(Another) Mini Lathe Speed Controller Problem

[Philip Coupland 3Participant @philipcoupland3]

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

[Philip Coupland 3Participant @philipcoupland3]

[David George 1Participant @davidgeorge1]

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

[MikeKParticipant @mikek40713]

Do you have a picture of the blown component?

[old martParticipant @oldmart]

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.

[Philip Coupland 3Participant @philipcoupland3]

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.

[Anonymous]

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

[Anonymous]

Posted by Philip Coupland 3 on 14/02/2022 21:41:41:

…why is so much electronics necessary to control the speed of an electric motor?

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

[noel shelleyParticipant @noelshelley55608]

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

[Philip Coupland 3Participant @philipcoupland3]

Posted by Andrew Johnston on 14/02/2022 21:42:28:

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

I’m in England.

[Les Jones 1Participant @lesjones1]

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

[MikeKParticipant @mikek40713]

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

[HuubParticipant @huub]

Posted by MikeK on 14/02/2022 22:50:31:

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.

[Philip Coupland 3Participant @philipcoupland3]

Thank you Les, that is very helpful information. I shall look carefully for the source of the short in the output to the motor.

Everything was working fine before I removed the controller box from the side of the lathe and, as I mentioned before, I just unscrewed the box but left all the wiring untouched while I worked on the mechanicals. This suggests that either some wiring has become physically damaged or a perhaps a piece of swarf has caused a short.

That’s a very good website that you linked to.

Philip

Posted by Les Jones 1 on 14/02/2022 22:41:32:

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.
**LINK**

Les.

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

[Philip Coupland 3Participant @philipcoupland3]

Thank you Mike

I agree, definitely a symptom, most likely of some fault introduced by moving things around while I was working on the headstock. I’m a careful worker but something must have been compressed or detached.

Yes, a new board looks likely. I’ve had the machine a long time and it was secondhand – Id guess its probably 20 years old. I’ve only been using it intensively for the last two years but still this pretty good service from a cheap lathe.

Philip

Posted by MikeK on 14/02/2022 22:50:31:

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

[Philip Coupland 3Participant @philipcoupland3]

Thanks. Yes, I must find that fault – the scenario of another blown board is something I’d prefer to avoid.

I’ve been tempted by the idea of a new machine but will persevere with this one for now. I’ve put quite a lot of work into it, to make it work properly. I’d really like to get a secondhand Myford but there’s no space for it.

Philip

Posted by Huub on 14/02/2022 23:03:58:

Posted by MikeK on 14/02/2022 22:50:31

[…] 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.

[oldveloParticipant @oldvelo]

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

[MikeKParticipant @mikek40713]

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.

[Ian ParkinParticipant @ianparkin39383]

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

[Philip Coupland 3Participant @philipcoupland3]

Posted by Ian Parkin on 15/02/2022 06:55:01:

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.

[Anthony KnightsParticipant @anthonyknights16741]

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

[Philip Coupland 3Participant @philipcoupland3]

Posted by oldvelo on 15/02/2022 03:51:29:

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

Thank you OldVelo. With your information, I've identified the blown part as a 'plug-in horsepower resistor (9838)' from KB Electronics. These are available in the US although the cost of shipping and import tax seems excessive. I'll see if I can find them in the UK.

[Philip Coupland 3Participant @philipcoupland3]

For future reference, a full range of horsepower resistors is available from Axis Controls in the UK (https://www.axiscontrols.co.uk/). I've ordered one and we'll see if it brings my lathe back to life. Its a plug-in so I won't even need to get the soldering iron out.

[John HaineParticipant @johnhaine32865]

I use a genuine KB controller on my CNC little mill and it has never given any problems even after the cutter was driven hard into the work, which just blew a fuse. On the other hand the "KB" controllers fitted to mini lathes etc seem to be very vulnerable. I suspect they are approximate but substandard copies of the real thing. A proper KB replacement could be more expensive but a good investment.

These sense resistors go in series with the motor and need to be matched to the motor power, which is why it's a plug-in component (though it probably gets hot too). I think these controllers keep the motor speed approximately constant by having in effect a negative output resistance which cancels out the motor's internal rotor resistance, so that the armature voltage is a good indication of speed. This resistor trims the output impedance by adding a small positive value to match the motor. The fact that it has blown up shows that it probably did have full mains voltage across it – since this could only come via the control semiconductors it's quite likely they have blown up too.

Several people here have used modern brushless sewing machine servo motors with electronic control and they seem to be a good solution and quite low cost.

[Ketan SwaliParticipant @ketanswali79440]

In addition to checks and suggestions made by various people, you may need to check the pre-load you have put on the bearings…. check how easy or difficult it is to turn your chuck by hand. It there is too much re-load, it will be difficult to turn the chuck, meaning that there is more power is required/drawn… which can result in certain components blowing.

ARC doesn't sell any KB control boards.

Ketan at ARC

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