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Thread: Unisaw would not shut off

  1. #1
    Join Date
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    Unisaw would not shut off

    Yesterday I shut off my TS. Had the dust collector running and one other tool also powered-up. With hearing protecting on I did not hear that the TS was still running, only saw the blade still turning. Turned off saw again but the blade kept turning. Hit the off button two more times still no help, pulled power cord out of outlet. Some info: Unisaw is model 36-817 about 12 year old. With Delta magnetic starter, 3hp operating on 220. No problem like this in its history. I can't fine anything wrong. I removed both wires from starter to motor, operated starter many times and it always returned to the off position after cycled. Checked freedom of on/off button movement on the switch cover but both are free. Starter seems to operate correctly but I'm hesitant to trust it. No idea why it stayed on. Any ideas, suggestions of anything else to look at??

  2. #2
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    Have you checked the magnetic pullout portion of the switch?
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  3. This is a tough to diagnose from your armchair, but if the starter has a holding circuit, the contacts could have been stuck. Or if the on/off switch has screw on contact blocks, they could be coming loose, not allowing the contacts to open when the stop button is pressed. I am making these assumptions based on 3 phase starter experience. I would think single phase 220v would be similar.....

  4. #4
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    {Have you checked the magnetic pullout portion of the switch?}

    Dan; Yes I did, and cleaned contact T1-L1 and T3- L3 they had no pitting or signs over load arcing.

    {This is a tough to diagnose from your armchair, but if the starter has a holding circuit, the contacts could have been stuck. Or if the on/off switch has screw on contact blocks, they could be coming loose, not allowing the contacts to open when the stop button is pressed. I am making these assumptions based on 3 phase starter experience. I would think single phase 220v would be similar.....}

    Doug; I did check both, on and off switches contact resistance both less then 1 ohm, and push button freedom of movement in case also ok. No loose screws and all terminals tight.

    Thanks for the help. If I do not find anything soon, I will just replace the whole unit just don't like to spend $200.00. But that's better than being unable shut your saw off when you need to. Tom
    Last edited by Thomas Hotchkin; 03-18-2013 at 8:15 PM.

  5. #5
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    HI,

    Three things come to mind: Stop PB, magnetic contactor, and related wiring.

    If the stop PB contacts did not open when you pushed it, then the magnetic contactor would not drop out and the motor would keep running.

    If the magnetic contactor somehow got mechanically jammed in the energized position, when the power was removed from the contactor, it would not physically move to the open position, and the motor would not stop.

    If there were a short between two wires in the circuit, somewhere between the power source and the high-side terminal of the magnetic contactor (not the neutral side) then the magnetic contactor would stay energized when the stop PB was pushed and the motor would not shut off.

    Since you did not find the magnetic contactor stuck in the energized position ( unless you caused it to drop out by gaining access to it and did not hear it do so) and since the stop PB seems to function correctly during you repeated tests with the motor disconnected from the circuit (I hope you tried things 20 or 30 times), chances are you could have a wiring issue.

    I would carefully check each wire for worn, cracked or broken insulation. You may have moved the wires enough to eliminate the problem during the checks you have already made.

    There is one other thing: The seal-in contact that is on the magnetic contactor. If the contact that closes across the start PB, when the magnetic contactor closes, stuck closed for some reason, then when you pushed the stop PB the contactor would have dropped out, but immediately picked back up when the stop PB was released. Normally you might hear this, but if other machines in the shop were running you might not.

    But since everything works now as expected, I would check each wire for broken insulation. If I found nothing I would try the start-stop sequence of the control as many times as needed to gain confidence that it will operate properly (100 times?) with the motor disconnected. And then if the problem did not return, I would hook the motor back up and use the existing control while paying attention to be sure the blade was stopping when I pushed the stop PB.

    You could also wire a pilot light to the motor leads that would come on when the motor is running and go out when power was removed from the motor when the stop PB was pushed. That is probably what I would do as that would give immediate feedback on motor status.

    Intermittent problems are difficult to find...

    You were pushing the STOP button right? Just joking...

    Just a few thoughts... Bill
    Too much to do...Not enough time...life is too short!

  6. #6
    I had the same problem with a 3 hp 220 Unisaw, magnetic starter. Kinda freaked me out when it happened. The start button (which is a momentary contact) had stuck in the pressed position, probably due to a bit of debris. The button is pretty closely shrouded. Only happened the one time, and I now vacuum out the face of the switch box when doing normal clean up in the shop.

  7. #7
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    Bill
    I did look the wiring all-over, all of it's in great shape, looks brand new. I took the magnetic contactor apart today all moving parts have freedom of movement. I very carefully looked at all switched contacts in magnetic contactor no visual arcing or burned contacts. I also found no lose wires at any terminal. I do like the idea of pilot light.

    Fred
    I took both on & off buttons apart, no dust or debris, both free to move in magnetic starter cover.

    Thanks again for everyone input and suggestions. I will put it all back together tomorrow, and report back. Tom

  8. #8
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    For your consideration:

    If I understand your post, pressing the off button did not turn off the saw. Unplugging it obviously did, and when you then plugged it back in, it stayed off until you pressed on, then it worked normally.

    Since killing the power to the relay (which is what the off button is supposed to do) opened the relay, I assert there's a high probability the flaw is in your "off" button failing to interrupt the current. Prior to spending a lot of money I'd replace the switch.

    Just my $0.02... YMMV.

    Jim in Alaska
    One can never have too many planes and chisels... or so I'm learning!!

  9. #9
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    Quote Originally Posted by Jim Neeley View Post
    For your consideration:

    If I understand your post, pressing the off button did not turn off the saw. Unplugging it obviously did, and when you then plugged it back in, it stayed off until you pressed on, then it worked normally.

    Since killing the power to the relay (which is what the off button is supposed to do) opened the relay, I assert there's a high probability the flaw is in your "off" button failing to interrupt the current. Prior to spending a lot of money I'd replace the switch.

    Just my $0.02... YMMV.

    Jim in Alaska
    Jim
    Both switches are non-stock Carling. Did take both apart and cleaned contacts. Also cleaned both switch actuators, still have not found and anything pointing to a failure. Did increase switch actuator spring tension by elongating spring on both on-off switch actuators. All back together and working great. Will keep my fingers crossed it stays that way. Thanks to all for your help. Tom

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