Unisaw 36-945 shuts off after 5-8 minutes

[Philip Berman]

Happy New Year to all

My 1999 unisaw has been shutting down on it's own for the past several weeks after running for about 5 minutes or so. There are no problems with startup, and the shutdown doesn't seem to be load dependent - I could be ripping 8/4 ash, 1/4 ply, or just standing there trying to figure out how to overcome my last mistake, it shuts down. I opened the control box, blew out the dust, checked all the connections and even checked the amp draw (~12A), all seems fine. Electrical box doesn't even feel warm, cord isn't warm and is plugged into a dedicated 220v outlet on the side of the 50" extension table. Saw starts and runs fine after allowing a cool-down of perhaps 15 minutes, although I can't detect any location that physically feels warm. I know there's a thermal cutoff in the control box although I don't know how to identify it, and the motor itself has 2 capacitors located in excruciatingly difficult locations to reach. I presume one is a start cap and the other a run cap. What's the most likely issue?

Thanks in advance for any suggestions

Philip

[Carroll Courtney]

By know means do I know what I am talking about but I do have a guess. Sounds like to me either your run capacitor is bad or the centrifugal switch is going bad that works with your start/run capacitors. When it comes to Unisaws that needs some repairs owwm.org is a good place for these type of questions. That website is devoted to old wwing machines that is American made only, nothing to join just sign up.

[Charles Lent]

How long does the blade coast after power off? My thinking on this is that bearing problems may be allowing the saw to run, but at high enough load to overheat the motor thermal protection. Does it start back up if you try it immediately after it shuts off? An overheat problem usually prevents the saw from restarting for 5-20 minutes while it cools down. Another thought is that the stop button contacts need cleaning.

These are just guesses, but worth checking in situations like this.

Charley

[David L Morse]

12 A is way too much for no load current on a 3 HP 230 V motor operating on a 240 V supply. Top 3 failure modes have already been listed in the previous posts. From most to least likely:

1. Stuck centrifugal switch.
2. Shorted run capacitor,
3. Bad bearings.

There are others but those should cover most high idle current problems that can be easily fixed.

[Alex Zeller]

My guess is you have a problem with your control box, not the motor. It's possible the electromagnet inside the contactor is going bad or the heater (assuming you have one). With a multi-meter and a schematic you should be able to pinpoint it.

[Philip Berman]

Thank you, all. I'm going to start with the control box and go from there.

Thanks again,

Philip

[Curt Harms]

Thank you, all. I'm going to start with the control box and go from there.

Thanks again,

Philip

That's where I'd start too. It needs a cool down period so something is overheating. I would think if the bearings were going bad causing excess drag you could tell that by rotating the motor with the power off.

[Michael Cuthriell]

There are motor current draw tables on line. You can compare your nameplate data and supply voltage to get a better idea of what your motor draw should be. Varying supply voltage at your house could make a difference. Fluke has a pretty good troubleshooting page that might help you determine if clutch, capacitor(s) or thermals are causing the problem.

[Bill Space]

How long does the blade coast after power off? My thinking on this is that bearing problems may be allowing the saw to run, but at high enough load to overheat the motor thermal protection. Does it start back up if you try it immediately after it shuts off? An overheat problem usually prevents the saw from restarting for 5-20 minutes while it cools down. Another thought is that the stop button contacts need cleaning.

These are just guesses, but worth checking in situations like this.

Charley

This is a key question, and easy enough to answer next time it trips off is you do not know now.

Bad bearings and/or capacitors should show up as over heating and a thermal trip on the motor internal sensor if your motor has one. IF the motor starts up immediately after tripping off it would seem unlikely that an internal thermal trip would reset itself that quickly.

[Stan Coryell]

I think David is right about it being a motor issue, unless the 12 amps is Full Load Amps.

If the 12 amps is athe unloaded motor, the problem is the motor. My guess is the run cap if it has one. I don't deal with many single phase motors, however I think that if the centrifugal switch failed to open the amps would be sky high. I also doubt it would take more than two minutes to overheat. The run windings may also be failing. I don't remember ever having a motor tripping overloads for bad bearings.

A picture of the contactor would be helpful. If it has overload protection see what they are set at.

I've never had a coil that would open after "heating up." Usually if the coil is weak it won't have enough oomph to pull the contactor closed. This is the ONLY time I would close a motor contactor or starter manually, and ONLY if I was absolutely sure there was not any chance of a phase to phase or phase to ground fault. Closing in on a fault may ruin the contactor or it may blow up in your face.