How to size capacitor for capacitor run fan motor

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't answer your question, but Digikey carries 7.5uF AC run caps. For $10 or so, better safe than sorry I always say.

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.

Hi!

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?

Hi!

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

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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.

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 .............

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.

I'm just a visitor here, but I agree with Michael's reply.

And Andy, why are you trying to figure out what size the cap should be when you said that the motor nameplate says 7.5 uF?

I thought your question was, is it safe to use 10 uF?

I think JW and Wild Bill answered those questions before your last post, and I would add that, though my info is 45 years old and probably for a different type of cap, I was told the real value for caps ranged from -10% of the rated value to +50 or +100%.

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I'm trying to figure it out because I want to know. I didn't think of that as smug, but I guess it is.

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I am trying to figure it out because I want to understand. I think of this as curiosity. Others call it smug.