Warco WM-16 Motor Speed Fluctuation.

Hello Guys (and Ladies).

For a while now my Warco WM-16 milling machine has suffered a motor speed fluctuation issue and sometimes just twitches or won't run at all. The machine uses the stock motor controller and DC brushed motor. New motor brushes have been installed.

When the motor doesn't run I can turn the spindle by hand safely a few degrees, unpowered, then restart and sometimes it will work. When running, the motor can be stable for a few minutes then suddenly increase or decrease speed or even run down and stop completely.

Searching the web I'm given to understand that speed control is handled by the SCR's and Diodes on the controller PCB. Is this correct? None of the ones on my PCB appear to be burnt out or overheated. I was also suspecting the speed sensor which I think is an optical device working through a holed flange mounted on the spindle.

If the SCR's and diodes are the likely cause, can anyone kindly verify if these parts from Mouser are ok to use? I believe the 'L' in the part numbers refers to the necessary isolated tabs?

S8010LPT as a replacement for the A69108.

D8010L

Thank you,
Ed.

Second time posting this. Microsoft Edge won't allow the SCAYT spell checker to close and all typing is lost.

Edited By Ed Duffner on 15/04/2023 15:55:07

As usual we jump in with both feet…..

My suspicion is with the motor, specifically the armature…either a bad segment or a poor contact leading to an open circuit winding.Check carefully at the commutator with a magnifying glass. You can check the continuity with an ohm meter…between the comm segments, the readings should be equal.

Firstly, check the speed controller by disconnecting the motor, then connect a 100Watt domestic incandescent light bulb in its place across the motor output terminals….The bulb should light up and be dimmable, every time you operate it…Do this many times, to prove its reliability.

If you feel able, strip out the motor, strip it down ( mark the motor end pieces relative to the main body! ) clean up the commutator with sandpaper, nothing else! Give it a wipe with a dry cloth and reassemble…

Check the brushes move freely in their boxes..

Put the motor back and try it again.

I’ve repaired many of these controllers where the motor has been the culprit every time….and repair isn’t cheap ( neither is a replacement)

So start with the simple things before reaching for the soldering iron…

Weird behaviour like this can be caused by a dodgy speed control potentiometer - it will of course work perfectly when you try checking it with a meter, so easiest just to test by substitution, they're cheap enough.

Thank you John, I went ahead and removed the motor, my day job is a service engineer for power-tools and small plant, armatures are generally replaced rather than repaired. The comm' segments are all flat and sound. I have cleaned them though with 1200 grit emery and IPA to remove any carbon dust, I think sandpaper is a little course.

Anyway, checking adjacent segments they measure 0.5Ω all the way around except for where the ends of the coil comes together. The two neighboring segments to the joint read about 1.1Ω an I can very slightly move the wire ends. I will see if I can gently crimp the joint back into place and retest.

The brushes are free to travel in their holders.

Tomorrow I'll see if I can test the controller with a lamp.

Andy, that's a good point about the pot', thank you sir.

Regards,
Ed.

Weird behaviour by the pot would show up with the lamp test…

Ok, a few minutes ago I made a video of the lamp failing whilst connected to the motor wiring. It would appear to me that there is a fault with the controller output.

The lamp used in the video is a standard 60Watt, I do not have a 100Watt lamp.

When my lathe (WM250) developed very similar symptoms I found that a resistor on the control board was not seated correctly. Once pressed in all back to normal.

Picture in album (of course may be different control board!)

Steve

Thanks Steve, it looks like the same board. I pushed in that resistor yesterday to make sure of that, it was another thing I found on the web. I think I need to remove the board completely and look for dry joints etc.

Cheers,
Ed.

Looking at the bulb demonstration, that looks very much like a dry joint or bad joint somewhere, as you suspect.

Doesn't seem to be an output stage problem, because the brightness is only varying very slightly - if an output stage was faulty, I would expect the bulb to be going to half brightness or off. i.e. a major difference. What we are seeing seems to be a minor difference, so I suspect a fault in the drive to the output stages, and would look around the speed control area of the board - start with the speed potentiometer itself, it might have a dirty or damaged track.

First thing to do is clean all connections with switch cleaner spray; so any wires held in terminal blocks, or ribbon cables or plugs joining the PCB. Then get some spray into the speed pot and exercise the pot over its full range, backwards and forwards. Allow to fully dry before testing. Then any integrated circuits held in sockets - be extremely careful if you have not removed ICs before, and earth yourself to avoid static discharge. Then resolder the solder side of the whole PCB in case of dry joints.

And then change all the electrolytic capacitors - they can dry out and stop being capacitors, which prevents the circuits they are in from working properly. If none of that helps, you will have to start fault finding the PCB at component level.

Apologies if I am teaching you to suck eggs !

Edited By John Doe 2 on 18/04/2023 10:06:53

Thank you John Doe 2, I hope to have another look at this tomorrow (Saturday). You have pointed out some additional things to look out for and action where necessary, very much appreciated, as is all the input.

Ed.

Had another look at this today. Found a corroded resistor on the power board, it's a 220KΩ and reads constant when tested with an ohm-meter, actually 223KΩ. I suppose it might be different when powered on and I will change it anyway. Below are some photos, of the power board and speed controller board?

All solder joints appear to be sound. I have yet to test the speed potentiometer and start cleaning socketed and plug-in components.

I don't think the 220k resistor is the cause - it appears to be part of the mains filter circuit, which would carry on working even if it was removed. I'd leave it alone.

It and the two rusty Allen head bolts on the heat-sink suggest condensation, but it could have happened in factory storage, as the rest of the it looks clean. Is condensation a problem in your workshop? It plays merry hell with electronics, causing all manner of weird faults. Rusty heat sink bolts might cause trouble if SCR1 and D15 where insulated from the heat-sink with a mica washer but they seem to be bolted straight to ground (normal).

Diagnosing fault electronics gets up my nose even though I have the gear. I'd replace the board if I couldn't quickly trace this to the pot, it's connections, or something obviously amiss on the board. Others have more patience!

Dave

Dave, yes condensation has been bad in my shed over the years. I now have 2 tubular heaters behind my lathe and mill. It's strange that it might have caused just two of the screws to rust? The nuts holding the power board to the casing are also corroded but not the screws, unless they are stainless of course.

Ed.

Does C101 have a bulging end? Hard to tell. I might also very carefully mark, then gently exercise those five blue pots; "Min Max" etc. It is possible that one of them has a dodgy contact with its track - especially if they might have been damp over time.

Put pairs of dots somewhere on each moving white part and the body with a permanent marker as a reference to get them exactly back to where they were. Probably not too critical.

Edited By John Doe 2 on 24/04/2023 17:07:25

By the way, only move or clean the pots etc with the power switched off - never with the circuit live.

I don´t know whether this is useful but could it have something to do with the set-up pots given that there was a bit of humidity.? (Manual for similar board)

C101 is ok, I can see what you mean though it does look bulged, just the way the light is catching the markings in the cap.

I will check the preset pots. I just received some electrical contact cleaner, ok for pot wipers too although the presets do seem to be very well sealed. All maintenance will be performed on a dead board.

Dick H, Many thanks for the manual link, that certainly is a lot of info to digest.

Regards,
Ed.

Picking up on John Doe's comment about dry joints, have a good look at the soldering and PCB tracks with a strong magnifying glass, both sides of the board.

Dry joints have a characteristic grey crystalline look rather than being shiny metallic. I've never seen one on an SMD board - the tiny components - but they seem more likely on wired components that go through the board like the big resistors, capacitors, connectors, etc.

Broken tracks are harder to spot - sometimes they develop a hairline crack that opens and closes randomly as the board heats up or is vibrated by the motor.

Both fixed by a dab of fresh solder. The track will have to cleaned back to copper first, which is a delicate operation requiring a steady hand and a scalpel. May be easier to trace the track back to something easier to solder to and bridge the whole section with a wire. Depends on the board - modern ones are often tightly packed with tracks running within a fraction of a millimetre of each other, not meant to be repairable.

Dave

I would very strongly urge you to do that before doing anything else with the boards!

As above - just replace it. They are 'wear parts' and don't last forever.

Just a quick update on this one. I've been waiting for a potentiometer from China which arrived this week. Fitted it all back together with a new pot' today ...and still the same.

Oh well, back to the drawing board as they say.

Ed.