think you've got enough of a difference there to cause a problem. > > William Can you hear a click as the motor gets up to speed? If you do, then the centrifugal switch is working and disconnecting the starting capacitor and the extra 2.5 uF isn't going to be a problem. The manfacturer uses the smallest value they can get away with as it saves them a few cents. Happy heating.
From: William R. Walsh on 31 Dec 2009 13:34
Hi! > Can you hear a click as the motor gets up to speed? I can now. The motor in question was treated badly and left outside in the weather. Rust soon covered the contacts of the centrifugal switch and all of the electrical contacts. It couldn't work properly, so I took it all apart and cleaned it. Even if you helped it, it didn't want to run properly. More often than not it would just wind down and start humming again. Then the switch worked, but the motor almost always needed help to start. After that it would run fine. So I started to look at the cap, and $7 later it was replaced. All problems were then solved. This is a converted fan, set up for standalone use wherever air flow is needed. I have another that uses a run capacitor. It is much fancier, with multiple speeds. William |