Mini lathe blowing fuse – time to change motor?

[Andy SevernParticipant @andysevern59482]

[Andy SevernParticipant @andysevern59482]

I've had a brilliant Warco Mini lathe for a few years now and recently it started continuously blowing the fuse. I've inspected the controller, all the wiring traced the PCB (KPWT-500) for a fault – I could do a whole thread on that – but can't really find a fault anywhere along the path that would cause a short or drain enough to blow the fuse.

Given the effort, I thought I'd just see if I could get a new board, but they don't make them any more, nor can I find anything suitable.

I do hear that the reason why they stopped making the board is to change from 240v DC motors to AC ones because this arrangement has a habit of blowing fuses.

And so to my question: Does anyone have any advice on choosing a new motor + speed controller board for a Warco Mini lathe (300×90 500w).

The separate controls board is the nifty digital one with the push button speed control and tacho readout which I really like over a boring speed knob. However, I'd be happy to build my own PWM speed controller electronics if there's a good PWM controller board out there.

Cheers!

[Bob StevensonParticipant @bobstevenson13909]

Does this lathe have a 'safety' chuck cover that cuts power when in the open (up) position? If it does then make sure this device is not wired around the spindle and has now worn thru the insulation thus making a direct short, like my Chester mini-lathe.

[Niels AbildgaardParticipant @nielsabildgaard33719]

Sewing motor mini lathe

Picture show my best advice for small lathe drive.

600W,240V and easy to find on web.

Only thing I will do different in future is to select a brand where Hall sensor/ foot pedal arm is a sepparate unit and not integrated in the power converter.

[SillyOldDufferModerator @sillyoldduffer]

Replacing the motor and electronics, probably easiest to use similar mini-lathe parts because they will fit. If I were doing it, Niel's modern sewing machine motor is a good bet. These are a far cry from the Universal motor fitted to Grannies Singer and much more suitable for lathes!

But don't give up on the old board yet. Relatively few faults blow fuses without causing visible damage to delicate components.

• Suspect 1: the mains suppressor capacitors. These are connected from Live to Neutral and from Live to Earth and are usually the first component on the board at the mains input end. For test purposes, the capacitors (which may be in a single block) can be removed; if the problem is a shorted suppressor, the lathe will then work normally. The suppressor should be replaced in due course, but they only cost a few pounds. Soldering iron needed.
• Suspect 2: Bob mentioned damaged insulation on the wiring to a safety switch, and similar can occur for other reasons. Look for nipped wires and protrusions on the underside of the board that might touch the chassis when the board is fixed down. Not unknown for protective plastic sheets to be missing, or damaged by an excessive protrusion, or for the mounting pillars to be too short, or swarf to get inside. Have a close look at the big heat-sink. Depending on the design, they may be grounded (no problem), or floating. If floating, they must not touch the lathe's metalwork, and anything that nearly touches is suspect. Once identified, these fault are usually very easy to fix.
• Suspect 3: a shorted power diode. Harder to identify, but cheap and not too difficult to replace, Soldering iron needed.
• Suspect 4: shorted SCR or power transistor(s) on the heat-sink. Harder to identify, and can be a pain to fix depending on the amount of disassembly needed.

Suspects 3 and 4 can cause a chain of damage to other delicate components on the board making it not worth fixing. Much depends on facilities and knowledge – an experienced diagnostician with test gear, spares to hand and soldering equipment is much better placed than someone with no knowledge of electronics.

Dave

[Ronald MorrisonParticipant @ronaldmorrison29248]

With no power applied, does the lathe turn easily ans smoothly? If not, don't mess with the board, check all bearings, especially those on the motor shaft. Any binding of the bearings will cause the motor to draw too much power and blow fuses.

[Ex contributorParticipant @mgnbuk]

A frequent cause of blown drive fuses on industrial dc motor installations was brush dust build-up around the brush holders. This caused "flash overs" to ground, usually when accelerating or deccelerating. The fuse failed during the flash over, but the dust build up that caused the failure could be burnt away by the flash – until the dust build up became too heavy. Sometimes the track of the arc could be seen, along with bright marks on the brass brush holders.

If you were lucky, the semiconductor fuses failed before the thyristors – later microprocessor drives were fast enough to respond so they shut down with an "instantaneous overcurrent" alarm before the fuses blew. Ingress of coolant mists gave the brush dust something to stick to & was very messy to clean out.

Might be worth blowing the motor out with clean, dry, compressed air. through the cooling air inlet holes. Maybe best to do this outdoors – the dust is fine & rather messy !

Nigel B.

[Andy SevernParticipant @andysevern59482]

Thanks for all the suggestions, really helpful.

Some follow-up:

* The motor is disconnected from the PCB so I'm ignoring that at the moment..

* Chuck guard switch seems to have recently failed in the ON position, but checking continuity AND resistance from both terminals to L, N and E show no anomolies (though I'll take that apart to fix it anyhow.

* Testing across the motor outputs shows pretty much the same as the braking resistor and the relays have successbully isolated both terminals from the 240v DC supply.

* No shorts across the rectifier from 240V AC > DC, Resistance there is 3.6M (happy with that)

* Resistance between the 240V ac input is 747R… that's a little low, bit still 321mA at 240v

* Nothing along the DC or AC pathways has a short to Earth, which I'd say would exonerate any supperssion caps – but messured them anyhow . I kow caps can fail short – I fixed a friend's XBOX which had a 100nF cap failed to short (A buqqer to trace that!)

now….. There's a high wattage 100R resistor with a cap in series across the Source and Drain of the mosft. I nearly missed the slight burns on that mistaking the wire-widings for black.brown stripes. BUT there doesn't seem to be anything that's going to pull enough current in that situation to burn the resistor or blow the fuse.

This looks like it's associated with a low value resistor that's making a divider for the current sense. No damage there and that even feeds into the op-amp throgh a 1/4w 10k res which even if THAT secrioin had failed wouldn't pull 6A without significant magic smoke.

….. scratching head. And yes, lothed to give up this board becasue it's areally nice design,

[SillyOldDufferModerator @sillyoldduffer]

Posted by Andy Severn on 06/12/2020 14:43:09:

…

* Nothing along the DC or AC pathways has a short to Earth, which I'd say would exonerate any supperssion caps – but messured them anyhow . I kow caps can fail short – I fixed a friend's XBOX which had a 100nF cap failed to short (A buqqer to trace that!)

…

….. scratching head. And yes, lothed to give up this board becasue it's areally nice design,

I like the sort of electronics problem where the faulty component is clearly indicated by smoke and flame!

I still suspect the suppressors though because although they often fail open circuit, and occasionally do the decent thing by exploding, they can also reform enough after a puncture to fool a low-voltage ohmmeter, and then break down at the first sniff of mains.

Dave

[Peter BellParticipant @peterbell11509]

I'm with SOD on this one and I'd be a bit suspicious of the 100R and cap in series,. Think its called a snubber and Iv'e come across a few of them over the years which have broken down in a spectacular way and left traces of burning in the process.

Occasionally on things like this Ive connected them ito the mains in series with a high wattage bulb or 1 bar electric fire to limit the current which has allowed me to see whats going on without too many distractions from blowing fuses and other dramas.

Peter

[Andy SevernParticipant @andysevern59482]

Agree with SOD Those caps are really the ONLY things that are energised at this point with the exception of the transformer for the current detect electroics.

(Yep 100R looks like a snubber)

Peter's suggestion of current limiting on a test is a good one, if I can limit it enough for the faulty component to "so the secent thing" then that'll be really handy. In racing drone building (my main speciality) we use what's called a smoke stopper – a bulb in series with the Lithium Polymer bsttery which lets a new build be tested without the risk of unpleasent LiPo reactions…. now those DO make a lot of fuss when shorted or punctured!! (I have some great videos of some batteries that we retired after crashes!)

[Anonymous]

Posted by Andy Severn on 06/12/2020 14:43:09:

I kow caps can fail short – I fixed a friend's XBOX which had a 100nF cap failed to short

That's true for ceramic capacitors. But X and Y capacitors are metal film and are specifically designed so that any short across the dielectric clears itself without the capacitor going permanently short.

The board can't be that nice a design as it's failed.

Andrew

[KnightriderukParticipant @knightrideruk]

Hi

Boards + motors are available on Ali – Express you will have to search but they are there.

Knightrideruk

Out of the darkness comes light………..

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