Possably a bad 3 phase motor?
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  1. #1
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    Default Possably a bad 3 phase motor?

    Back story about 10 years ago i set up my 9c south bend with a 1.5hp baldor motor and VFD due to a missing counter shaft and it works great! Its all serpentine belt drive with a fixed ratio to give 720 rpms at 60 hz. Giving a full range of speeds with back gear and enough torque.

    The VFD is a Teco Vector drive. The motor i picked up at a scrap yard for $20 so it could possibly be bad. I just got caught up on some projects and have some time fix it.

    When i hooked it up 10 years ago the motor would not start with no belt on it. It would just grumble and jerk back and forth. But when i put the belt on it runs fine. Comes up to speed and seems to run fine. But about 1 in 10 times i turn it on it acts weird. It will hesitate for a second and then try to accelerate hard to full speed normally causing the belt to slip. If it flip it off and back on it will normally start fine. It has been doing this since it was put together 10 years ago. Not really a problem but ive used it a lot lately and it go me thinking i might have a bad motor and woudl like to test it. It also seems to hum under a heavy load.

    If i have a bad phase would the motor start with a VFD?

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    Quote Originally Posted by bugman53 View Post

    When i hooked it up 10 years ago the motor would not start with no belt on it. It would just grumble and jerk back and forth. But when i put the belt on it runs fine.
    That is so very strange.
    Teco drives sort of bullet proof but parameters. Why did it not like the initial not loaded back then?
    The start check on any such motor is a simple ohm check of equal unhooked. The fancy people have megs and while I have such it is so rarely needed and so many miles overrated.
    No bad smell or other hint?
    Bob

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    bad bearings. its completely by chance that when you add the belt it pulls the rotor to the good side of the bearing so the rotor don't drag on the stator.

    as for the rest of the weirdness, no idea.

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    Quote Originally Posted by CarbideBob View Post
    That is so very strange.
    Teco drives sort of bullet proof but parameters. Why did it not like the initial not loaded back then?
    The start check on any such motor is a simple ohm check of equal unhooked. The fancy people have megs and while I have such it is so rarely needed and so many miles overrated.
    No bad smell or other hint?
    Bob
    No bad smell, It seems to run fine up to speed.

    It seems if the HZ are below 10 hz it acts strange. It is supposed to instant reverse where it ramps down and then changes direction. It will not do that as it ramps down it gets down to 10hz or so and then just slows down and then speeds up and will sometimes stop and then jerk and spin the same direction but will never reverse.

    I'm thinking a bad phase and some how the VFD ramping up the speed slow gets it started and it is running on 2 out of 3 phases. Ill ohm them out tomorrow. at low speed it is just acting a fool. I never run it below 20hz running but at start up or reversing it seems to be having a problem.

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    I checked out the windings today, All show 2.9-3 Ohms between any 2. And none show any continuity to ground.

    Could it have a bad rotor?

    The bearings feel smooth.

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    Three phase motors are dead simple, I am guessing the trouble is in the set up of the VFD. There should be a couple of different modes that the drive supports. Try a volts/Hz mode and make sure the other parameters are configured for your specific motor and see if the problem goes away. I had a similar issue when trying to use a vector mode on a different drive.

    Hope this helps.

    Ben

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    Quote Originally Posted by bugman53 View Post
    It will hesitate for a second and then try to accelerate hard to full speed normally causing the belt to slip.

    If i have a bad phase would the motor start with a VFD?
    1. Described the way my VFD does it's motor run test. A static followed by a dynamic tuning operation.

    2. How bad is bad. How much VFD do you have until it faults. For $20 you might have something working, or some smoke.

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    Quote Originally Posted by bugman53 View Post
    I checked out the windings today, All show 2.9-3 Ohms between any 2. And none show any continuity to ground.

    Could it have a bad rotor?

    The bearings feel smooth.
    Using a multimeter? Knowing there isn't a direct short to ground at low voltage is a start, but a proper test between windings and ground needs a megger. And connection resistance variability in the multimeter lead/meter/probe/motor winding leads prob exceeds the winding resistance you're trying to measure. But you have eliminated any glaring problems with the motor.

    I'd look hard at the VFD settings, were it me. Which it ain't

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    A short to groumd would either blow the vfd or trip it on overcurrent.


    Sounds to me like an intermittant connection on one phase. Many vfds dont care and will not trip for an error in this situation.

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    It just dawned on me this is a 1hp VFD and a 1.5 HP motor, The nameplate says 1hp but the model number and amp draw indicate 1.5 hp. I remember this was from an air compressor and it must have been tagged as such due to the application. When i set it up i put all the parameters in to the VFD and it even allowed to set the HP to 1.5. If i run it hard like drilling with a large 1in bit with the speed turned down for more than 10-15 mins it will give an over load fault But that is to be expected as it is actually overloaded. The only problem is jerky inconsistent speed below 10hz and at startup. My guess is VFD setting and the undersized VFD. We will find out soon.

    So i bet this has something to do with the low speed problems. So i just ordered a 2hp Teco VFD to replace it. It should be here next week. I needed a VFD for my belt sander anyway so ill move the 1hp one over to that and its a win win.

    Im going to take the motor apart and check it out and probably replace the bearings while i am in there anyway. If it does the same thing its got to be an bad connection on 1 of the phases or a bad rotor bar or two.

    I do not have a megger but i do have an oscilloscope and a pretty good range of test gear. I might could rig up a high voltage source and use the scope to check for leakage? My meter will test up to 40Mega ohms and it was showing no connection to ground.

    I think yall are right though it is a problem with the vfd settings.

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    when the motor is running properly check the voltage of each phase againt ground and between oneanother
    If that is consistant check the amps with a clamp on ampmeter These readings should be consistent too If the voltage is consistant but amps are not the motor is bad

    Peter

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    Just for future reference, a small transformer can be used as an improvised megger. 110 to 440, for instance. Hitch the 440 across the windings and ground. Plug the 110 side in and listen closely (stethoscope is a plus) for a snap when connecting or measure current in the 440 lead. Hi amperage from possible shorts in the 440 side are limited by the transformer windings.

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    I installed the 2hp teco VFD today and it runs great! It looks like the 1hp vfd was having problems starting the 1.5hp motor. It could have been a setting in the old VFD but all is well now! The motor seems to be fine.

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    The lathe is up and running great with the new vfd now!

    Turns out the problem was in the VFD.
    I put the TECO 1hp vfd that came off the lathe on a belt sander with a 1hp Baldor motor that i had. Factory reset the VFD and set all the parameters for the motor. Completely different setup, all new control panel and it does the same thing on the belt sander about 1 in 10 times it starts it move forward then stops and tries to reverse and then just hums and does not start and acts erratic. Flip it off and back on and it runs fine 9 out of 10 times. I found several other threads with this same vfd and similar problems. It must just be a bad vfd from day one.


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