do i need a new capacitor?

[matt tennessen]

when I start up my contractor's table saw, the motor (which was working just fine earlier today) hums, the table vibrates a little, and nothing else. my internet searches suggest that it might be as simple as a bad capacitor or as bad as burned out windings. anyone care to weigh in and guess? I'm probably going to track down a replacement capacitor tomorrow or monday but if I get a rash of responses telling me the motor is done, then I won't bother. thanks!

[jim sauterer]

cheapest thing to replace.most likely if it hums thats what it is.just my experience with motors.i have a cap tester takes the guess work out.hope thats all it is.

[Ken Fitzgerald]

Matt,

While it might be the starting capacitor, it might just be a dirty clogged centrifugal switch. A lot of motors have a starting capacitor that insures the motor starts in the right direction then once the motor is up to speed, the centrifugal switch removes the capacitor from the circuit. Check your motor. On the end that doesn't have the arbor there is probably a removeable cover. With the saw disconnected from power, remove the end cover, check for a centrigual switch. Blow out the switch area with a little compressed air. Reinstall the cover, connect power to the saw and see what happens. Often, it's just sawdust built up in there that prevents the switch from closing and putting the capacitor in the circuit. Thus, the motor doesn't start or can start in the wrong direction.

IF that isn't the problem, I'd go for a starting capacitor.

[Dan Friedrichs]

This is a super-dangerous idea, BUT...

Turn the saw on (letting it hum), and give the blade a little "push" in the right direction with a stick or something. If it's a problem with the starting capacitor or centrifugal switch, the saw should start up and run fine (until you start it again).

Starting capacitors often contain a liquid electrolyte that will dry out, necessitating a replacement.

[Tom Veatch]

Matt, as Ken mentioned, check out the switch even if the capacitor is bad. It's been my experience that often a capacitor fails because the switch fails to open after the motor starts.

Start capacitors are generally for intermittent duty only. Unless the switch opens to take them out of the circuit, they are energized the entire time the motor is running. All it takes is a minute or so of continuous operation and they are toast.

So, even if the capacitor is bad, check out the switch to be sure it's functioning properly, i.e. opening at or before the point at which the motor is up to speed. Otherwise, your new capacitor will be ruined the first time you run the motor for any length of time more than a few seconds.

[Chip Lindley]

Capacitor could be the cause, only way to know is to test or replace. If motor windings are burned you would usually see smoke or smell burning insulation. Ideally, the internal overload (or external reset button) would trip before the motor was fried.

Otherwise, I have to wonder if your motor is TEFC (Totally Enclosed Fan Cooled, with a fan shroud at the rear) or Open design (with open slits in ends or sides for ventilation) If Open type, debris could have caused the starter winding contacts to become stuck. Disassembly of the motor is called for. The start winding contacts are on a disc at one end of the stator. You will see how the spring loaded disc slides up and down the shaft to engage the wiper contacts.

When a single phase motor is started, centrifugal force of the turning stator causes these contacts to open when full RPM is attained, and close when the motor is shut off. This is the *click and rubbing sound* you hear just after the motor is powered off. If the starter contacts are stuck open, you have no starter winding for the capacitor to engage on start-up. The motor will not start, only hummmmm.

Cleaning the shaft of gunk until the disc slides freely on stator shaft should solve this problem. Do not lubricate, just wipe clean. Good Luck!!

[matt tennessen]

hey its been a month but i wanted to say thanks to all who gave their wisdom and knowledge to my quandry. I ordered a new capacitor shortly after the fact and then was away from my shop for awhile. Today I found out that I was sent the wrong capacitor but upon your advice, I gave my motor a much needed cleaning (specifically the centrifugal switch) and with the old capacitor still on it gave it a whirl and now its running like a charm. The cleaning inspired me to clean and lube up the rest of the saw and its better than ever. Sometimes those little hiccups are the motivation for neglected maintenance. Thanks again, you saved me a trip to the repair shop.