PM66 capacitor exploding

[Jesse Busenitz]

I replaced my (i believe starting... 200 something MFD) capacitor on the saw here a month ago as it suddenly just died on me. That seemed to fix it for a couple weeks and then it started acting up. Seem like it was making the start up sound and staying at that pitch if that makes any sense. It then wouldn't work at all so I took it apart, and couldn't find anything wrong so I put it back together, and it worked for a while and started acting up again. Today I had just put my dado in and was plunging up through a new insert, and BAM!!!!!! the capacitor shredded and sent a shower of fiery parts all over! Thankfully I had just cleansed my shop and there wasn't piles of sawdust all over and was able to stomp it all out( didn't wan't to wast the new fire extinguisher...., but I'm glad I finally got one rated for electrical!) Anyways.... anybody else have this happen? Did I hook up the capacitor wrong? Can a capacitor run and be hooked up wrong? I need your help you electric wizards! I use this saw almost every day, and wan't to get it up and running but not necessarily to explode again.

Jesse, who's taking a breather after all the excitement.

[Bryan Wiesendahl]

Could be a few things...

1) Make sure the new cap is the right voltage. I'm assuming it's the starting cap?

2) The centrifugal switch could be kaput, or intermittent, or on its way out, causing the capacitor to not disengage when the motor gets up to speed.

3) The motor could not be getting up to speed, and the capacitor could not be disengaging because of this. I would think you would notice this while cutting, but you never know...

I'd check those things.

[Jesse Busenitz]

I matched the old cap when replacing it the first time(I'm assuming the first one was original but I need to double check the motor# and make sure it is correct)

Just checked the cent switch and nothing appears out of line

And the motor definitely seemed full speed

[David L Morse]

...just put my dado in and was plunging up through a new insert, and BAM!!!!!! the capacitor shredded and sent a shower of fiery parts all over!...

Once the motor is running the centrifugal switch is supposed to open and take the start capacitor out of the circuit. If you have a run capacitor it stays in circuit. If you only have one capacitor then it is a start capacitor and should have been out of the circuit when it blew. This would point strongly to the centrifugal switch causing the failure by not opening (you did say that the motor was up to full speed, right?). That would allow continuous current through a component that's only rated for intermittent use, thus overheating and then boiling the electrolyte and then BANG. How did you test the switch?

Does your motor have one or two capacitors?

[Jesse Busenitz]

Once the motor is running the centrifugal switch is supposed to open and take the start capacitor out of the circuit. If you have a run capacitor it stays in circuit. If you only have one capacitor then it is a start capacitor and should have been out of the circuit when it blew. This would point strongly to the centrifugal switch causing the failure by not opening (you did say that the motor was up to full speed, right?). That would allow continuous current through a component that's only rated for intermittent use, thus overheating and then boiling the electrolyte and then BANG. How did you test the switch?

Does your motor have one or two capacitors?

The motor is a 3hp Baldor with two capacitors.... I have no way to test the cent switch since I'm missing a capacitor, but it didn't look like it had excessive wear. Not sure how I can test it as it's internal. I tested my run cap and it's suppose to be 12 MFD and my tester just goes back and forth from 12 to 13 I think..... I'm inclined to replace it to.

[Jesse Busenitz]

If my run cap was bad the start cap would stay on correct? But I would think I could tell in power/speed of the motor not?

[Rod Sheridan]

No, the motor would run without the run cap.

If the start cap failed most likely situation is a bad centrifugal switch.

Regards, Rod.

[David L Morse]

If my run cap was bad the start cap would stay on correct?

Not correct. The start capacitor is switched based upon rpm. The run capacitor mainly affects power factor and efficiency at light loads. It improves torque at heavy loads. Synchronous speed is the same with or without the run capacitor.

[Allan Speers]

I matched the old cap when replacing it the first time(I'm assuming the first one was original but I need to double check the motor# and make sure it is correct)

Just checked the cent switch and nothing appears out of line

And the motor definitely seemed full speed

You may have matched the values, but did you get a cap specifically rated for high inrush current?

[Phil Thien]

I agree w/ the others. Here is a well-done article on the subject:

http://www.eurobodallawoodies.org.au/motorfaults.htm

Hopefully you can get to the centrifugal switch w/o too much trouble. But I think the only way for most start caps to go bad when the machine is running is if the RPM slows down so much that the centrifugal switch is kicking the start cap back into the circuit. That would be a lot of load on a 3-HP motor.

[Jesse Busenitz]

As I stated the cent switch appears to be in working order.... nothing stiff, or any excess wear marks. Points look decent, probably could use a little filing.... How do you check a internal cent switch?
I got a new start cap, can I do anything to test the start cap once the saw is up to speed?

[Allan Speers]

Jesse, did you read my post? All caps are not the same.


Also, did you by any chance use an old cap? Electrolytics dry up, even sitting nos on the shelf. There's more to a cap than just capacitance. You have to also check used ones (and older nos ones) for ESR and leakage.

[Rod Sheridan]

As I stated the cent switch appears to be in working order.... nothing stiff, or any excess wear marks. Points look decent, probably could use a little filing.... How do you check a internal cent switch?
I got a new start cap, can I do anything to test the start cap once the saw is up to speed?

Yes, put a clamp on ammeter on one lead of the cap and make sure that as soon as the motor is up to speed the switch opens and the capacitor current is zero.

Start capacitors are meant for intermittent duty, a few seconds should be it for the capacitor being in the circuit......Regards, Rod.

[glenn bradley]

I cleaned and cleaned and cleaned my cent-switch but, kept blowing caps. Swapped the cent-switch and haven't had a problem since. If the cent-switch is failing to open there is a definite "different" sound during startup. You have to catch it quick because if the switch stays closed, the cap won't last long . . . we're talking a number of seconds. YMMV.

[Jesse Busenitz]

You may have matched the values, but did you get a cap specifically rated for high inrush current?

So how do I know if it rated for the inrush? I told the guy what motor it came off of, so I think he would have given me the correct one but I could be wrong.

So I put a new start and run cap on it and it seems to be working. I can here a definite click after I shut the motor off so the cent switch is moving. I'm going to try and test it as Rod suggested.