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Mill capacitor centrifugal switch issue?

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My new mill is down. So while I'm waiting for Mr. Technician to arrive, I figured I'd post & solicit some advice. It was running fine & then it seemed to me the motor started making a noise that didn't sound right. Then it tripped the breaker. The motor noise didn't really sound 'mechanical' like grinding or friction, but I'm no expert & maybe this was wishful thinking. Anyway, no heavy load or anything like that. Maybe a couple hours of cumulative running time. A few repeat tries of this startup with & no progress. If I start it in neutral it ran a tad longer (but always rough sounding) then trips breaker. If I put it into gear it trips breaker almost immediately. The motor is a 240V single phase 3 hp.

Tech visit number #1. <insert self ass-kick here> The first thing he notices is my panel breaker is 15A & the motor is 20A. DOH! Nowhere in my purchasing discussions did the topic of available amp duty come up. Its not even specified in the brochure or Taiwan manual. Now I am smarter about such worldly matters & will never make this error again. I will ask for motor plate specs up front. Anyway, I'm told start-up current can be higher than running current, so this needed to be dealt with first.

So I just finished installation of new 40A breaker on my panel, ran a dedicated AC 8/3 ( 8 gauge 40A) armored cable direct line & 240V 40A outlet to my mill area & new range style plug on the mill. Now I can use this outlet or at least this upgraded line for a TIG welder one day. Measure 240V at the panel, 240V at the plug, so far so good. I plugged the mill in, flicked the switch on for 2 seconds, it started immediately. But to my ear the exact same kind of ratchety sound. I started again, ran it for about maybe 5 seconds and then BANG. Pretty sure the capacitor blew because I think that's what lives in the little cover pod on the motor side & where lots of magic smoke came from. Well, crap!

This got me Googling motor capacitors & related problems. I found this article & similar ones that suggest if the centrifugal switch is faulty, then it can overload the capacitors. https://cdn0.grizzly.com/manuals/g2529_m.pdf
Maybe the capacitor was bad or incorrect spec. We will never know because it surely must be shrapnel. Do you think its possible that the centrifugal switch migrated or slipped out of position & was the main culprit all along? Do you think my low 15A breaker configuration was tripping early & prolonging the inevitable when it finally had full 40A & went pop?

I cant see any detail in my 'abbreviated' user manual for the motor itself. Do you think my centrifugal switch / related hardware might be on top of the motor can where it could be readily examined & replaced without hauling that big muther motor off? Anything else you can think of that could explain this situation? Can windings & such be diagnosed on the machine?


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Could it be that the motor is wired for 120V and you're feeding it 240?

Or maybe the wrong value capacitor was installed at the factory.
Well, I guess open the capacitor box up and see what's in there. Maybe you can still read the values. I think the voltage rating should be 370V for a 240V system
The Jin Shin motors are made in Taiwan and yours has a production date of 2018, so should be more reliable given it is essentially new. Unlikely to have the start switch fail at this point, but you can take the motor apart and inspect the points to see if they are burnt or sticking. If it is a newer machine it should be covered under warranty. Take off the motor cover and the capacitors should be labeled if still intact or refer to the manual which should give you the information.

Please note that one of the limitations of single phase motors is that the start capacitor will go quickly with frequent start stops, as the electrolytic capacitors will overheat quickly. It is unclear if the switch might have been damaged from a wrong size breaker if it did not trip immediately with start up. If the motor does not come up to full operating RPM quickly due to low voltage, the starter switch would not disengage and the start capacitor would fail. The capacitors must be AC rated and the same or higher voltage value with a uF rating of probably +/-20%. They should be available from the manufacturer, size is often an issue of finding replacements that will fit under the cover.
Sorry with the late pics boys, I was doing laundry. Had to get the brown stain out of my pants LOL. Here are the caps under the right hand cover.
I'm not electrically savvy but is W.V. 125 VAC on the left hand (blown = starter) capacitor incorrect voltage? The right hand one is 250 VAC.
I'm not even going to comment on the blue electrician tape.


Date codes all attest to this being a new motor - parts are only now a year old. I'd say it's time to contact the dealer, rep, company that sold this to you.

Just food for thought: Installing a bigger breaker where a suspected problem is only lets more smoke out & allows for more damage.....
Would you concur that the W.V. 125 VAC labelled on the blown = starter capacitor is incorrect / insufficient voltage? If its a 240V motor, I am just assuming both capacitors should be 240+V right?

Brockwood, I think the job of the panel breakers is to protect the wiring under a short circuit. A motor will draw what it will draw. A 15A motor in a 50A outlet will just draw 15A. But a 20A continuous motor that draws say 30A on startup inrush will pop a 15A breaker. There is another level of protection on the mill itself that has to get installed before its CSA certified.


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Since the capacitor is in series with the winding, the theoretical voltage across it might be in the 120V range at Asian 220V power, but at the 240+ volts that we have here in North America it might be a bit light. So if that cap was marginal to start with, I could see it failing.
So you're saying this particular capacitor its taking a single line ?phase? off 220, so 220/2 = 110V in Taiwan. It is rated 125V so has 15V headroom.
But I have 240V, 240/2=120. So 125-120=5V headroom. But if it was lower spec to begin with or degrading, then that's why it bent boom?
I'm still confused, then why the recommended McMaster 250V capacitor?
Both the starter and ruin capacitprs should have a nominal voltage rating about double the intended - 250V for a 120V circuit, and 450V for a 24V circuit - this is pretty much standard in a Blador or other NA made motor...
Ya, I don't know if this generic info corresponds to my configuration or not, but it kind of infers same voltage both caps, no?

I'm sure Technician will know right off. I'm just biding my time.... over Christmas & New Year break..... chewing my knuckles in chip making anticipation...
Why cant I type my motor manufacturer specs into Internetland & get the dang schematic?


A voltage of 125VAC is not unusual for a start capacitor (which is blown), the start coil only see's 1/2 the voltage. The issue is often one of size, a 250VAC capacitor would be significantly larger. The run capacitor is usually oil filled and a rating of 250VAC is acceptable but marginal. Looking up a comparable Baldor motor 2 Hp 120/240VAC they spec a 125VAC electrolytic 850-1020 uF start cap and a 370VAC 15 uF oil run cap. The size "uF" of the caps is motor specific. I would be surprised if you could fit any higher volt caps under the metal cover, but I would try to buy higher quality motor capacitors. You run capacitor is a MPP (Metallized Polypropylene Capacitor) solid type which are often used for run capacitors, although the oil filled ones tend to be more durable. Once again, an oil filled 60 uF 370 VAC capacitor would be much bigger. Yo will often see much larger capacitor covers on some motors like Baldor's.

Since this is not an old motor, and the capacitor dates are recent, I do not feel it is an issue of deterioration seen with electrolytic's that sit unused for years. It is also possible that you have a faulty start switch, it doesn't take more than a few seconds for start capacitor to blow if the switch doesn't kick out, so it also needs to be checked.

I pulled the cover of my 1HP Baldor motor -the only one easily reached - and the start capacitor is rated 300V and the run capacitor is rated 450V. This is, however a 30 year old TEFC motor, so more current designs (pun intended) will differ.
It is true that most dual voltage US spec motors only subject the start leg to half the supply voltage when running on 240 volts ac, so a 125 volt start cap will survive- for what the mfg. considers an acceptable lifetime; a few years, maybe less.
And yes, it's often a size/voltage issue which sets a limit of capacitors you can stuff in there- unless you mount it outside the cover(s).
Unless the centrifugal switch is stuck or broken, most likely the cap failure is just an infant mortality- it happens
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it is generally a (UK) rule that any start/run capacitor should have a working voltage (Wkg) of double the supply voltage, so for a 220/240v supply, the Wkg should be 450/500v. 125 is NOT enough! the capacitor will have to deal with transient supply spikes of many times supply voltage, and the big bang is the classic failure mode of an under rated capacitor!
The placard does not indicate the motor is split phase so I would assume a standard single phase motor. The start cap will be switched in parallel with the run cap. Parallel capacitors add the capacitance but the voltage across each capacitor will still be 240V not 120V.

125V capacitor is not enough for a start cap, when I had my single phase lathe both start and run caps were 400v. My lathe used to eat start caps and had the motor replaced 3 times under warranty. I got so sick of it blowing I removed the start cap altogether, the lathe was not starting under load so the run cap was enough to get the motor going albeit a little slower.

I would also have the tech replace the start switch, if this sticks it WILL kill the start cap in less than 2 seconds.
It is unclear as to the wiring of the capacitors in the motor circuit, so I would not assume that the capacitors are in parallel. Given that Jin Shin is a major Taiwanese motor manufacturer, I would not second guess their specifications for the capacitors. As I noted, Baldor (and a number of other motor manufactures) spec a 125VAC capacitor for the auxiliary start wiring as shown above. When the capacitors are run in parallel the Baldor motors spec a 250VAC electrolytic and a 370 VAC oil capacitors. The assumption is the start capacitor is in the circuit for a very short period of time, which may not have been the case in this situation. There is also a percentage of initial failure. Typically I see a much higher capacitor failure rate of the Chinese single phase motors, but rarely the Taiwanese motors. The Jin Shin motors are used in many of the PM mills and SB/Grizzly mills, and I haven't heard of any single phase motor failures. Given that the wiring connections to the capacitors do not look like something from the factory, it is possible that they may not be the original caps but they are both of Taiwanese origin which suggests they may be OEM? So some further details as to the brand of mill and if it was purchased new or used might shed some light on the discussion.

If you look at HVAC/Compressor motor start electrolytic caps rated at 400 uF 250 VAC you are looking at something in the 4 3/8 in x 2 1/16 can size, a 60uf 370VAC oil motor run would be a similar size. My compressor motor has a large compartment for these size of capacitors, but I have not seen this on mill/lathe single phase motors. I would start by contacting the mill manufacturer to get replacements capacitors/verify specs, and also checking the start switch.
Thanks. Its a very new model 935-VS1 mill of Taiwan origin. Very similar to a PM-935 but distributed & serviced through a large machinery company here in Canada where I live & they have handled the line for some time. It has maybe an hour of actual run time & still under warranty. The technician has been booked for a service call in the new year. I thought it would be beneficial to post in the meantime in case there was some similar experience or diagnosis. Thanks for the constructive comments. Hopefully this will be resolved soon & I will be sure to report back.
I would also have the tech replace the start switch, if this sticks it WILL kill the start cap in less than 2 seconds.
Now do you mean the main Fwd/Off/Rev switch box (upper left) or the centrifugal switch inside the motor can?


The only switch that will kill your start capacitor is the centrifugal switch. It is almost always located near the armature.
Now do you mean the main Fwd/Off/Rev switch box (upper left) or the centrifugal switch inside the motor can?
No that is the power switch. The capacitor switch is inside the motor and disengages when the motor is at a preset RPM.
I recognize the Grizzly G0796 is a completely different mill to mine so apples & oranges motor wise too. I'm envious of their parts detail which strangely I still cant seem to locate for my motor despite it 'appears' to be a well known industrial name. What I was wondering out loud, is it reasonable to expect the centrifugal switch assembly to be located deeper in the motor, like beneath the rotor assembly - versus on the top side with the fan cover off?

I'm reading some not-so-good articles about start circuit windings potentially being damaged as a result of a failed centrifugal switch. Can the windings be tested in-situ for that? I guess all this will be revealed once the pro arrives, but just wondering out loud.


The switch is within the motor case- replace the start cap and see if that takes care of the problem. If not the warranty should cover motor repair or replacement
Success. At least so far so good. The starting capacitor was replaced with the exact same model / part number / spec. All the wiring & connections were checked over from outlet plug through CSA box. Centrifugal switch is located near top of motor by removing cover plate, then plastic fan, its under there. Contacts looked pristine, open/close conductivity checked, wiring terminal nice & tight. De-gunked the slight leakage form blown cap. The blue tape job over the wire was just wrapped over a legitimate connector plug underneath, so wasn't a bad thing after all.

Flicked the switch & away she ran. No more gravely, cogging sound. Ran it for several minutes through speed ranges, repeated on/off & fwd/rev. Yay! So not sure if it was a dud capacitor to begin with and/or or my under-amped breaker flicking off was somehow aggravating it. But I hope this is behind me now.

Technician provided a tip that on these kinds of motors you should not flick the switch on & off quickly. Not that can think of a good reason to ever do this but the point is to allow the centrifugal switch do its job uninterrupted. That's the audible click you hear shortly after starting, it happens in a blink if everything is working properly. Another comment was look for any sign of corrosion on things like contacts & terminals. Sometimes between storage & the ocean journey over this can happen.

Thanks to all who helped. I look forward to more machining & less diagnosis & remediation!
MY 20 cents worth.
Your motor, 2.2kw @ 220, about 10 amps.
A circuit breaker is to protect your wiring, cabling in the building. A normal domestic C curve circuit breaker...well read this....

If you wish for motor protection, you need to provide that for each individual motor., which also, accounts for start up current x time automatically. either thermal or electronic. If your mcb was still 15A, I wouldn't worry about it. But with the 40 Amp bugger, you have no motor protection at all.
My understanding is the yellow reset switch box thingy shown in post #9 is the requisite CSA combination thermal / current overload protection that the re-seller must install to make it compliant. CSA = Canadian Standards Association (electrical). Beyond that I'm not sure how it works internally but I did notice it was also clicking off when the 15A breakers tripped. So I think that box prevents the scenario you are mentioning where the motor could draw the full line amps.

As mentioned, a 15A main panel breaker does me no good if the 20A FLA motor draws say 25 or 30A even for a brief instant at the beginning of every start. Either I was just on the bubble of it even allowing a start, or it had entirely to do with capacitor, or the issues were kind of aggravating one another I cant say. If I'm off base on any of this let me know, that's the purpose of sharing & learning.
An MCB has a time delay curve for any given current. The normal start up time is what, <1 second at possibly a tad > 30 amps, on a 15amp MCB, should be OK.
Technical, but explains what is going on.
The full load current of your motor when working will maybe approach 15 amps, but would need to be stalled to go above it.
But look, if you are game, leave the 40amp MCB in there. It is no skin of my nose.
This all about what I'd do. An of course, I have never made a mistake in my life. ( Typing with stub of 1 finger)
Thing is, it seems you are up and running. So start making some some chips mate.
And Yes, like all the safety blokes, glasses condoms, gloves and everything. Pain in the bloody arse, I reckon. But shoot, a parachute would be nice at 40,000' when the plane blows up?
I wish I understood electrons better but I don't. Now that its up & running running, I have a clamp meter I believe can be set to record maximum as well as continuous. I'll flip the switch & convey what it reads.
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