Hope it's the capacitor

[dennis thompson]

My dust collector ( yes it's Harbor Freight) stopped working. When I turn it on I just get a hum. The motor turns freely but doesn't run. Do you think it the capacitor?
Thanks

[Mike Henderson]

Sure sounds like it. A start capacitor is cheap so it's a good thing to start with. About the only other possibility is a problem with the centrifugal switch but most likely the start cap.

You probably don't have access to do this, but the test for a bad start cap is to spin the shaft and, while it's spinning, turn the power on. If it spins up to operating RPM it's usually the start cap.

Mike

[Lee Schierer]

Sure sounds like it. A start capacitor is cheap so it's a good thing to start with. About the only other possibility is a problem with the centrifugal switch but most likely the start cap.

You probably don't have access to do this, but the test for a bad start cap is to spin the shaft and, while it's spinning, turn the power on. If it spins up to operating RPM it's usually the start cap.

Mike

I agree with Mike. However, if you can blow out the fan end of the motor with compressed air first it would clear any dust of of the switch.

[Mikail Khan]

Sometimes they get bulges when they fail. Do a visual inspection.

MK

[Brian Holcombe]

They can be tested for capacitance. I do this now with a multi meter rather than replacing parts at random.

[Bill Dufour]

I know you can test them for resistance slowing going up as they charge then reverse polarity and see it again. Can a standard VOM really see if they are good or not. I always assumed there was a high voltage arc that a meter would not see.
Bil lD

[Brian Holcombe]

My meter tests for capacitance, I don’t know what it’s measuring to provide the capacitance number but it provided one.

[Mike Henderson]

Those starting capacitors are very inexpensive. In my opinion, the best way to test for a bad capacitor is to replace the old one with a known good one. If you're wrong and it's something else, you haven't lost much and you'll have a new capacitor on your motor. It's usually very easy to change the capacitor - they're usually outside the main part of the motor and often have push on lugs on the wires.

Mike

[Brian Holcombe]

So, how do you know your known good one is good if you can’t test it?

[Mike Henderson]

So, how do you know your known good one is good if you can’t test it?

It's tested at the factory before shipment.

Mike

[Brian Holcombe]

No kidding, Mike. Great, so keep stock of every capacitor in your shop and just throw them away when they might be bad rather than finding the actual problem. Seems like that might get more expensive than checking them?

[dennis thompson]

Those starting capacitors are very inexpensive. In my opinion, the best way to test for a bad capacitor is to replace the old one with a known good one. If you're wrong and it's something else, you haven't lost much and you'll have a new capacitor on your motor. It's usually very easy to change the capacitor - they're usually outside the main part of the motor and often have push on lugs on the wires.

Mike

Mike you were right. I bought a new capacitor, $10, as you said it was cheap, fixed my problem. Thanks for the advice.

[Mike Henderson]

No kidding, Mike. Great, so keep stock of every capacitor in your shop and just throw them away when they might be bad rather than finding the actual problem. Seems like that might get more expensive than checking them?

Err, no. I would go purchase a new capacitor when I had a motor exhibit the problems of a bad capacitor. Why would you want to keep a supply in your shop? Doesn't make sense to me unless you cannot stand any down time, even the amount of time necessary to run to the store and buy a new capacitor. But capacitors on good quality motors generally last a long time. But then, you'd have to have a lot of other spare parts on hand because other things can fail besides a starting capacitor. To really make sure you don't have any down time you'd also have to have a spare motor. Now that would be expensive.

Induction motors are pretty simple. When they fail to start you can go down the list, starting with the most common problem, which is a failed capacitor (assuming you checked and have power to the motor). Next on the list would be a problem with the centrifugal switch. Very uncommon would be burned out stator, but you'd probably smell that one (it's a very distinctive smell).

Bearings could be a problem but you'd probably find that when you spun the rotor by hand. You can usually feel and hear bad bearings. Not much else to go bad.

If I had to estimate probabilities, I'd guess that 80 to 90 percent of the time when a single phase induction motor will not start (just hums) it's the start capacitor. That's why I usually recommend putting in a new one as a first low cost test. In general, they're not very expensive.

Mike

[One more point: If your tool has a high quality motor in it, it probably has a high quality start capacitor and the cap will last a long time. If your tool has an inexpensive Chinese motor in it, it probably has a cheap capacitor in it and the capacitor may fail early in the life of the motor. If you replace it with a good quality capacitor, it will probably last a long time.]

[Brian Holcombe]

Downtime is expensive, I have spare capacitors for all of the single phase motors in my shop. Throwing out good parts just because they are cheap is a great way to never learn how to diagnose the issue. It’s great until you are throwing out $50 capacitors that take a few days to arrive for the sake of saving $75 on a diagnostic tool. And if your guess is correct, great. If not, then your back to square one having wasted hours with a machine that is down for days.

[Mike Heidrick]

I have more money in my meter than many have in their saw. All a perspective.