Hi Folks, this is a preliminary, looking for anyone who has come across a similar issue before, before I start trying to reverse engineer the PCB etc. (Which I'm not a big fan of as it would really need live testing.)

I have a 230V AC 50Hz (single phase) Optimum Lathe, which has a variable speed control - their own inverter I guess.
After a few years of light use the motor is now running erratically - see the video (only the motor is running, with the gears disconnected):

- It stutters and splutters as it suddenly accelerates, then decelerates .. while it is supposed to maintain constant speed. The problem has actually developed over time, as it would occasionally have very "fits" of this behavior before, but now it is constant. The actual running speed is supposed to be preset by a pot. on the control panel of the machine.
It sort of suggests some sort of mechanical (?) feedback issue, as I would have thought that a purely electronic fault would not be quite so intermittent and would not tend to get worse over time. There is a hall effect sensor on the back of the motor, but is this really likely to cause a fault? Plus To really test this I need to have the lathe running and all the wires to buttons, interlocks etc. are all a bit short, so live testing would be rather precarious, and if there is a quick obvious fault I would prefer to try fix this first.
(The motor is (probably) a 3 phase one, so its unlikely to have brushes).
I am in contact with the technical support at optimum, but so far they have not really been able to assist.
(The PCB photo is just a preliminary one while I see if anyone has had any similar experience, before I get deeper into it!)

 

There is a hall effect sensor on the back of the motor

Only one? The motor plate indicates three Hall outputs (Sa, SB, SC). A 'scope on those outputs might give a clue as to the problem.

The motor is (probably) a 3 phase one

Yes. The circuit board has outputs marked U,V,W going to the 4-core cable (which presumably connects to the motor?).

 

Only one? The motor plate indictes three Hall outputs (Sa, SB, SC). A 'scope on those outputs might give a clue as to the problem.

Yes. The circuit board has outputs marked U,V,W going to the 4-core cable (which presumably connects to the motor?).

Hi Alec, yes I guess its a 3 phase hall sensor or some such. And the 4 core cable you see is indeed the motor cable.
The big trouble is its all tightly packaged, so I cant get at the back of the motor (and connections) unless I take the whole end plate of the lathe with gears off, and I can only just get the control PCB out of its box while the cables are all connected (which are threaded through the machine, and are needed to run it), its also pretty heavy so would need supporting. Its a little hairy doing any testing without really spending considerable time at it. So im trying to get a better idea of what might cause the problem first, (and get some courage up! - theoretical repairing!! )
I figure I might be best adding an extension to the sensor cables and checking the outputs first.
I guess its unlikely to be the driving IGBT transistors themselves (?) as it does seem to have small spurts of regular speed so its not like it is missing a phase entirely.

 

If the problem is intermittent it might just be due to a loose/damaged connection which is affected by vibration. Have you tried unplugging/replugging any connectors and checked for wire chafing? You said wiring is threaded through the machine. Hopefully each hole it passes through has a grommet to protect the wiring?

 

If the problem is intermittent it might just be due to a loose/damaged connection which is affected by vibration. Have you tried unplugging/replugging any connectors and checked for wire chafing? You said wiring is threaded through the machine. Hopefully each hole it passes through has a grommet to protect the wiring?

Hmm a good point indeed.. I will have a check for that!

 

I’d be looking for a loose switch and/or wire

 

You could try Model Engineers' Workshop (UK Forum). VFD crops up regularly.
https://www.model-engineer.co.uk/forums

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