7x16 brushless motor fault question.


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Thread: 7x16 brushless motor fault question.

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    Default 7x16 brushless motor fault question.

    I recently picked up a barely used Micro-Lux 7x16 mini lathe. Unfortunately it appears to have an issue I did not catch when I was looking at it prior to purchase. I turn the lathe on with the speed dial set to zero and slowly move it forward. Most times the lathe will either fault and not start at all or turn maybe 1/8th of a revolution and fault. If I take a piece of wood or something and gently nudge the spindle to help it start turning it will run fine. From some other threads I thought maybe the spindle bearings were too tight and causing it to stall. I loosened the motor and removed the belt to see what would happen with no load at all. Exactly the same thing. The motor will turn just a bit then stop and fault. If I "help" it start turning it will run fine although it does appear to "lurch" when running slowly. It may also so this at high speed but I can't tell. I'm not sure how these motor work but I expect I am about to learn. I also don't know a lot about 3 phase motors but if this were a three phase motor it feels like it is missing a phase. I'm not even sure a 3 phase motor can run minus a phase but I say that because for about 2/3rd of the rotation it seems fine then for the final third it really slows down until it gets back around where it takes off again for two thirds or so of the rotation. This is what I mean by appearing to "lurch" along at low rpm. Since everything else mechanical is out of the equation by removing the belt it must be either the motor of the control board, my question is which one? I have yet to pull out the control board but I'll do that next. It will be a while before I can get back to the lathe so I thought I would post here and see if any of you know have seen this before and know where to look. I do have a multi-meter so I can conduct some test if necessary. I'm not sure if this is relevant or not, but it appears to reach 2500 RPM at about 2/3rds on the dial. I do not have the built in tach, but I use a handheld reflective tach. I thought maybe the shiny spindle didn't offer enough contrast to the piece of reflective tape but I get exactly the same reading if I put black electrical tape around the spindle and under the reflective strip. Thanks in advance for any ideas.

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    Community Moderator Jim Dawson's Avatar
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    Default Re: 7x16 brushless motor fault question.

    A brushless DC motor actually is a 3 phase motor with a permanent magnet rotor. Yes, a 3 phase motor will run minus one phase, but it won't start without spinning it up first. This is the way static phase converters work, they manufacture the 3rd phase for starting only.

    It sounds like you may have lost a drive output. It could also be one of the Hall sensors has failed, but not likely. Check your wiring connections to make sure everything is OK there. You can check winding to winding on the motor to see if one is shorted or open. You can also measure the phase to phase voltage on the motor when it's running. Be careful probing live wiring!



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    Default Re: 7x16 brushless motor fault question.

    Things like this will drive me crazy so I broke away from something else so I could look at the lathe a little more. I opened the panel to reveal the controller board. Nothing obviously wrong, nothing appears to have been hot and everything is seated. I unscrewed the board and lifted it out and everything looked good on the bottom as well. I couldn't see any obviously bad solders, a few of the top side mosfet solders looked slightly questionable but not really bad. I manually pushed on all the connectors to ensure they were seated even though thy looked like they were. The three power wires to the motor were screwed down securely. All in all everything looked fine. Before putting it back together I set everything such that nothing would short and fired it up, works perfectly. Motor is smooth as silk and starts every time. I'm hopeful that one of the connections was just not seated but I suppose there could be some bad solders. I'm going to put it back together and make some chips. Hopefully problem solved, if not I guess I'll pull the board completely out and scrutinize the solders more carefully.



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    Default Re: 7x16 brushless motor fault question.

    I must not have refreshed the page because I didn't see your reply before I posted my own. I'm almost positive I was down a phase but handling the board seems to have solved it. That leads me to believe bad connection or bad solders. None of the solders look bad, but I will go through them more extensively if the problem returns. Only one connector wasn't perfectly seated but I couldn't tell if I caused that during disassembly or not. Time will tell, right now it runs perfectly.

    Last edited by tomshane; 12-13-2016 at 09:51 PM. Reason: spelling


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    Default Re: 7x16 brushless motor fault question.

    I forgot to mention that in addition to the motor smoothing out the RPM is now correct, 2500 RPM when the knob is turned all the way up.



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7x16 brushless motor fault question.

7x16 brushless motor fault question.