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Small induction motor hesitancy

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Small induction motor hesitancy

Postby Eric the Viking » 17 Sep 2021, 10:13

No sniggering please, this is the motor on my "Dakota" wet grinder (AKA the "Tormekkalike").

It's been neglected for a while - just used/abused - so I've taken time to do some maintenance. Mostly concentrated on the main shaft - de-rusting, greasing the bearings, etc. but...

The motor isn't happy. On startup it stutters and causes the machine to vibrate (does this even under no load whatsoever). Left to its own devices, it does settle into smooth running usually, but sometimes not.

Its bearings are in ridiculously good nick - if off-load it runs on for ages after you cut power to it, but not always.

So might a faulty capacitor cause these symptoms? It's a very small induction motor, rated at 110W @ 240V. The capacitor is 4uf @400V (rectangular potted block), which I have no way of testing*.

The only other thing I can think of is a dodgy connection to the motor: it's on a "pendulum" mount - the shaft is horizontal and the motor hangs down from a parallel shaft so it can swing towards or away from the rubber tyred wheel that's on the main grinding wheel shaft (it's a friction drive). Imagine the idler drive of a record turntable, turned on its side. Because of this arrangement it's easy to run the motor under no load at all, and the vibration/stuttering is independent of the load the motor is under.

I'm just about to put an AVO on it to see if I can prompt a poor connection by wiggling leads - nothing visibly wrong from inspection. I'll post some pics later if that helps (or if I find the cause isn't the motor wiring!).

Ideas welcome.

E.

PS: If anybody can recommend good brands of motor cap. or ones to avoid please say. The current one has a date code of 2011, so probably doesn't owe me anything...

(*well I don't have a capacitance meter to do it)
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Re: Small induction motor hesitancy

Postby 9fingers » 17 Sep 2021, 10:55

Almost certainly a permanently connect capacitor motor or Split Phase" in the jargon.
These tend to have poor starting torque but from what you say, the mechanism is not likely to need much torque. I'd suggest trying a different capacitor. Nothing much to choose between brands really - they can all fail. Do check the new one will fit. Often that sort of value will be a block type rather than tubular.

Post a photo of the markings if you are in doubt.

hth
Bob
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Re: Small induction motor hesitancy

Postby Eric the Viking » 17 Sep 2021, 11:23

Thanks Bob.

I have the existing cap next to my elbow as I type:

4uf, 400V, "VIM" brand, with Lucar connectors and potted in a rectangular package around 1.5"x3/4" (1.25"tall). There's an unused 'ear' on the bottom edge of the pot for a mounting screw, but that doesn't matter.

It's not attached to the motor, just stuffed into the junction box inside the machine, so there's probably room for anything in a similar package.

The manufacturing date code is either 2009 or 2011, so it's probably at least a bit 'tired' by now.

I've had a careful look at the wiring and can't see any obvious reason for failure. There's what looks like high-temp sleeving over all four motor wires, and that's done so as to give some strain relief. Nothing looks scorched or cracked and the sleeving is still flexible.

I'll order a new cap and see how it goes.

Thanks again,

E.
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Re: Small induction motor hesitancy

Postby sunnybob » 17 Sep 2021, 11:35

As a mechanical problem rather than electric....
If the motor has been resting against the rubber wheel for a long period of time, its most likely put a dent in the rubber, causing the same juddering effect as a car tyre that has stood for so long its been flattened at the bottom.

I have the JET clone, and I saw this possible problem almost at once, so I fitted a long cable tie around the motor, cut a slot in the front of the machine, threaded the tie through and then put a small hole in the tie.
Now when I have finished using it, I just pull the tie through from the outside, which holds the motor clear of the rubber wheel, and slip a split pin through the hole in the tie to hold it there till next use.
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Re: Small induction motor hesitancy

Postby Eric the Viking » 17 Sep 2021, 12:00

Thanks Sunnybob, but it's been spotted here too: for years I have hooked a bit of galvanised fencing wire round the motor shaft, to pull it away from that wheel when it's not in use. I think the Jet oes are probably from the same factory - the stropping wheel and drive wheel arrangements look exactly the same as mine.

Also on the agenda is replacing the bearings on the main shaft. They're not terribly worn, but I can get a stainless version with rubber seals, so that would be ideal for the wet wheel end, which has some superficial rust around the outer race and isn't watertight. I'm tempted to leave it for now, as I've greased it well, so it won't rust for the time being.

There's also the issue that the main shaft is weirdly out of balance too. It's not obviously bent, but will roll to a specific spot if you just rest it on a flat surface. I can't see* how you could (easily) machine the two ends to not be concentric, but it looks as though they might have done that somehow. It could be re-made in stainless, but where does one stop with these things?

E.

*I used to drive a capstan lathe as a teenager (holiday job), with the bar fed through the headstock. I guess if the chuck or collet was seriously misaligned, AND the machining operations were done in one pass it might be possible to have the tweo ends not concentric with the shaft, but there are drilled centres at each end, implying that at least some operations were done separately. I don't have the means to put an indicator along it to see where the runout is, so I'll probably never find out what's wrong.
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Re: Small induction motor hesitancy

Postby Eric the Viking » 18 Sep 2021, 09:00

Quick update: I ordered a replacement motor run capacitor from Amazon (3-pack was cheapest and I have another similar small induction motor to refurb, too). That should be here later today.

It's the work of a few moments to install it and reassemble, I hope: the new one is not quite the same dimensions, but there should be ample space in the junction box, and it will have Lucar lugs, so I don't even have to re-make the wire terminations.

If that smooths out the motor behaviour it's a win, if not, more disassembly and a more careful look at the wiring will be needed to find any damage.

The motor is skeletal - very little protection for the coils, so it might easily have been dinged or scraped during manufacture. This is the second machine: the first unit's motor burned out in less than a week, and although it was sent promptly back, Rutlands were very tardy replacing it. In retrospect it might have been the run capacitor on that one, but I didn't diagnose it properly as I wanted a prompt replacement.

E.
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Re: Small induction motor hesitancy

Postby 9fingers » 18 Sep 2021, 09:14

You can do a rough comparison of capacitance at mains voltage by putting each in series with a filament light bulb. The brightness should be comparable. A reasonably powerful lamp will show differences better, if there are any, such as a 500 watt halogen work light.
I can't think of another electrical issue with a free turning motor that would fit your symptoms and improve with time other than a bad connection which presumably you checked first.
You did mention rust on the wheel spindle before. that and poor starting torque from this style of motor would/could be something that would improve with run time.

Good Luck
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Re: Small induction motor hesitancy

Postby Eric the Viking » 18 Sep 2021, 09:36

Advice much appreciated - I'll do that test when thespare arrives, as I haven't been through it with a meter, only a careful visual inspection.

There's also an annoying rocker switch, with one of those transparent rubberized covers. It doesn't operate with a positive click - there seems to be too much friction in it - and it might be making poor contact. I'll put a meter on that too, and intend to (silicone) grease it if I can get to the right bits, but I think I also have a spare of the same size that doesn't have the cover, so if I can't get it working properly I'll fit that instead.

E.
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Re: Small induction motor hesitancy

Postby 9fingers » 18 Sep 2021, 10:02

Eric the Viking wrote:Advice much appreciated - I'll do that test when thespare arrives, as I haven't been through it with a meter, only a careful visual inspection.

There's also an annoying rocker switch, with one of those transparent rubberized covers. It doesn't operate with a positive click - there seems to be too much friction in it - and it might be making poor contact. I'll put a meter on that too, and intend to (silicone) grease it if I can get to the right bits, but I think I also have a spare of the same size that doesn't have the cover, so if I can't get it working properly I'll fit that instead.

E.


Be careful, that cover Is quite important with water sloshing about. Almost better to have no switch and use the wall socket than risk water in switch problems.

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Re: Small induction motor hesitancy

Postby sunnybob » 18 Sep 2021, 10:40

That switch should have a solid click when operated with a firm prod of the finger.
I think youve just found another likely cause of erratic operation.
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Re: Small induction motor hesitancy

Postby Eric the Viking » 18 Sep 2021, 15:10

:text-goodpost: So it seems (just seen your post!).

I've just had an unhappy-half-hour disassembling the switch, and discovering one of the contacts is somewhat scorched, cleaning, greasing very slightly around the ballbearings, then p*****g about for ages trying to get rockers and BBs back where they belong. Grease helps with this as it sticks the BBs down fairly well.

I couldn't be bothered to go up and fetch my AVO, but using my old DMM there's now about 0.02 Ohms across one pair and 0.03 across the other (the scorched one was quite high resistance beforehand).

There's now a satisfying click without the rubber cover, soggy noise with it in place. But the panel hole is recessed to take the extra bezel round the switch (wot holds the rubber bit), and the switch floats about without that part. The trouble is that the BBs don't exert enough pressure on the pivoting contacts if the switch doesn't throw to the full amount, and the clear cover impedes that. I suspect that's why there's been excessive arcing - it's opening too slowly.

I think I'm going to trim the clear rubber away from around the actual switch, leaving the bezel in place (so that the switch isn't impeded by the cover), and just make sure my hands aren't dripping wet when I use it.

I'll save up for a gert big toggle switch instead.

You do get the feeling with a lot of this Chinese stuff that if they cut any more cost from it, there would only be air in the box when it was delivered.

And of course the new capacitor isn't exactly the same shape, so doesn't want to fit where the original did.

Ho hum...

PS: Bob I do take your point, but the case is well earthed and there is an RCD on the circuit too. And I seriously intend to swap it for a proper toggle switch with a rubber cover at the first opportunity (which will come from Farnell or RS, and hopefully not China!). You'll be pleased to know I was actually using "Radiospares" silicone grease - 'recommended for valve bases and turret tuners, etc.'. An almost full tube my dad left me...
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Re: Small induction motor hesitancy

Postby sunnybob » 18 Sep 2021, 15:42

I dont know if yours is exactly the same as mine, but the switch is on the back of the machine as you use it. I got so fed up reaching over the wheels that I now just turn it on and off at the wall. the motor is so small current wise that the wall socket will never get overstressed.
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Re: Small induction motor hesitancy

Postby Eric the Viking » 18 Sep 2021, 15:51

sunnybob wrote:I dont know if yours is exactly the same as mine, but the switch is on the back of the machine as you use it. I got so fed up reaching over the wheels that I now just turn it on and off at the wall. the motor is so small current wise that the wall socket will never get overstressed.

Yes it's just like that, and probably because I couldn't see the switch, just felt for it, I didn't really notice how reluctant to move it was becoming.
I probably can use the wall switch in due course, but it's a bit hard to get to. And, as I said I will fit a proper toggle switch as soon as practical, as this is just annoying beyond words.

I also just found out the new cap uses a smaller Lucar connector (wasn't obvious from the pictures). The old one seems to work fine though, and yes, I really should have checked the switch before sending off for another cap. I've been inside those switches before, so ought to know better <grrr>. At least I don't have to make off new ends on the leads...
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