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View Full Version : Ahh, so this is where the magic smoke comes from!



DICKEYBIRD
04-07-2019, 11:34 PM
Buddy up the street comes down with a crude pencil drawing of a couple aluminum mounts for some automated machine he's working on at the plant. No sweat says me, I have a stick of 2"x3" 6061 on hand & I'll whip 'em out for you in no time (for the usual weekend special price of course.)

About a 1/4 way through the 1st cut on my chop saw, it started smoking, I let off the trigger & it kept going with a crazed mind of its own. By the time I yanked the plug, the whole shop was full of very stinky magic smoke. I tore it down & the motor field windings are toast. I don't see any other damage at all. I wonder what happened. I know it's a wood cutting saw but I don't see any chips shorting any thing out. I guess it'll have to be scrapped since parts for chinese saws are probably non existent.

Hmmm, it's got a 5 1/2 to one gearbox on it (planetary I think). I wonder if a treadmill motor could be grafted on to slow it down & have a poor man's cool (not cold) saw?

https://oi57.photobucket.com/albums/g227/DBAviation/Chop%20saw%20field_zpsrtjirjp8.jpg (http://s57.photobucket.com/user/DBAviation/media/Chop%20saw%20field_zpsrtjirjp8.jpg.html)

lakeside53
04-08-2019, 12:07 AM
Yep... burnt up the field winding. Did you bog it down with a dull blade?

Happens... I had a high end Dewalt compound mitre saw that I was given like that, except the armature was burnt up too. $180 in parts and like new, but I just "had" to fix it ;)

J Tiers
04-08-2019, 12:41 AM
Interesting that the field has considerably thinner wire than the armature. It is very obvious in the picture. Yet, in general, the same current passes through both. Perhaps the coils are in parallel, which would have 1/4 the dissipation in each.

The coil is also only half-burnt, with a section well toasted, and the rest appearing nearly untouched. For one of the coils, the center part is burnt, which one can understand, as tt would have less heat sinking, but the other coil is burnt in a different pattern.

DICKEYBIRD
04-08-2019, 01:12 AM
Yep... burnt up the field winding. Did you bog it down with a dull bladeNo, it had a fresh blade made for cutting aluminum & I was cutting at a steady rate without a lot of pressure...just letting the blade do the work. I'm guessing the field windings fretted through the varnish & then,POOF! It was given to me about 4 years ago so it doesn't owe me anything. I'd like to find another one of the same size so it'll bolt-on to the custom sliding mount I built for it. It goes into a cramped spot & stores out of the way but slides out for use easily.

DICKEYBIRD
04-08-2019, 07:38 AM
Thinking a bit more about the failure, I think the main power section of the switch stuck "on". The saw has a dynamic brake circuit which has contacts in the switch that shorts out the field during coast-down, yes?. With power still being applied and the brake circuit kicked in, POOF!

RB211
04-08-2019, 07:58 AM
I bet someone sells replacement parts, if it is a name brand.

vpt
04-08-2019, 08:49 AM
Whats the silver wire wrapped around the rotor wires by the commutator?

Glug
04-08-2019, 09:10 AM
Thinking a bit more about the failure, I think the main power section of the switch stuck "on". With power still being applied and the brake circuit kicked in, POOF!

Clumsy bastard.

Interesting failure mode. I think most of us would have reacted similarly, and wouldn't have thought to keep the trigger pressed while trying to unplug the skitzo tool. For some reason, I think a switch failure causing this would piss me off more than the outright failure of a cheap winding.

J Tiers
04-08-2019, 09:16 AM
Whats the silver wire wrapped around the rotor wires by the commutator?

String.

It is there to help keep the wires and commutator from flying out at high RPM. Many have several times that much wrapping.

Doozer
04-08-2019, 09:18 AM
Why are you guys so fascinated with treadmill motors????

-D

DICKEYBIRD
04-08-2019, 10:48 AM
Why are you guys so fascinated with treadmill motors????Speaking for myself, because I have one, they're (relatively) light & powerful, easily speed-controlled & in this case run much slower than the original motor (intended use: metal cutting) and the most important reason, CHEAP.:)

MaxHeadRoom
04-08-2019, 10:51 AM
Whats the silver wire wrapped around the rotor wires by the commutator?

Not typically wire but strong thread and varnished.
I always keep an eye out for local T.M.'s that in some cases are offered free as users often want them gone if they quit.
Max.

chipmaker4130
04-08-2019, 11:44 AM
. . .I don't see any chips shorting any thing out. . .

Chinese motor magic smoke is encapsulated at extremely high pressure and contains a catalyst that increases that pressure by the day, sometimes by the hour. Given time it will burst the containment element regardless of what you or anyone else does.

andywander
04-08-2019, 12:14 PM
Thinking a bit more about the failure, I think the main power section of the switch stuck "on". The saw has a dynamic brake circuit which has contacts in the switch that shorts out the field during coast-down, yes?. With power still being applied and the brake circuit kicked in, POOF!

If the field winding was shorted out as a brake, with AC power still applied, the current from the supplywould flow through the short, and likely blow a breaker.

I don't think it would affect the field winding at all.

loose nut
04-08-2019, 02:43 PM
If you had a failure that let all the smoke out then cam't you just buy a bottle of Liquid Smoke and top it up?

Corbettprime
04-08-2019, 04:39 PM
Gee, all the stores here only carry hickory and mesquite, no electric or chineseium.

Jim Stewart
04-08-2019, 05:06 PM
Is the motor British?
http://www3.telus.net/bc_triumph_registry/smoke.htm

-js

PStechPaul
04-08-2019, 06:37 PM
It looks like the field might not be very hard to rewind. The armature looks like it's OK.

Corbettprime
04-08-2019, 06:42 PM
On the serious side, does anyone use an electric motor rebuilder?

vpt
04-08-2019, 07:01 PM
It looks like the field might not be very hard to rewind. The armature looks like it's OK.

Sure does look simple. I rewound a motor not long ago and it came out fine. This one looks dead simple, I'd give it a go.

J Tiers
04-08-2019, 07:10 PM
On the serious side, does anyone use an electric motor rebuilder?

Not for something like this.... Rebuilding is very expensive, and usually the shop will not even take in a motor under several HP, unless it is a special that is NLA and you need exactly that configuration.

For that motor, if you count turns, use the same size wire, and do a reasonably careful job, you should be able to get it working, assuming you get the switch problem taken care of.

kf2qd
04-08-2019, 10:38 PM
Field coils like that are really easy to rewind. Unwind ist and count the turns on each half, and whether it is on winding or 2 in prrallel. Find a motor rebuild shop and get a length of the same size wire and rewind. Have done it for a table saw that I killed. Went on for many more years until I killed the the rotor.

J Tiers
04-08-2019, 10:49 PM
PAY ATTENTION TO DIRECTION THE WINDING WAS WOUND IN and duplicate when you rewind..... that sets relative magnetic polarity, and that has to be right or you will be losing smoke again. Or spinning backwards.

DICKEYBIRD
04-10-2019, 09:55 AM
Update: Rewinding the field was out of the question with all the pressing projects I have going on so I ordered one of these from Amazon: https://www.evolutionpowertools.com/us/build/mitresaws/r255sms/ Amazon is scary good. I ordered Monday afternoon & it arrived Tuesday around noon!:eek: I guess they have a warehouse around here somewhere. Home Depot shows it for the same price ($205.04) but I was too lazy to drive over & get it there.

It looks pretty darn good for the price & comes with a steel cutting blade. It runs @ 2500 rpm which is great & should be great for my needs. I'll report back after making a few cuts.