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?
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...