3HP Table saw flips breaker on startup

[Matt Michaud]

Background: We recently pulled our table saw out of storage (sat unused for about 1yr). It ran great for a couple days and then started flipping the breaker. It is 220V (though voltage in the building typically runs closer to 200V) on a 20A breaker. The space is unheated and it is below freezing most nights, but this was never an issue in the past. The table saw ran great in the same conditions for 3yrs.

Problem: When the on button is pressed you can hear the breaker immediately flip; the motor spins the blade about 1 rotation. Occasionally, if the table saw has sat for a while, the motor will start and run fine. However, it will fail to start every time after that.

Attempted solutions: We have switched breakers - no change. I have checked the centrifugal switch - contact is good (and the motor never gets up to speed to disengage). I have switched for a brand new capacitor - no change.

[Charles Lent]

How easy is it to spin the blade with no power connected? The bearings in the saw arbor or motor could be the problem. Check the belts too. If the centrifugal switch in the motor that is in the start capacitor circuit isn't closing as the motor slows to a stop, then the start capacitor is not in the start winding circuit of the motor during start-up. The motor will draw significantly higher current for a longer time in this condition and would trip the breaker.

There is a plastic or fiber collar on the motor shaft that is moved by the centrifugal fly weights and this collar in turn moves the centrifugal switch. If it cannot slide freely on the motor shaft, it may not be returning to the position needed to close the contacts of the centrifugal switch. A polishing of this area of the motor shaft using a fine emery or crocus cloth strip (used like a shoe shine rag) and then a very thin coat of oil on the shaft where the collar slides, will usually correct this problem. My guess is that the shaft has developed a thin layer of rust in this area from the saw's time in storage. You don't want to leave any excess oil on the shaft that might fly off when the motor runs. Just apply a very thin coat.

This problem is compounded by the low line voltage. It should be 240-250, not 200.

Charley

[Bill Dufour]

What is FLA on the motor. I no longer believe HP ratings on motors thanks to Sears vacuums.
Bill D

[Bill Dufour]

have you tried a rope start before turning on power? Removing the blade will reduce starting load by a good amount.
I would install a 30 amp breaker and see if the motor runs. If it does let it run for a few seconds and keep checking. for overheating. If it is good let it run for 5-10 minutes to knead the old grease. Then put the old breaker back in place.
Don't have a spare 30 amp breaker..remove the drive belt and follow the same procedure.
Bill D

[Matt Michaud]

If the centrifugal switch in the motor that is in the start capacitor circuit isn't closing as the motor slows to a stop, then the start capacitor is not in the start winding circuit of the motor during start-up. The motor will draw significantly higher current for a longer time in this condition and would trip the breaker.

Charley, thanks for the response. Lets apply some numbers to the rotation. If the motor is going to reach 3000rpm, there is going to be some minimum rps to generate enough centrifugal force to open the start circuit with even a brand new centrifugal switch. Lets say that's 1000rpm (feels right). When I say the blade rotates maybe 1 rotation, I am not exaggerating - the breaker flips immediately. The motor maybe got up to 100rpm for that fraction of a second. I can't imagine that the saw ran long enough to need the switch to disengage.

Yes, the low voltage is not ideal and draws extra amps. The building is 208Y 3ph so we will be adding a transformer to create a true 220 subpanel, but we have successfully run 220V pumps, dc, bs, and ts on the 208V leg for years.

The blade rotates easily when off.

[Malcolm McLeod]

The 'law of halves'.... Can you run anything else on this saw's circuit? Does it trip? A long shot, but could you have gotten on a fast trip breaker (typical to protect sensitive electronics)? Can you run saw on another CB (even the 30A, as Bill suggested?) If this is all OK, then the supply 'half' is apparently OK. The low voltage would be my biggest concern.

Now cut the remaining 'saw side' in half... is the problem in the electrical supply half? Or, in the motor half? Could the cord or supply line from CB to saw be damaged? Check resistance and for L1<>L2<>Gnd short? If the saw tripping is intermittent, supply line damage could be culprit - depending on when/how the line is moved or positioned before each run attempt?

Belt may have gotten hard (if equiped)? I had a mower that would stall when the cutter deck engaged - after it sat unused for 18 months. Sounds like you have the motor knowledge down .... so maybe above is just a couple other things to consider?

Edit: Yeah, a short on the supply side SHOULD trip before the saw is turned on, but I have seen equipment - and their electrical supply - do strange things in my life.

[Matt Michaud]

The 'law of halves'.... Can you run anything else on this saw's circuit? Does it trip?

Yes, after the TS started tripping the breaker I ran our 3HP shaper on the exact same outlet. Never a problem. Supply line damage is an interesting idea, but the "fails on the second attempt" aspect seems odd. Literally, I started it - it ran. I cut nothing, just turned off the saw and once it came to a stop, tried to start it again. It will then trip the breaker every time until it sits for a long time. No cord got moved, no load applied. I just have a thin kerf 7-1/4 blade on it.

Bill, I believe I have a 30A breaker for a grinder I can swap out. Shop Fox manual says 18A, but I haven't looked at the motor plate to see if it says anything else. Starting it without the belt is another interesting idea I haven't tried. Will have to google rope start, not something I am familiar with (but that's fine).

[Rod Sheridan]

It’s most likely a failed centrifugal starting switch.....Regrds, Rod

[Lee Schierer]

It’s most likely a failed centrifugal starting switch.....Regrds, Rod

I agree, if the centrifugal switch isn't closing or not making good contact, the motor would appear to be a short circuit, which would trip the breaker.

[David Kumm]

You might have to pull the motor and have it tested. If not the switch, there could be a short in the windings. I've had that blow a breaker periodically before completely failing. Dave

[Bill Dufour]

Bill, I believe I have a 30A breaker for a grinder I can swap out. Shop Fox manual says 18A, but I haven't looked at the motor plate to see if it says anything else. Starting it without the belt is another interesting idea I haven't tried. Will have to google rope start, not something I am familiar with (but that's fine).[/QUOTE]

Rope start as used on rotary phase converters for first testing. Wrap a rope or thick twine around the shaft or pulley and give motor a good pull. Then, once rope is free, turn on the switch. Same way old lawn mowers got started before rewind starters. If you can get if fast enough it will run even if starting circuit is bad.
Probably a way to do it with a electric drill and a rubber grinding wheel.
Bill D

[Richard Coers]

Take the belts off and try it. Bearing grease in the spindle bearings may have hardened during storage.

[Charles Lent]

See my post #2. The centrifugal switch is sticking, but moving back slowly to connect the start winding and capacitor when it sits unused for a while, thus working properly on the first start-up but failing to move back to re-connect the start winding and capacitor after first run, so it fails at the next start-up. Clean the motor shaft where the centrifugal switch collar slides and lubricate it. I have serviced a lot of motors in my life.

Charley

[Tom Bender]

Table saws have tiny pulleys to allow maximum depth of cut. That makes the belt eat power to bend and straighten it. Yours may be too old and stiff. See if you can get an X series belt, (with notches on the inside). It will spin easier.