How to size capacitor for capacitor run fan motor
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From: Andy on 31 Dec 2009 15:49 On Dec 31, 11:34 am, "William R. Walsh" <wm_wa...(a)hotmail.com> wrote: > 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 So, let's suppose you have a motor, and the nameplate is damaged to the point you cannot see what size of cap is required. Where do you go from there? I'm sure there must be a means of establishing that you do have the correct cap. I found one document which states that "the voltage across the cap should be about 5-10% higher than the voltage across the winding, with the rotor locked." Has anyone any other rules of thumb?
From: William R. Walsh on 31 Dec 2009 21:50 Hi! > So, let's suppose you have a motor, and the nameplate is damaged to > the point you cannot see what size of cap is required. Where do you > go from there? I'd start by checking the cap that came with the motor. In the case of the fan I'm talking about, the capacitor was clearly labeled as to its ratings. If that wasn't possible, I'd try looking the motor manufacturer up on the web (either through their web site or from a web site belonging to someone who sells those motors) for more information. You could also try telephoning the motor manufacturer. Still nothing? I'd have to visit an electric motor repair shop at that point. (That's the only way I know of if everything else is missing or does not pan out.) William
From: Andy on 1 Jan 2010 08:58 On Dec 31 2009, 7:50 pm, "William R. Walsh" <newsgrou...(a)idontwantjunqueemail.walshcomptech.com> wrote: > Hi! > > > So, let's suppose you have a motor, and the nameplate is damaged to > > the point you cannot see what size of cap is required. Where do you > > go from there? > > I'd start by checking the cap that came with the motor. In the case of the > fan I'm talking about, the capacitor was clearly labeled as to its ratings. > > If that wasn't possible, I'd try looking the motor manufacturer up on the > web (either through their web site or from a web site belonging to someone > who sells those motors) for more information. You could also try telephoning > the motor manufacturer. > > Still nothing? I'd have to visit an electric motor repair shop at that > point. (That's the only way I know of if everything else is missing or does > not pan out.) > > William This can't be that difficult that we are all dependent on the keepers of the secret capacitor knowledge. I prefer to understand rather than to ask someone and then believe them. I want to know how to solve this riddle myself. What is the basis for capacitor sizing? There must be some relationship between the amount of current in the main winding and the current in the secondary winding. If someone on this site doesn't beat me to it, I will figure out how to size a cap and then post the solution.
From: Wild_Bill on 1 Jan 2010 16:39 There are many small motors in use that utilize a low capacitance value capacitor. The most common type smaller than 1/8 HP is the PSC permanent split capacitor type motor. PSC motors have 2 windings connected in series (3 connections or wires), and don't have centrifugal switches for starting. PSC motors are commonly labeled with; thermal and/or impedance protection. The method used to determine if a motor is a PSC, is to measure the resistance of the windings. A PSC motor's windings resistances will be nearly identical for boh windings. The value of the capacitor affects the torque and speed of the motor. I don' know of any formula for determining the (best) value of the capacitor. Years ago, there was a reference to a capacitor value selection chart, related to PSC motors used in HVAC servicing, IIRC. The chart suggested capacitor values for different sized motors, for increasing torque and/or speed. Many of the small PSC motors that I'm familiar with will operate with different capacitor values, and a couple of them use different capacitor values to make the motors operate at different speeds (low-medium-high). One PSC motor I was using for a machine application actually ran much cooler when the cap value was changed to a higher value (about 3x the manufacturer's specified value). I'm surprised that your motor didn't run with just one 5uF cap connected. Marked values on caps can vary as much as 20% from the actual measured value, but 10% is probably a much more likely worst case situation. The best indicator from a standpoint of harm, is probably going to be the motor temperature. If the motor doesn't run excessively hot, there wouldn't be any reason that the cap's value could harm the motor. -- Cheers, WB .............. "Andy" <andrewkgentile(a)gmail.com> wrote in message news: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? > > thanks
From: Michael A. Terrell on 1 Jan 2010 17:29
Andy wrote: > > On Dec 31 2009, 7:50 pm, "William R. Walsh" > <newsgrou...(a)idontwantjunqueemail.walshcomptech.com> wrote: > > Hi! > > > > > So, let's suppose you have a motor, and the nameplate is damaged to > > > the point you cannot see what size of cap is required. Where do you > > > go from there? > > > > I'd start by checking the cap that came with the motor. In the case of the > > fan I'm talking about, the capacitor was clearly labeled as to its ratings. > > > > If that wasn't possible, I'd try looking the motor manufacturer up on the > > web (either through their web site or from a web site belonging to someone > > who sells those motors) for more information. You could also try telephoning > > the motor manufacturer. > > > > Still nothing? I'd have to visit an electric motor repair shop at that > > point. (That's the only way I know of if everything else is missing or does > > not pan out.) > > > > William > > This can't be that difficult that we are all dependent on the keepers > of the secret capacitor knowledge. I prefer to understand rather than > to ask someone and then believe them. I want to know how to solve > this riddle myself. What is the basis for capacitor sizing? There > must be some relationship between the amount of current in the main > winding and the current in the secondary winding. If someone on this > site doesn't beat me to it, I will figure out how to size a cap and > then post the solution. This isn't a 'site' It is 'Usenet', even if you are using the lame 'Google Groups' interface. There are formulas to calculate the required capacitor. You need some basic information to use them, so stick your smug attitude where the sun doesn't shine. -- Greed is the root of all eBay. |