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Old 12-19-2022, 06:34 AM   #1
CptChronic
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Default Carpet overlocker motor gradually increases speed by itself

I have an overlocker for stitching carpets and once I turn it on and leave it on the slowest setting, the speed will slowly increase by itself. When this happens, the voltage between the speed increase potentiometer, increases by 0.001V. I've stripped the thing down and the circuit board looks fine. Could this be a faulty potentiometer? Or something else? There is some soot inside the housing near the transformer and drive belts

Pictures here: https://imgur.com/a/sUjRJrU
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Old 12-19-2022, 02:07 PM   #2
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Default Re: Carpet overlocker motor gradually increases speed by itself

Go for the simple things first. Check the current it is drawing from the socket. Maybe it needs to be taken apart, cleaned and lubed?
You can check the pot with your DMM and verify for smooth operation.
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Old 12-19-2022, 04:59 PM   #3
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by CapLeaker View Post
Go for the simple things first. Check the current it is drawing from the socket. Maybe it needs to be taken apart, cleaned and lubed?
You can check the pot with your DMM and verify for smooth operation.
Thanks for your reply, I did check the potentiometer and when increasing/decreasing the speed (with the thing turned off) it shouldn't smoothly increase/decrease in ohms, it often jumps right up before settling on the value it should be.

What do you mean by socket?
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Old 12-19-2022, 07:29 PM   #4
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Default Re: Carpet overlocker motor gradually increases speed by itself

Someone else has been in here doing repairs. The main speed control IC Atmel U2010 is in a socket and many connections resoldered etc.
The IC socket looks like it's cracked or pins did not go in straight? The triacs need a heatsink so I can't see them taking much abuse.
The speed control potentiometer resistance should not jump around unless it is dirty or burnt inside. I would disconnect any two of its wires and check with an ohmmeter.
The motor's commutator has a lot of carbon on it, I take fine sandpaper and scrub that off. Most of this looks new.

If you add a pic of the board's backside, I could figure out the calibration trimpots VR2, VR3 what they do.

There's a loose metal washer in the picture, shake it out.
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File Type: jpg washer_bzzzt.jpg (135.0 KB, 14 views)
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Old 12-20-2022, 05:44 AM   #5
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by redwire View Post
Someone else has been in here doing repairs. The main speed control IC Atmel U2010 is in a socket and many connections resoldered etc.
The IC socket looks like it's cracked or pins did not go in straight? The triacs need a heatsink so I can't see them taking much abuse.
The speed control potentiometer resistance should not jump around unless it is dirty or burnt inside. I would disconnect any two of its wires and check with an ohmmeter.
The motor's commutator has a lot of carbon on it, I take fine sandpaper and scrub that off. Most of this looks new.

If you add a pic of the board's backside, I could figure out the calibration trimpots VR2, VR3 what they do.

There's a loose metal washer in the picture, shake it out.
Ah yes I didn't spot that.
I've just had it running with a thermal camera and only one of the triacs got hot and a few of the other parts, only ~50C so nothing crazy. The 2 resistors above the IC went to 300C after a few minutes though!

I stripped down the potentiometer, it looks alright inside but gave it a good clean. It did seem to run a little better but the speed wasn't solid, at least we can cancel that one out.

I'll get the motor commutator cleaned up and get you a picture of the boards rear next.
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Old 12-20-2022, 06:01 AM   #6
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by CptChronic View Post
Thanks for your reply, I did check the potentiometer and when increasing/decreasing the speed (with the thing turned off) it shouldn't smoothly increase/decrease in ohms, it often jumps right up before settling on the value it should be.

What do you mean by socket?
I wonder if the motor is gunk ed up, kinda working harder until it warms up a bit. You should see that reflecting in the power consumption of the motor. To monitor the current consumption the easiest was is to pug that machine into some power monitoring device on the power mains plug / socket. If you got a insulation tester, it could tell you the condition of the winding inside. What about taking that belt off and try spinning that motor by hand? I should spin like very easy.
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Old 12-20-2022, 06:07 AM   #7
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Default Re: Carpet overlocker motor gradually increases speed by itself

Ok so here are a few pictures https://imgur.com/a/9mlfzEb

I have traced what the 2 resistors are attached to, they're attached to 3 pins but only 2 of them are connected. 1 goes to the motor the other into the yellow relay which then is connected to the front panel which houses the on/off switches, the counter and the control with on/off trigger and speed control. Not sure what part it is connected to though. I get no continuity when flipping the switches, increasing the speed etc.
Here's a couple of pictures of the front panel https://imgur.com/a/7gxIe8B you can see the blue and brown wires entering it at the bottom
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Old 12-20-2022, 06:11 AM   #8
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by CapLeaker View Post
I wonder if the motor is gunk ed up, kinda working harder until it warms up a bit. You should see that reflecting in the power consumption of the motor. To monitor the current consumption the easiest was is to pug that machine into some power monitoring device on the power mains plug / socket. If you got a insulation tester, it could tell you the condition of the winding inside. What about taking that belt off and try spinning that motor by hand? I should spin like very easy.
Can I test it with a multimeter? I don't have a power monitoring device or insulation tester unfortunately.
I darent take the belt off for fear I won't get it back on, it does seem to turn quite easily though

Edit: it does make sense though, this could be why the resistors are getting so hot right?

Last edited by CptChronic; 12-20-2022 at 06:13 AM..
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Old 12-20-2022, 06:26 AM   #9
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Can I test it with a multimeter? I don't have a power monitoring device or insulation tester unfortunately.
I darent take the belt off for fear I won't get it back on, it does seem to turn quite easily though

Edit: it does make sense though, this could be why the resistors are getting so hot right?
Yes it does... also the amount of dirt that the motor sucked in. Just look at the motor itself... it sucked tons of dirt.
I wonder that when the motor is warmed up, if it keeps its speed and current consumption.
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Old 12-20-2022, 06:33 AM   #10
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Default Re: Carpet overlocker motor gradually increases speed by itself

I think most DMM's have a 10A fuse. But since I don't know what DMM you have, I can't say. You have a current clamp? Other than that, a el cheapo power monitoring device will do just fine for this. I don't really think you will get around not taking that belt off the motor at some point. You can also try current consumption with load (belt on the motor) and without any load (belt off). Maybe something binding up whatever that belt drives.
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Old 12-20-2022, 07:20 AM   #11
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Yes it does... also the amount of dirt that the motor sucked in. Just look at the motor itself... it sucked tons of dirt.
I wonder that when the motor is warmed up, if it keeps its speed and current consumption.
Possibly, I haven't let it get that far as those resistors were at 300C and rising which is already way over their max temperature, guessing it's a 3W resistor.

Yes my multimeter is 10A, I don't have a current clamp either, I didn't know those things existed so will pick one up soon anyway, they look like a great addition to the kit.

I've just ordered a power monitor, theyre on offer at the moment, £16 from Amazon...bargain! That'll be here tomorrow so I'll check the current once it arrives.

Thanks for the help so far, I really appreciate it
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Old 12-20-2022, 10:56 AM   #12
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Default Re: Carpet overlocker motor gradually increases speed by itself

A lot of the "basic" test equipment is cheap to get and doesn't break the bank for the occasional electronics tinkerer.
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Old 12-20-2022, 02:05 PM   #13
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Default Re: Carpet overlocker motor gradually increases speed by itself

OP can you give us a make/model of this carpet overlocker?

I would confirm the speed control potentiometer is ok and also its wiring to the board.
The hot running resistor pair gives power to the controller IC.
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Old 12-20-2022, 02:27 PM   #14
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by redwire View Post
OP can you give us a make/model of this carpet overlocker?

I would confirm the speed control potentiometer is ok and also its wiring to the board.
The hot running resistor pair gives power to the controller IC.
I can't find a model number, there really doesn't seem to be much info for these on the Internet but it's this one here: http://www.carpetsewingmachines.com/...pping-machine/
It's make is Kompakt

I have stripped down and cleaned the potentiometer, it's alot smoother now.

As the resistors getting that hot normal?

Thanks for the help as well redwire, I really do appreciate it
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Old 12-21-2022, 02:29 PM   #15
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Originally Posted by CapLeaker View Post
I think most DMM's have a 10A fuse. But since I don't know what DMM you have, I can't say. You have a current clamp? Other than that, a el cheapo power monitoring device will do just fine for this. I don't really think you will get around not taking that belt off the motor at some point. You can also try current consumption with load (belt on the motor) and without any load (belt off). Maybe something binding up whatever that belt drives.
Ok so I've got one of these power testers you plug into the wall, the amp reading started off around 0.9A and slowly trickled down to 0.82A and stayed around there.

Would this still point to the motor dragging before warm up? I let it run for around 8 minutes, the speed increased and decreased marginally all this time but it was slowly gaining speed overall. https://imgur.com/a/FWJ5zKS

Really don't know what to do at this point
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Old 12-22-2022, 01:43 PM   #16
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Default Re: Carpet overlocker motor gradually increases speed by itself

I'm not sure what resistance readings you now get from the speed control potentiometer. I wondered if its wiper was open-circuit and the speed control board floating with no speed command coming in? Do the switches work OK or is it stuck running?

The two 24k resistors R16, R17 will run quite hot, they each dissipate around 2.2W each which is a lot. But not 300įC, I'd guess around 100įC is a max.

As for what's wrong with this thing, it's either the speed pot or the IC or the calibration or (less likely) the electrolytic capacitors. You'd have to take voltage measurements while the board is powered up, which is no fun. The IC needs around 14VDC power.
It's hard with no wiring diagrams although the same speed controller scheme is used on many power tools. This overlocker one just has a soft-start (many seconds to ramp up motor speed) and variable speed. There is no load compensation or tachometer circuit.

I would pen-mark the trimpots VR2, VR3 location and give them a turn to see what they do. I can't follow their PC board traces running to which IC pins to figure them out. Note this board is hazardous live so use an insulated screwdriver...

At this point I'd likely put in a new IC and make sure the triac is OK.
You could short the speed pot forcing it to minimum speed (resistance) or maximum speed to see if the IC is responding at all.
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Old 12-22-2022, 04:11 PM   #17
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Default Re: Carpet overlocker motor gradually increases speed by itself

I’d point at either the pot or the motor. The motor should come apart. But before I’d take the motor apart I’d warm it up then start it. The warming up is to get the lubricant what ever is in there left a bit more runny first place. Keep in mind that all that dirt went through he motor, the windings, the bearings, the lube, etc.
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Old 12-22-2022, 07:04 PM   #18
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Default Re: Carpet overlocker motor gradually increases speed by itself

Why do you think it's the motor? There's such a reduction factor on the pulleys I don't think it drawing more current would cause the speed to creep up.
An idea would be to substitute the motor with an incandescent light bulb as a load for the control board and see if it works as a dimmer.
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Old 12-22-2022, 09:36 PM   #19
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Default Re: Carpet overlocker motor gradually increases speed by itself

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Why do you think it's the motor? There's such a reduction factor on the pulleys I don't think it drawing more current would cause the speed to creep up.
An idea would be to substitute the motor with an incandescent light bulb as a load for the control board and see if it works as a dimmer.
That is a great idea to try! I have to admit, that I didnít think on subbing the motor with a light bulb.
My motor theory went like this: as the motor warms up the dust infected old dry grease in the bearings gets a bit thinner and the motor is easier to turn. Cold motor works harder and higher current consumption. Warm motor turns easier and less current consumption. I am wondering once the motor is hot, is it being stable in the rpm department. As I understood it the motor is going faster slowly, but at some point itís got to stop speeding up. Plus my assumption that it doesnít slow down.
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Old 12-23-2022, 07:36 AM   #20
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Default Re: Carpet overlocker motor gradually increases speed by itself

If this motor controller is like the ones I have work on before one pot control adjust the lowest speed possible then there is one that either adjusts frequency or torque if had three pot controls not including the one on the outside of the enclosure would control the fastest speed

If it has three pot controls and one slow speed and one controls the fastest speed they are in proportion to each other in other words when you adjust one it affects the other adjustment

If it the type that I have described above the way you would adjust it is as follows turn the outside pot control to minimum control position and then adjust the slowest speed pot control then turn the outside pot control to maximum position then turn the fastest speed pot for the maximum speed needed to do the job then go back to the slowest speed and see if it functions correctly or not
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