mini lathe speed control

Hi all.

Having followed John Smiths excellent thread on the speed control board, circa 2010, and having run my lathe for around 10 years after the fix, I had another pop/bang and blew the fuse. I replaced both MOSFETs, forward/reverse switch and Pot. The problem is the lathe now works in reverse, i.e. switch on at the wall socket, the lathe runs at top speed. If I then switch on via the pot, then turn it clockwise, the speed reduces and or increases as "normal"

This is a clark cl300 with the old CL250J board. Previously I had to solder in various links where the circuit board tracks had breaks. It has run for about 10 years since, though.

I have a suspicion it could be the MOSFETS which I have miss wired.

I would be most grateful if someone could point me towards the likely explanation.

Regards Dave

On the assumption the motor is a permanent magnet type I think you have reversed the two wires to the motor.
I think you have also reversed the connections to the two outer pins on the speed control potentiometer.

Les.

Starting the lathe with anything other than Zero rpm selected is reckoned to be bad for the board.

I would try changing over the wires on the outer pins of the pot as a starting point.

Hopefully that will sort the "star at high speed" problem.

If the motor still runs in reverse, as suggested, changing over the motor wires should solve that,

But I would feel inclined to work on the pot first, and then on the motor wires next.

Howard

Hi Les.

Thank you for the rapid response. I'll check out the relevant wiring when I get back.

Should be straight forward to correct.

My apologies to John Swift. It was his excellent work which enabled me to fix the problem before

Regards Dave

Hi Howard.

Thanks for your reply.

It does seem to be my miss-wiring of the pot.

It was with great trepidation that I switched on for the first time, to be rewarded with a screaming motor.

Regards Dave

When I first read your post I though forward and reverse were the wrong way round as well as the speed pot working the wrong way. Ignore my comment about reversing the wires to the motor.

Les.

Edited By Les Jones 1 on 03/08/2022 18:14:45

Hi Les.

No problem

Regards Dave

My 7 x 12 needed a new speed control pot/switch many years ago and I got the pot/switch from Maplins. I had to reverse the switch action because it was off to on when it needed to be on to off. The proper switches can be obtained from ARC.

Hiya.

I did source both the pot and the fwd/reverse switch from Arc.

I committed the gravest sin by not transferring the contacts, like for like to the new pot.

I did so religiously, with the fwd/reverse switch.

Best regards Dave

Hi all.

Thanks for the suggestions, but I started again from scratch, having sourced a bulk supply of MOSFETs.

I have ended up with a working lathe, jumping various dodgy tracks on the PCB.

The problem is, with a 2amp motor and 2amp fuse, any more than the lightest of cuts results in the fuse blowing. Would this be a rectifier or capacitor at fault?

Logically, I should just replace the board, but I do have the bit between my teeth!

Well done you for getting it to work at all! My first thought is the fuse is wrong. Is it a like-for-like replacement? Possibly the ones being used are too small or a slow-blow type is needed.

Doesn't seem likely a faulty capacitor or rectifier would start the motor without blowing the fuse. It suggests the board is providing power, but overloads when the amps needed to make a cut are taken.

Not good at diagnosing electronic faults myself because I hate doing it; for some reason makes me impatient very quickly. However, forced at gunpoint I might try isolating the fault by letting the motor run unloaded for a few minutes whilst watching for bubbling, discolouration or smoke. If none appears, power off and run fingers over the components looking for hot ones.

What test equipment do you have? An ammeter across the fuse holder would show if the board is pulling excessive current.

Dave

Hi S.O.D, Thanks for the support.

After I repaired the board, back in 2013, it ran fine for a few years with a 2amp fuse, till it started blowing them, mainly when parting off. At that time I actually started fitting a 5amp quick blow, which again worked fine for several years, till it went pop recently. Strangely the fuse in the lathe remained intact, it was the mains plug fuse, also 5amp, which blew. It doesn't seem right that I should fit a bigger fuse than the motor rating of 2amp.

You are assuming that it is the board that is at faultt - have you considered that it might be the motor ?

Your motor has been run for several years and, being brushed, will doubtless have an accumulation of carbon dust inside. If this dust builds up to provide a path from the brushes to the motor frame, shorts can occur. On industrial sized motors this can lead to some very spectacular bangs when it happens. Modern industrial drives are pretty good at shutting themselves down when a short is detected, but armature fuses did blow occasionally if you were lucky. If unlucky, the semiconductors (usually thyristors) failed to protect the fuse.

If you don't want to strip the motor, try removing all the brushes & blowing through each brush hole with dry compressed air. Best to do this outdoors ! You can check the brush condition while they are out & it is preferable to mark the brushes to show their original location & direction they came out.

Nigel B.

I don't have any knowledge of the driver board in question, but if it is now popping a 5A fuse when a 2A used to be OK, then obviously something is drawing more current.

My immediate thought is electrolytic capacitors on the driver board. These can 'dry out' over time, and could easily cause excess current to be drawn. I would look at checking and replacing any duff electrolytics - if you are into that level of repair. If not, a new board would be much easier and possibly cheaper in the long run - assuming the motor is not partially shorted.

I would also check the current draw with the board powered up but the motor at rest to see if that seems sensible.

Edited By John Doe 2 on 27/08/2022 10:25:22

It has been suggested on other threads on this subject, that the control board can be tested using a 60 watt incandescent (not LED) lamp as a load instead of the motor. If the lamp dims and brightens with the speed control, the board is probably OK.

Personally, I am fed up with fragile Chinese electronics. I recently had to change the motor on my CL300, only to have the control board fail a few weeks later. I have now almost completed converting my machine to 3 phase inverter control using a package from Newton Tesla. Expensive, but should last me out.

Best of luck. Anthony

Hi all.

Many thanks for the suggestions and support, all of which have been most helpful.

Having dismantled the motor and cleaned it, (there was a little swarf inside), the lathe seems to be running OK. The next hurdle will be some parting off of aluminium bar and tubing for my project. Doesn't sound too heavy, but only time will tell. Steady as you go, I suppose.

Dave