# Mill motor doesn't stop when switched off



## FanMan (Feb 14, 2015)

I have an older Jet 15 mill/drill.  It's always worked fine, but lately it doesn't stop turning right away when switched off.  I can switch it off, and it takes some time (anywhere from less than a second to 5 seconds or more) before I hear the relay click and then the motor stops.  I'm sure I can troubleshoot it myself, I do a lot of electrical work, but I haven't had the time yet and if somebody else has had this issue it could save me some time looking.  Obviously it's a potential safety issue.


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## f350ca (Feb 15, 2015)

Probably a faulty relay. Had to replace the forward one on my Taiwanese lathe. Sometimes would release sometimes not.

Greg


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## hermetic (Feb 15, 2015)

This can be caused by the armature of the relay becoming magnetized enough to hang on to the pole peice for a few seconds when the energising current is cut, or in a purely electronic circuit, a capacitor holding a charge rather that dumping it through a resistor. If it is the relay type, and it is easy to replace  that may cure it, but the replacement relay may be prone to the same type of magnetization further dowm the line. and also there could be some leak current going through the relay coil that is enough to make it hang on that bit longer than it should do. On balance, if you are aware of it, and it is not a safety issue, I would live with it, and monitor it to see if it gets any worse. It may not.


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## UncleRuss (Feb 15, 2015)

Little more info please.  Is it run from a VFD?  Is there a brake unit on it?  Did you change or modify anything?
Picture of control cabinet might show someone what all is involved.


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## cathead (Feb 15, 2015)

Increasing the relay spring tension might do it.  I did that on my band saw and
that solved the problem.


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## John Hasler (Feb 15, 2015)

FanMan said:


> I have an older Jet 15 mill/drill.  It's always worked fine, but lately it doesn't stop turning right away when switched off.  I can switch it off, and it takes some time (anywhere from less than a second to 5 seconds or more) before I hear the relay click and then the motor stops.  I'm sure I can troubleshoot it myself, I do a lot of electrical work, but I haven't had the time yet and if somebody else has had this issue it could save me some time looking.  Obviously it's a potential safety issue.



Are you hearing a relay or the centrifugal switch in the motor?


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## atunguyd (Feb 15, 2015)

Get it sorted out sooner rather than later. This could be very unsafe. Imagine having the spindle grab a piece of your clothing and start to pull you in and even though you hit the of switch it won't stop?


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## FanMan (Feb 15, 2015)

It's a single speed 120V motor, no brake, no mods.  It's definitely the relay that's holding in after the switch is turned off.  It's not the switch because it happens with the switch disconnected (the switch is switching the 24V power from the transformer to the relay coil).


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## John Hasler (Feb 15, 2015)

FanMan said:


> It's a single speed 120V motor, no brake, no mods.  It's definitely the relay that's holding in after the switch is turned off.  It's not the switch because it happens with the switch disconnected (the switch is switching the 24V power from the transformer to the relay coil).



Replace the relay ASAP. (I'd take it apart and try to service the spring and the contacts first, but I'm crazy.  And stingy.)


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## seasicksteve (Feb 15, 2015)

I had a similar issue on my lathe. The screws that secured the contactor for the drive motor to the electrical enclosure had become loose, I tightened them up and it continues to work just fine. I thought I had a bad contactor so I sourced a replacement, now I have a spare.


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## amuller (Feb 15, 2015)

Something is hanging up that contactor.  Could be electrical, mechanical, magnetic.  Check it out!  Sometimes very rough contact pads can stick together mechanically.


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## Wireaddict (Feb 16, 2015)

I've seen a lot of relays & motor starters stick like this & benefit from cleaning.  If your relay or contactor isn't hermetically sealed try cleaning it & the armature & make sure the armature is free.  I've seen coils get hot & their plastic cases warp, making the armatures bind.  Also check the armature return spring & see it the armature's magnetized, not likely but possible.  Another unlikely possibility is contact welding: if this is happening I doubt if the relay would ever drop out without intervention.  If the armature's binding & won't free up, a return spring's missing or broken or the contacts get welded replace the relay.


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## FanMan (Feb 17, 2015)

Looks like the problem was something sticky, looked like grease, on the contact surfaces of the magnet pole pieces.  Might've been from a dead bug, too, there was a bit of white fuzz from some kind of insect nest inside the relay.  Pushing it together manually with a finger didn't squeeze it hard enough together to stick much, but the coil pulled it in hard enough to stay stuck for a little while.  I haven't put in back in the mill yet but it works on the bench so I'm confident I'll have no problem.


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