Posted on 02/12/2019 8:01:16 AM PST by Paul R.
I've been all over the Internet and am having trouble finding definitive answers regarding a couple motor capacitors.
The first cap (capacitor) is in a PSC (Permanent Split Capacitor) motor, and is rated @ 7.5 uF, 370 VAC. Researching this one was a bit of a "journey", and I believe I've answered most of my questions correctly -- I hope!
Basically, it appears to me that the specs for a cap in a PSC motor are similar to those in an electric motor that uses a "run" capacitor, since in both cases the cap is in use anytime the motor is running. The PSC design eliminates the need to have a "start" capacitor which gets switched in for starting, and out for running.
Further research tells me that the PSC cap is usually a 370 VAC (or more) oil filled type, or a polyester (Mylar) cap, to meet the continuous duty @ line voltage requirement.
So... I have a PSC motor in a sump pump and the motor capacitor has evidently failed, leaving a pool of light oil (much akin to Castor Oil) in the motor. So far, this all makes sense. However, initially the capacitor seems to have only suffered a modest drop in value, from its rated 7.5 uF, to about 6.6 uF. DF (dissipation factor) was approx. 0.015. (I measured these values using a very reliable-for-me inductance-capacitance meter, and I confirmed the capacitance using a Fluke 8060A VOM using a couple different resistors to vary the "discharge load" from that of the meter itself.
Note however that I did not try to do any measurements at "working voltage" - a slightly more involved endeavor...
Anyway, would a drop from 7.5 uF to 6.6 uF be enough to keep a PSC motor from starting at all?
A couple days later (this evening) I re-measured, and got 5.4 uF, and 0.019 for DF. Hmmm... An unstable capacitor? THIS capacitance value I can believe might prevent the motor from starting. Is it possible the capacitor is "drying out" further, now that it is not in a sealed pump housing?
I cannot find an exact replacement for this capacitor online. The closest I can find is a considerably larger polyester cap of the same ratings. I believe it will just barely fit. If so, that's ok with me, as I have to believe the beefier polyester cap would be much more reliable. (Self-healing, no oil to vent out...)
My remaining question on this one is how is the old capacitor so small? (1.75" x 1.4" x 0.94"). I have a number of oil filled caps around, and I checked various others online -- it seem as though for this rating it should be about twice as large as it is. Has the technology of making oil filled capacitors improved greatly in recent years? Or did the mfgr push the internal layers' thicknesses too small, in order to save on materials & final cost? (A sump pump is a hell of a place to skimp on something like that!)
I also found some "oil filled" polypropylene capacitors - something I'd not heard of before. Is this a new type? In any event, the ones I found are even bigger than the polyester caps, and won't fit.
The rest of the motor & pump itself looks great, spins smoothly, etc. -- like it could go on functioning for many years. The pump itself - made by "Flotec" - uses a very interesting electronic sensor (as opposed to a float switch) to detect water level, and ultimately set off an alarm if need be. There's nothing to wear out except the cap, possibly small caps in the control "brain", the bearings, and the impeller itself (which looks great after several years of use.)
---
The second motor is in a small electric powered brush & leaf chipper. I assume the capacitor in question here must be a start capacitor, as it is only rated at 200v (no "AC" indicated, although it HAS to be a non-polar cap.) The capacitance is 45 uF, and that's where I am a little puzzled, as most start capacitors are over 100 uF. The 45 uF caps I am finding online are RUN capacitors, with at least 370 VAC ratings, and are considerably larger dimensions than this cap. The capacitor is housed externally to the motor: Given that the cover for the old cap matches the old cap's size, this is a bit problematic! I suppose I could bite the bullet (cost) and buy a polypropylene or polyester capacitor*, and find or finagle / fabricate some sort of larger cover for it.
*I did find electrolytics and oil filled types, but they were as or more expensive than the PP caps, and the oil filled were, if anything, bigger. Again I ran into what seem to be (castor?) oil filled polyester or PP types - something I've never heard of, before.
Also again, the rest of the device is in very good condition and should run a LONG time, if the replacement cap is a correctly rated polyester or PP type.
What do you all with background in this, think?
Smother Brothers routine:
“I fell into a vat of chocolate and nearly drowned.”
“Wow. How did you get out?”
“I yelled, “Fire, fire!”
“What? Why did you yell ‘fire?’”
“Well, nobody was gonna come if I yelled, “Chocolate!”
Agreed...to my knowledge the large cap is only used to ramp up voltage/current to start the motor. It’s better to replace it then fret over the capacitance reading!
Ooops wrong thread
My conspiracy theory about this is that the mfrs use substandard caps so when they give it up, likely from heat over long periods of time, most people will have to buy a new motor/alternator/etc.
Interesting. I'll have to look into that. However, I do know polyester and PP are often spec'd as "run caps". I'd think if the ESR (internal resistance) is low enough, reasonable current should not be a problem. Of course, some of the tradeoffs are capacity, working voltage, ESR, and size. If capacity and voltage are set, then you have ESR vs. size (essentially)...
Like the maintenance supervisor I had who had to make a run for an emergency part. He called ahead to be sure it was ready. When he went to the counter, it was the wrong part.
The clerk checked the part’s number and said, “Well it was only off by one.”
Just go to Tractor Service Center and they will match your old sump pump for half what Lowes, HD or other big box stores will sell it to you for. It costs about $75-150 for an electrician to wire it you are still lost.
The cap is used to shift the phase of the current to create (IIRC) a rotating magnetic field.
I’m definitely not “lost”. But I am very curious about “oil filled poly caps” (never heard of them B4), certain failure mechanisms, and maybe dodging typical mfgr skimping.
I’m not so sure that’s only a theory!!!
Except they are almost all too big (dimensions).
A true start cap would be in the 200 mfd range (177-217 for instance) depending on the HP of the motor.
That's what I figured. But then this 47 uF 200v cap in the chipper motor makes no sense, unless it's a case of serious skimping.
Do you mean “Tractor Supply Company”? A search for “Tractor Service Center” turned up nothing useful - unless I need a tractor fixed.
TSC’s prices look “comparable”. House brands suspect until proven. Lowes & Home Depot often cheaper for same model of brand name pumps.
Thanks for all the replies! Will check back in a couple days: duty (elderly Mom care) calls...
By the way I did bookmark All About Circuits. Thanks
Generally +/-10% of the rating is considered ok, otherwise replace. Residential HVAC industry commonly uses 370VAC caps on blower motors, these are oil-filled and usually oval in shape. I’ve seen motors start well outside of the 10% range but they usually don’t get to full RPM. Zero Uf=no start.
This similar to what you have?
https://www.homedepot.com/p/Packard-370-Volt-7-5-MFD-Motor-Run-Oval-Capacitor-TOC7-5/204417183
I recommend the Flux Capacitor in the 1.2 gigahertz range.
Also, I am not an electrical engineer, but I did stay in a Holiday Inn Express and watched Back to the Future on Cinemax.
Checking back in 1 more time...
That’s close but physically too big.
Closest I’ve found so far:
It is a little bigger than the orig, and polyester vs. oil filled “something” for the orig cap, but I think it’ll squeeze in.
I may break apart the old one when I get back home.
Best answer is switch over to three phase.;-)
If by ‘almost too big’ it will fit, do it.
I have no idea what a chipper motor looks like, voltage, horsepower or anything else.
My experience is with HVAC, household items, pump motors, etc.
We often used what is called a ‘hard start kit’ on stiff single phase AC compressors. Basically a start cap and electronic relay that is hooked up in parallel with the run cap.
Check Graingers for your capacitor?
TSC or whatever store is cheaper. In my area, TSC is definitely lower.
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