(Another) Mini Lathe Speed Controller Problem

To briefly introduce myself, I’m hobbyist whose metal work interests are mainly related to motorcycles. My tiny workshop is crammed with many machine tools, including the lathe that is the cause of this request for help.

I have recently added roller bearings to the headstock and wanted to test the running of the drive belt so plugged it in but on turning the machine on there was a flash and a pop and that was that. On closer inspection inside the control box it was found that a large rectangular component marked 120V had the back blown off. I don’t know what this component is, my knowledge of electronics having stalled at the battery and bulb stage.

I hadn’t touched the electronics at all, merely unscrewed the control box and moved it to one side, leaving everything connected apart from the two earth wires going to the headstock, which I had reconnected.

I’m wondering if I could replace blown component (assuming I can identify it and find one) or is it likely that a whole lot of other stuff is gone. I’m also concerned to find out what’s caused the problem - if I end up spending a fail amount of cash on a new circuit board I don’t want to ruin it as well.

With thanks, Philip C

PS If I can figure out how, I’ll post a pic

Hi Philip welcome to the forum.

This should help how to post pics etc.

https://www.model-engineer.co.uk/forums/postings.asp?th=103028

David

Do you have a picture of the blown component?

Welcome to the forum Phillip, you will need to checkout the method of posting photos, then somebody will probably be able to help. Failing that, have a look at the ARCeurotrade link top right on the page, they have spares for this type of lathe including electricals.

Thank you for the guidance on posting pics.

Here is the smoking wreckage below - the broken off piece came off the back of the rectangular white component.

One question occurs to me to ask is why is so much electronics necessary to control the speed of an electric motor? For example, an electric drill usually has the facility to control its speed without all this.

There is a picture of the board in the OPs album. It's a rather odd component, apparently on its own PCB. It's definitely flipped its lid. Looks a bit like a power resistor, but why 120V? Alternatively it could be a MOV. It would help to know if the OP is in the UK or the US?

Andrew

Electronics has moved on from batteries and bulbs.

The board runs off AC mains, not a low voltage battery, so there needs to be input surge protection, rectification and filtering. It's also possible that the motor types are different.

Andrew

First thought is a capacitor rated at 120v. Some sort of relay ? Just guesses ! Noel.

I’m in England.

I think the component WAS a ceramic resistor that is used to sense the motor current. I think the output connections to the motor must have been shorted out putting the full rectified mains voltage across it to cause it to fail in the way it has.

I think the board is a KBLC speed controller board and the resistor that has failed is R21.

This is a link to information on the board.
http://andysmachines.weebly.com/variable-speed-controls.html

Les.

Edited By Les Jones 1 on 14/02/2022 22:50:45

That blown component is likely a symptom, not the problem...Something else failing would have caused that component to fry. I just looked at the control board for my own mini-lathe and it also has one power resistor like yours (which is probably what yours is). Diagnosing and fixing this is going to require someone with electronics experience, and equipment, though. I'd help you out if you were near me, but...you're not.

Looking at the back of the board may give clues for other fried components. Also, looking at the top side carefully. Capacitors are common culprits in switching circuits, as cheap Chinese ones eventually fail (although mine haven't) and this causes further components down the line to fail. But, again, this would require testing.

You might get lucky posting on the groups.io minilathe group...Perhaps someone had the same thing happen and found the bad component. But my guess is that you're going to wind up buying a new control board.

-Mike

I agree, and this can be an expansive repair. If you can't find the cause the new board will probably also be gone in seconds.

You could change te motor and control by an AC motor and VFD or an AC Servo motor and AC servo controller. I know it is not the cheapest option but it has to be considered if you can't find the cause.

I do not know how old and worn the lathe is, but maybe is replacing the lathe also an option.

Hi

The control board looks like a KB Electronics AC to DC motor variable speed control.

The white rectangular block is a horse power restrictor The component that was damaged was probably fitted for some reason piggy back in parallel to the hp resistor.

Spares and an extensive range of information is available from KB Electronics.

Another source is available from Surplus Center

Click on the link Surplus Center have spares available to match the motor horsepower.

I have used Their  Minarik control on DC motors  and found the satisfactory once setup to match the motor power

Edited By oldvelo on 15/02/2022 04:00:16

If the short was caused by a motor short, like Les said, and you've fixed that problem then I would definitely solder in a replacement resistor. A very inexpensive repair if it works. Worth a shot before going for a replacement board.

On the photo is that a piece of curly swarf just under the resister framed by the red wires?

Hello Ian, well spotted! Despite my efforts to keep the workshop tidy all manner of swarf, splinters, chippings gets everywhere. It's a very small space and bits go everywhere.

Later today, I'm going to thoroughly check the output to the motor for shorts - it would be a relief to find one and know the cause of the problem.

That definitely looks like a power resistor to me. I suspect it is mounted above the main PCB because it gets HOT in normal operation. It would have blown up if subjected to excessive current caused by a short circuit. Did you trap the motor wires when you re-assembled everything? Well worth a try replacing the resistor before splashing out on a new control PCB.

Good luck