935 centrifugal switch

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petertha

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Have any of you had to adjust the centrifugal switch on your 935 mill? For example, alter its position if its somehow migrated, or adjust spring tension, or anything else that might be impeding how its supposed to switch? Looks I am having what I suspect is the same start capacitor issue. I have already replaced one. Now maybe after another 300 starts since the new one the motor is making a cogging, almost grinding like sound on initial start. I know this sound all too well, exactly like before. No heavy operations, just had a drill chuck mounted when it happened again. After a few tries in high, low, neutral with same sound, the electrical protection box kicked off power to the motor because it must have saw high current or something it didn't like. Now maybe this cap was also defective & came from the same batch, TBD. Seems low probability though. Please note this is not a PM mill, its a 'very similar' model 935 VS (very new). Was hoping to draw on any knowledge from PM owners, but I will post elsewhere if its deemed NA.

I found this vid that got me wondering if the underlying problem might be the centrifugal switch & not the cap. The motor is obviously not a mill but around 3:20 when I see what the centrifugal switches generically look like, makes me wonder. There are springs & tension washer & components held in place with screws etc. My switch (and everything else) was checked out the first capacitor episode & concluded it was activating freely, no burned points, electrically isolated open & close position. It looked pristine, but that's not really the same functioning under operating conditions... which I'm not even sure how one would test.

There is no mechanical drag on the motor but is there something we should be checking electrically in that regard? Getting very frustrated.

 
If the capacitor is going bad, the motor will not spool up and disconnect the points and draw heavy current and dump the breaker.
If the points are welded together and the cap is good, the motor will spool up but continue to draw heavy current and dump the circuit breaker.

The simplest way would be to take a look at the points in the motor first and see that they are not welded together and in good condition.
If that is the case, the capacitor is probably defective. If you have to replace the capacitor, get a good quality one. Testing with
a Volt-Ohm-Milliameter will detect open or short circuits but not detect the overall quality of the capacitor.
 
Thanks, here is the post of episode-1 .... which I was hoping was permanently behind me.
https://www.hobby-machinist.com/threads/mill-capacitor-centrifugal-switch-issue.75557/

The consensus at the time aside from wiring & panel upgrade was a reputable & common JinShin motor & correct factory capacitors. Everything is stock & low run time. So all was running fine & looking good until episode-2 recently. I guess I'll know the story on the capacitor next week, but my bet is that it will turn out to be degraded or already dead.

This might be getting ahead a bit, but say I found a 'better' capacitor to just try. I am assuming it must be behind a cover like pic? ie thats th epurpose of the cover - to protect from flying shrapnel if caps go bang? My understanding is there may be caps of similar or better spec but longer/fatter than the form factor constrained by the cover dimensions. Its still under warranty so I guess we can go through a few more but I'm concerned its some other issue that is taxing the satrt capacitor to begin with & 300 starts from now playing the same violin.

SNAG-3-16-2019 0002.jpg
 
The box is basically a protective cover for the capacitor . I have seen them leak but not explode.
I'm thinking capacitor at this point too...
 
Not uncommon to find a brand new capacitor bad right out of the box, it's certainly possible. Very rare you would see that when motors (and caps) were made here. Now, anything goes- out
Another thing to check along the same lines is that the centrifugal switch isn't sticking shut and not disengaging- poor tolerance in machining the sliding parts of the mechanism, or weak plastic parts cracked and binding could cause it
Mark
ps if you can go higher on the voltage rating the capacitor would last longer- unfortunately the space under the cover is usually a limiting factor- higher voltage rating means larger diameter or length or both on motor starting caps. Also, mounting the cap away from the motor's heat will lengthen it's life.
 
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Also, no reason you couldn't mount the capacitor(s) on the wall behind the machine, with extending wires. Easier access, cooler, and no limits on size or voltage rating
 
That's good to know, even if for a temporary experiment.

Just wondering out loud to any PM-935 owners, is your motor same as mine? Is your start cap also 125VAC (one on the left blown from episode-1)?
I know others have chimed in that their Bridgeport or whatever is 250v start & run & that's my issue. MKSJ commented on this issue (post #14) saying 'it depends' 125V may not necessarily be incorrect. But now I'm curious & why I'm asking specifically about a PM-935.
https://www.hobby-machinist.com/threads/mill-capacitor-centrifugal-switch-issue.75557/

I'm told this cap/motor combo has been the exact same on my particular vendor machine for past decade with practically zero issues. Apparently mine likes to eat them though. Or considering the replacement cap came off its sister machine landed same time possibly 2 duds from a faulty batch. Anything is possible I suppose. That's what makes me think replacing centrifugal switch might be worth it. Ideally one make one change at a time to know who the bad actor was but right now I just want to get back to machining & I might not see a repeat issue until 300 starts from now anyways.
Grrr...

IMG_7807_edited-1.jpg


mill motor specs.jpg
 
The 125 volt rating is intentional since the cap is supposed to have only 120 volt imposed on it even if the machine is wired for 240.
But it's a weak point in the design, very little safety factor
 
(I'm not electrically savvy if it wasn't already obvious LOL).
You are saying if I got a different cap, same capacitance rating but higher voltage rating it would give me more head room? Is there any downside purely from an electrical standpoint? You already mentioned the likely different physical size, but if I found a solution around that, worth a try?
 
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