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  1. #1
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    Default VFD stopped working

    I recently got a Teco Westinghouse VFD for my surface grinder and wired it last weekend. Everything worked great for about 3 days, then I try to start the machine and the motor clicks but won't start. I didn't have time to look very closely then so I unplugged it and left it alone. I tried to start it a couple days later and I got an error from the VFD "OC-A" which means Overcurrent trip. I don't have any ideas to try to resolve this. Is there something wrong with the VFD or my wiring?

    The motor is a 220v 3/4hp 3 phase 3450rpm and the vfd is a 1hp 110v input, 3 phase 220v output.
    Thanks

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    First thing....

    Disconnect the output wires from the unit. Turn on and make it start.

    If it does NOT do the OC, then check your wires. Odds are there is a short from phase A to earth.

    If it DOES do the OC, then look carefully for any stray wire strands inside, shake it out, and try again. If still does OC, it is as dead as Kelsey's nuts, so check your warranty.

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    Quote Originally Posted by JST View Post
    First thing....

    Disconnect the output wires from the unit. Turn on and make it start.

    If it does NOT do the OC, then check your wires. Odds are there is a short from phase A to earth.

    If it DOES do the OC, then look carefully for any stray wire strands inside, shake it out, and try again. If still does OC, it is as dead as Kelsey's nuts, so check your warranty.
    I was wondering why you thought it would be Phase A, and I think you are interpreting the "OC-A" as meaning Phase A. That doesn't mean that, it means Over Current on Acceleration (as opposed to OC-C for Constant Speed or OC-D for Decel).

    Other than that, your advice is dead on. Isolate the problem first to the VFD or the motor circuit. If the motor is unwired from the VFD and the fault persists, a transistor is fried and the VFD is a door stop.

    (I had to look up "Kelsey's nuts", funny! I did some work at a Kelsey Hays wheel factory nearby in the late 70s, it's the same Kelsey. I however had never heard that expression before. Apparently it was a favorite of Tricky Dick Nixon back in the day, but he twisted the original meaning of "Tighter than Kelsey's nuts", referencing the lug nuts of his wheels. His saying "Deader than Kelsey's nuts" could only refer to an anatomical reference..., but he was the POTUS so I guess nobody argued the point with him!)

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    yeah I did assume it was a phase reference. Plausible, useful, apparently not true.

    I guess the ID as accel, etc could be useful, I'd suspect the phase ID to be more so, and perfectly possible to do. I figure operator would know what it was doing, but I suppose for an unattended conveyor, etc, it might not be otherwise obvious.

    picked up the saying from a boss long ago.

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    Quote Originally Posted by JST View Post
    yeah I did assume it was a phase reference. Plausible, useful, apparently not true.

    I guess the ID as accel, etc could be useful, I'd suspect the phase ID to be more so, and perfectly possible to do. I figure operator would know what it was doing, but I suppose for an unattended conveyor, etc, it might not be otherwise obvious.

    picked up the saying from a boss long ago.
    Most inexpensive drives like these don't have individual phase current sensors, they sense current on the DC bus (cheaper) and calculate the AC current, assuming it is balanced going to the motor. The drive might not even know if one phase is disconnected, other than the fact that the motor would not start the next time. In fact, it's entirely possible that's what is happening here; a bad connection worked for a while but no longer does, so the motor is single phasing and cannot start, tripping on OC as it attempts to.

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    I have a Teco that wakes up cranky sometimes. I'll get an over-load fault one day and not the next. When I get the fault, I have to increase the accel. time to get the motor to start. Some days 1 second is good, other days it takes 3 seconds. I am sure one day it will not start at all. I keep a spare.

    Bill

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    Quote Originally Posted by Bill in PA View Post
    I have a Teco that wakes up cranky sometimes. I'll get an over-load fault one day and not the next. When I get the fault, I have to increase the accel. time to get the motor to start. Some days 1 second is good, other days it takes 3 seconds. I am sure one day it will not start at all. I keep a spare.

    Bill
    That actually sounds like more of a bearing problem or something else mechanical that is binding up.

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    Quote Originally Posted by Jraef View Post
    Most inexpensive drives like these don't have individual phase current sensors, they sense current on the DC bus (cheaper) and calculate the AC current, assuming it is balanced going to the motor. The drive might not even know if one phase is disconnected, other than the fact that the motor would not start the next time. In fact, it's entirely possible that's what is happening here; a bad connection worked for a while but no longer does, so the motor is single phasing and cannot start, tripping on OC as it attempts to.


    I've seen them with current sensors on two out of three outputs. Have not seen any with just one on the DC. Current sensors are cheap these days. I guess the drives are even cheaper.

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    Quote Originally Posted by Jraef View Post
    That actually sounds like more of a bearing problem or something else mechanical that is binding up.
    I'll take a look. I have not noticed any new noises, etc. coming from the lathe. I ran a job on it Saturday and it seemed normal. Many starts and stops with short cycle times.

    I can get the over-current fault when I try to start the lathe the first time in the morning. I will reset the VFD without changing any parameters. The lathe will start on the next try and be fine for the rest of the day.

    I don't want to hijack this thread. I suspect the VFD as being faulty since this is the unit that got crunched about 4 years ago when the lathe fell on it.

    Thanks,

    Bill

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    To the OP: Can you try spinning the grinding wheel manually in the correct direction, then engaging the VFD (with your other hand behind your back )? (Not as an ongoing operational mode, but for diagnostic purposes.)

    Chip

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    I finally got to come home for the weekend (I go to school about 2 hours away) and work on the grinder. I disconnected the motor from the vfd and did not get the OC-A error, wired it back to the vfd and got the error.


    The wheel spins easily so I don't think that anything is bound.

    Does this mean that there is something wrong with the motor or could it be the wiring?

    Thanks

  12. #12
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    Quote Originally Posted by emorymcdougald View Post
    I
    Does this mean that there is something wrong with the motor or could it be the wiring?
    Thanks
    Yes to both or either.
    It's possible that as the VFD can impart considerably higher voltages into the windings, that it has caused the old insulation in the motor to fail, and now there's a short.

  13. #13
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    Quite likely the wiring, since it is something that is "new and changed". Of course the motor could have a problem, but wiring is the first place to look. Check it over carefully. It's very easy to get a wire strand hanging out somewhere, although of course I have never done that

    Also, wires can be nicked on sharp edges, cutting the insulation and causing a short. Of course I have never done that either.

    Look in the motor wiring box, and at all the rest of the connections and exposed wire ends. If you have a meter, disconnect wires at both VFD and motor, and check wires with ohmmeter, from wire to wire, and also from wires to ground / enclosure.

    Check also from motor wires to motor case.

    You do not need to disconnect the ground wire itself, leave it connected and verify that it DOES have continuity to the enclosure and motor case.


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