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Bench grinder troubleshooting

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Speedy Builder503/02/2020 07:57:21
2104 forum posts
146 photos

Now pose the question. How did that wire burn out ? It is hardly likely the a grinder would stall? Re check the start contact switch and its mechanism. You have a new capacitor so that should be covered ok. Should be good to go after that.

Gene Pavlovsky03/02/2020 08:04:09
91 forum posts
77 photos

Builder5, I would like to know this, too. Perhaps it is not burnt out, but was defective from the beginning? Do you mean the centrigufal switch? I had disassembled it, and found no obvious issues with any of the parts. I wiped them all with a bit of light oil and put it back together. All the moving parts move fine by hand. How it works in actual operation is yet to be tested...

Kiwi Bloke03/02/2020 08:32:55
461 forum posts
1 photos

Great! Perseverance pays off!

If the connection was poor from the outset, it would have tended to run hot and oxidize - a vicious cycle until failure. Not an uncommon scenario - thinking in general, not just motors. To re-phrase what I said before, all connections are suspect until proven otherwise.

I wouldn't worry about temperature. Old stator windings would have been bound with linen tape. Modern, warm-running, totally-enclosed, fan-cooled motors can run with a case temperature around 60C. I think that exposing the insulating varnish to various solvents, resins and other chemicals is living dangerously. Epoxy should be safe enough, however. Beware silicone resins also - most evolve acetic acid during the curing process. So-called 'Neutral Cure' types are available, however.

Brian Morehen03/02/2020 09:11:46
124 forum posts
11 photos

Use some cable Ties to hold your repair . Looking at your pics I guess the windings are solid hbecause of the age they used to dip these in a type of shellac and then bake in a oven to dry . Most new motors today I have seen Are just wound and tied or laced together and are not shellaced . Sounds very good will now work ?. Motors today cannot be rewound because they weld the cases together . A throw away society


SillyOldDuffer03/02/2020 09:31:34
6313 forum posts
1382 photos
Posted by Gene Pavlovsky on 03/02/2020 08:04:09:

... Perhaps it is not burnt out, but was defective from the beginning? ...

That's my feeling Gene. Just a guess, but compared with the rest of the motor's wiring, that joint is exposed. Looks like it was soldered, so maybe you have a wire nicked when cut, then baked in flux, left in tension, and open to slow oxidisation for 20 years in storage. No problem passing inspection when new, but time worked on the flaw until it broke. Maybe if the motor had been used heat would have stopped the corrosion. Or maybe vibration would have broken the joint within months and the grinder would have been a warranty failure. We shall never know.

Sounds like you've found and fixed the fault - well done!


Circlip03/02/2020 09:46:39
1197 forum posts

" Most new motors today I have seen Are just wound and tied or laced together and are not shellacked ."

This was the problem with the original "Cheap" machinery imports from Taiwan, (and subsequently from China,- using the original manufacturing spec sheets). First job was to change the unimpregnated wound motors for a UK one.

Regards Ian.

Gene Pavlovsky03/02/2020 09:54:36
91 forum posts
77 photos

The connection of the green wire (the other end of the start wiring) also looks somewhat oxidized (as did the black wiring going to the main winding). Should I inspect the red (actually pink) wire connections? Should I resolder all the oxidized-looking connections to improve future reliability?

David Davies 803/02/2020 11:11:36
125 forum posts
9 photos


I have used Dow Corning 736 silicon sealant to secure winding tails after digging around for star points on 3 phase motors. This has a max temp of 315 degrees C.

As an aside I also used the same product to re-secure/seal the glass window of my kitchen oven door. It has lasted over 5 years with, sometimes, an inside temperature of over 220 degrees when baking bread.



Gene Pavlovsky03/02/2020 15:30:09
91 forum posts
77 photos

Couldn't find the DC product locally, but I ordered a sensor-safe (usually that means neutral cure) high-temp silicone by "K2". Last thing to decide whether to investigate the red/pink wire connections, and whether to try to re-solder all the other connections (that would require some active flux, RMA flux didn't work at all for the green wire joint). Or not mess with them and just cover everything with sealant?

Brian Morehen03/02/2020 18:46:48
124 forum posts
11 photos

Silicone may be O/K unless you need too remove same , Just brush a load of yacht varnish on your repair.

Has always been O/k when I have done similar repairs,


Brian Morehen03/02/2020 18:46:50
124 forum posts
11 photos

Silicone may be O/K unless you need too remove same , Just brush a load of yacht varnish on your repair.

Has always been O/k when I have done similar repairs,


Gene Pavlovsky03/02/2020 19:31:57
91 forum posts
77 photos

I've found automotive silicone comparatively easy to remove in the past. Unfortunately I don't have any yacht varnish, and it's not so cheap to buy either. I didn't think regular (non high-temp) varnishes would have significant temperature resistance?

I'm probably overthinking it - I bet with my (probably light) use the grinder would be fine with any kind of stuff, e.g. regular acrylic lacquer... But since I already ordered the silicone, that's what I'm gonna use.

John Paton 103/02/2020 22:30:34
281 forum posts
17 photos

Modern fittings with risk of high temperatures use glass braid sleeves. You can salvage these from all manner of items especially light fittings. I would be tempted to put lengths over the heatshrink just to make sure if you think the joint may come in contact with a bit that matters. The sleeping is normally white in colour or creamy coloured if it has got warm in service. Especially common on halogen light fittings.

Gene Pavlovsky02/03/2020 16:14:09
91 forum posts
77 photos

I finally had time to work again on the grinder (work, family, vacation)!

In the end I decided to replace all the wiring, as all of the joints looked somewhat oxidized and not that well made.

I've bought a length of SIHF-J 3G1.5 cable (180C-rated silicone insulation) and used the individual wires to replace the old ones. I decided against heat-shrink and instead used self-fusing F4 silicone tape which I had.

I used high-temp sensor-safe (supposedly neutral cure, although it did smell similar to the regular acetic cure one) automotive RTV silicone sealant, and then some thick-ish polyester thread to tie things up (hopefully it won't cause problems later).

While I was at it, I replaced the power switch with an electromagnetic switch (KJD17).

I didn't have the time to put the grinding wheels back on, but it starts and runs (and stops) fine now! I can hear the centrifugal switch during start and stop, it sounds different from what I imagined (from reading it described as clicking), but now I know how it should sound like

Thanks to everyone for your encouragement. At some point I was about to throw everything in the bin, and it's only due to positive messages I persevered, managed to track down the issue, and in the end get a working grinder. Although a quite dear one, by the amount of my hobby time invested







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