Any experts here on capacitors for PSC or other motors?

Same info. as "body" of thread. Definitely NOT an excerpt!

- - -

I've been all over the Internet and am having trouble finding a 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?

I think you might have better luck over at All About Circuits. Or call Digikey or maybe Grainger.

I can’t imagine that small difference in capacitance would make the motor not start. The only purpose of the PSC cap is to develop enough of a phase difference between windings to get the motor to start. By staying in circuit all the time the idea was to produce greater reliability than a start capacitor that is switched out (which actually works just fine in my experience).

What is the tolerance of the original cap? I’d bet 6.6uf is within the tolerance range for a new 7.5uF cap. The difference you measured after a day could be due to residual charge that interfered with the measurement...did you discharge it first?

Which is to say, you’ll have to look elsewhere if I’m right. Substituting the closest available cap you can find is probably the only way to know for sure. You’re correct about the voltage rating. Why not contact the manufacturer?

I’m certainly not an expert. But the only cap I have is the MAGA brand. Size is 7.25”. Does this help?

Not an expert on motor starter caps, but I do know that another important parameter to a cap, besides capacitance value and working voltage, is current capability.

I believe the oil filled caps are capable of dissipating much more heat from high current flows than a Mylar cap of the same capacitance and working voltage rating.

You have a very common, inexpensive run capacitor. Go to Grainger.com and type in 2MDV6 and you should be viewing the capacitor your talking about. Instead of messing around with the leaking capacitor just buy a new one and replace it.

How much is a new sump pump? If this is a full submersible, get a complete new pump and don’t fool around with replacing the capacitor for safety’s sake. You want a complete watertight unit.

Oil in the water can indicate a shaft seal failure on a sealed submersible. Repair is not practical except at a shop with a high dollar pump.

The only thing we ever bothered with changing run capacitors on have been blower motors, compressors, etc..

JMHO.

Those guys are too technical.
You get more fun answers on FR.

“...get a complete new pump and don’t fool around with replacing the capacitor for safety’s sake.”

Smart call. If OP is not married and lives alone, gopher it! Otherwise, face the music when it fails. You will be blamed by the wifey for trying to go cheap when it fails, even if over the normal operating lifetime. Just buy a new pump! It’s worth the extra cost in the long run.

Common rule of thumb, you can tell when a capacitor is bad but you can’t tell if it’s good.
IOW a meter isn’t always the answer and caps are so inexpensive, replace it. That’s a common size.
You could go up a little on the capacitance but not down. A 440 VAC rating would be fine also. That’s a run capacitor.
Any AC supply house would have the cap.

A true start cap would be in the 200 mfd range (177-217 for instance) depending on the HP of the motor.

Did you check Mcmaster-carr? They have a pretty good selection.

Did you try Granger for a source for the original cap?

Hi, bigbob. Thanks for the reply!

Your thought about the capacitance agrees with my own initial thinking. But, everything else looks fine. Granted, the original is +/- 5% tolerance, and the cap did mysteriously(?) drop to 5.4 uF, using the same measuring equipment and techniques, a few days later. Regardless, I am wondering if “something weird”, but not “destructive” happens to the cap under voltage / load.

(Yes, I did discharge the cap B4 testing. Don’t want to risk a $200 plus LCR meter or my Fluke 8060A VOM!) (This LCR meter, BTW, uses analysis of an AC signal at either 120 Hz or 1kHz to make the measurement. It is not a true “bridge”, however.)

I do have some other caps I could parallel to get the right rating, as an experiment (but those will never fit inside the housing).

The mfgr.’s website is not very helpful as to customer service issues (I think they got bought out & are under another corp. now.) I may try a “general” contact. Then again, I’d like to put in a more reliable type cap, if possible. I strongly suspect the original was “pushing it”, construction-wise, to try get the cap value and voltage rating at a price point (although this is NOT a cheap sump pump, at $250 from Home Depot). There actually IS some (some) room for a physically bigger cap. A PP or Polyester (PET) @ 300VAC or better should last almost forever.

BTW, The chipper was Harbor Freight. If I spend tons of time on it, maybe I can get hooked up with a Chinese engr. Maybe. Yeah, I know...

“You have a very common, inexpensive run capacitor. Go to Grainger.com and type in 2MDV6 and you should be viewing the capacitor your talking about. Instead of messing around with the leaking capacitor just buy a new one and replace it.”

I agree, best answer.

I’m an Electrical Engineer.

I would go buy a new pump.

There’s no oil in the water. Just INSIDE the housing. The seal is in great condition & should go back together fine. The intent is to see if I can put in a more reliable cap.

I have lots of experience with air-tight and also gas (natural and propane) tight. Plus some experience with submersible pumps. So long as corrosion / deterioration of surfaces / materials is not a problem (they look great), water tight is much easier. The oddities with the motor cap are a bit new to me though. Usually the failures are more obvious / drastic.

That said, I’ll set the reassembled pump in several feet of water for several days & then put it in a large bucket & test it. Moisture inside should trip the GFCI outlet.

I’m going to get a new pump anyway. But I suspect “skimping” on the caps happens on all but the most pricey. Motor caps seem to fail often. This is more of an engineering project to see if I can improve the thing, and will give me a backup.....

(Yeah, there’s already a “battery backup” pump installed, too.)

Oh, I’ll replace it, but I’d like to find something better. I suspect the original was miniaturized a bit too much.

OTOH, 2MDV6 appears to be a bit too big (dimensions). I will recheck.

If you are having caps fail on a regular basis on the same motor you need to investigate the power quality of the circuit that supplies the power to it. AND run a megger on the motor.

I’m not after “cheap” (at least for the cap.) I’m after learning about these devices (esp. the caps — poly film plus oil is new to me) & also improving the reliability! My experience & research (practical and online) with the failures (overall) says a new pump may fail sooner, anyway.

Wifey started off as an engineer, so, she somewhat understands the mindset. Plus, the basement is to my shop, so, no impact on her. Except me griping at whoever spec’d the crappy Chinese capacitor. (Not all Chinese parts are poor - I just think in this case the mfgr skimped on this.)

Oh, maybe I should clarify! This is not a repeat problem with the same motor. I was talking about cap failures in many different electric motors, over the last 45 years or so. It appears to me the motor mfgr’s often skimp on the caps, relative to the rest of the motor.

You are of course 100% correct about the other checks — all looks great.