Interesting intermittent fault in universal motor

Thread Starter

jamesd256

Joined Sep 19, 2018
8
I have a Scheppach plunge saw.

If I pick it off the shelf and do a test spin, or a cut, it always fires up and works perfectly. If I then go for a second spin or cut, it practically always refuses to come on. After a delay ranging from a minute to 5 minutes, it will come on again, but normally only for one cut.

I have stripped it right down looking for a thermal cut out resistor, but there doesn't seem to be one. The only component visible is a safety capacitor (TNS-2TH) wired in parallel with the neutral mains input and microswitch.

Could it be a short in the windings? How do I check for this? As far as I can work out that capacitor should not stop it coming on at all, even if faulty.

The brushes look fine, and the switch is not exhibiting any faults testing the terminals.

Thanks for reading!
 

MaxHeadRoom

Joined Jul 18, 2013
19,057
Do a resistance check across the input conductors assuming it has no variable speed unit etc and the motor is directly powered.
Test continuity inside the unit at the switch output also.
Max.
 

ebp

Joined Feb 8, 2018
2,332
Sometimes small thermal cutouts are buried in the windings or at least wrapped under tape around the winding in the loop that extends from the stator. Usually such a thing would be connected between one of the brush holders and one end of the field winding. If you can get a clear look at the brush holder that isn't connected to the line cord or switch, try to see if there is any difference between the wire to it and the wire used for the field winding. Sometimes sleeving is used for insulation and you can slide it back just a little to see the wire.

If it is overheating so quickly, you aren't being mean to it and the bearings are OK, it may well mean there is a short in a winding.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
Thanks for the responses

Do a resistance check across the input conductors assuming it has no variable speed unit etc and the motor is directly powered.
Test continuity inside the unit at the switch output also.
Max.
There is no variable speed unit. The motor is directly powered. By input conductors, do you mean on the commutator? I'm not an engineer, so if you could explain in more detail?

I think I probably misunderstand you, but I have tested basic function of the switch. When pressed, resistance goes to 0, and I cannot find a fault. I guess you mean something else though, so apologies for my ignorance.

Sometimes small thermal cutouts are buried in the windings or at least wrapped under tape around the winding in the loop that extends from the stator. Usually such a thing would be connected between one of the brush holders and one end of the field winding. If you can get a clear look at the brush holder that isn't connected to the line cord or switch, try to see if there is any difference between the wire to it and the wire used for the field winding. Sometimes sleeving is used for insulation and you can slide it back just a little to see the wire.

If it is overheating so quickly, you aren't being mean to it and the bearings are OK, it may well mean there is a short in a winding.
The wire from the mains into the field windings on the stator is pretty hard to move/look, as it's buried in the stator in a nylon sleeve, which is impossible to move. Likewise exiting the field winding to the brush holder, and no thermal resistor visible.

The weird thing is, I can do a short spin, or a long cut through a whole 8x4 board, and on the first cut, it never cuts out, so logic tells me it's not a purely thermal cut out, because I've never had it happen during a powered spin. Odd.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
If it is a thermal sensor, then you should feel some overheat of the stator, unless the sensor itself is failing.
Max.
It's hard to say because I never felt for heat around the motor housing after a cut out. My instinct says it's not overheating, because it seems unrelated to the time of operation, and has never cut out during a cut even doing a 8' rip.

Does this saw have a soft start feature ? The line of thinking is that a heat problem in this part of the circuit may only effect starting.

Les.
There is no soft start. The capacitor I mentioned is the only visible component in the circuit. I could not determine from googling whether this is a start capacitor or a safety capacitor. Is it worth replacing this? They are a few £'s on eBay.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
Apparently applications for this capacitor are EMF input filtering and 'AC Motor Suppression'. I presume this is to decrease EMF and clean the supply to the motor. . It's a 275v 0.33uF. I'm more skeptical about replacing this helping now
 

LesJones

Joined Jan 8, 2017
2,325
Universal motors do not use a start capacitor. I did not understand your description of how the capacitor is connected. Can you trace out the schematic of the wiring so we can see how the capacitor is connected ? In post #4 you say you have tested the switch. Most power tools I have seen have a double pole switch. Did you test both poles ? I think you should do the test that Max suggested in post #2 by testing the resistance between the pins on the mains plug when it is in the fault condition and with the trigger pulled in. (Or connect an ammeter in series with the mains supply to see if it draws any current when you try to start it in the failed condition. One other slight possibility is a winding on the armature going open circuit when it is warm. If it stopped with this winding between the brushes it would not start. Turning the blade a small amount would move it to a position where the open circuit winding was not selected. This in not very likely as an open circuit winding would cause a lot of sparking on the comutator.

Les.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
Universal motors do not use a start capacitor. I did not understand your description of how the capacitor is connected. Can you trace out the schematic of the wiring so we can see how the capacitor is connected ? In post #4 you say you have tested the switch. Most power tools I have seen have a double pole switch. Did you test both poles ? I think you should do the test that Max suggested in post #2 by testing the resistance between the pins on the mains plug when it is in the fault condition and with the trigger pulled in. (Or connect an ammeter in series with the mains supply to see if it draws any current when you try to start it in the failed condition. One other slight possibility is a winding on the armature going open circuit when it is warm. If it stopped with this winding between the brushes it would not start. Turning the blade a small amount would move it to a position where the open circuit winding was not selected. This in not very likely as an open circuit winding would cause a lot of sparking on the comutator.

Les.
Thank you, now I understand the test requested by Max. It's 11 Ohms with the switch engaged while in the fault condition.

I tried moving the commutator along in case it's a particular winding, to no avail. But logically this seems unlikely as it would mean we're always landing on that winding after every first spin (unless I'm good at roulette). I also tested input conductor resistance at different blade positions and it is consistent.

Forgot to mention: after re-assembly it worked first time, and I got a slight 'new-motor' smell, second spin, it did actually cut out while engaged, which I don't think it's done before, so maybe I made things worse.

Edit 2: Well trying again after 5 minutes, this time letting the first spin last a good 30 seconds, and for the first time since I've owned the saw, it came back on. It also keeps coming back on for numerous tests! Seems to be fixed, just from a tear down :)
 
Last edited:

Thread Starter

jamesd256

Joined Sep 19, 2018
8
Incidentally did you purchase this saw new? If so how long has it been used to date?
Max.
I bought it on eBay listed as new. It normally goes for £150, but it was listed as £60 including all the rails and other bits. I suspect it was a return. I've used it for a few jobs but always had to wait for it to start working again - super frustrating.

Final update. So it exhibited the fault again. After being tested sitting still over and over without fault, I moved it down to the track, and it did not come on. So I pulled the trigger and kind of moved around in the air and it kicked in. After many more tests, when it cuts out, I just have to move it around in the air and it comes to life. It's definitely not overheating.
 

MaxHeadRoom

Joined Jul 18, 2013
19,057
If it measures 11 Ohms with the switch engaged while in the fault condition. I would expect to see or hear something!
Sounds like it may take a little more in depth trouble shooting, such as eliminating the power cord and switch all together and feed the motor direct somehow after a partial strip down.
Max.
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
If it measures 11 Ohms with the switch engaged while in the fault condition. I would expect to see or hear something!
Sounds like it may take a little more in depth trouble shooting, such as eliminating the power cord and switch all together and feed the motor direct somehow after a partial strip down.
Max.
I see thanks. Doesn't that fit with the fact that it does come on with a bit of a jog?

I may continue with this. Is it safe to run the motor with no load? It would be easier to test with the blade and plunge housing removed to investigate further. For now it works. Kind of.
 

Ylli

Joined Nov 13, 2015
776
You got 11 ohms when it was in the fault state. Can you try to get a reading when it is in a state that you believe will be functional?

Are the brushes free to move in their holders?
 

Thread Starter

jamesd256

Joined Sep 19, 2018
8
The motor is ok with no load.
If it comes on with a sharp movement that points to intermittent connection somewhere.
Max.
It's not a sharp movement. I rotate the saw in the air through a few planes. It starts to bite then comes on continuously. Once it's running, further movement does not really interrupt it very easily, but it does cut out if you wave it around. I think maybe vibration is involved, and as you say there is an intermittent wiring issue.

You got 11 ohms when it was in the fault state. Can you try to get a reading when it is in a state that you believe will be functional?

Are the brushes free to move in their holders?
I clamped the unit and cable to the bench, spun it, measured input conductor resistance, plugged back in and confirmed it spun again, and 11 ohms again.

I stripped down the whole motor to the brushes, stator and commutator, and it is all running freely.
 

MaxHeadRoom

Joined Jul 18, 2013
19,057
It's not a sharp movement. I rotate the saw in the air through a few planes. It starts to bite then comes on continuously. Once it's running, further movement does not really interrupt it very easily, but it does cut out if you wave it around. I think maybe vibration is involved, and as you say there is an intermittent wiring issue.
.
Definitely points to a intermittent cord?
At least the first item to replace.
I have picked up heavy duty power cords when they are on sale for this very reason, they include the molded power plug also.
Max.
 
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