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Run capacitor quality (residential A/C compressor)

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dick_freebird

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Every other year or so the A/C service tech for the "maintenance
plan" claims I need a new run capacitor. "Getting weak", they
like to say. Yet I do not see them applying equipment that would
give them any useful readings.

So I am suspicious beyond the usual suspicion of "incentivized"
repairmen.

What I'd like to ask, is if anyone here has an experience based
opinion of the most reliable grades & mfrs of basic, 45 / 7.5 run
capacitors for compressor motors. I do not believe a run cap
should degrade in a year or two, used to be "just there, forget
it" (back when engineers ran electronics companies instead of
MBAs).

Dude wanted to charge me $273, for what looks like sells for
$12or $15 (370V vs 440V stamped). There's a lot of room in
that gap, to spend on quality instead of (mendacious?) labor.

Lots of mfrs, how to know what's "good for life"?
 

I'm not sure of the physics/chemistry behind it but they do degrade. Most will have an 'hours' rating on them to indicate how long they should operate before the value drops out of tolerance.

Over time the value drops and eventually the motor draws more current, eventually stalling. It is measurable with a capacitance meter but I guess maintenance man doesn't know how to check them. The ones in water pumps here need changing about every three years, the pumps are switched to keep tank levels up and the usual symptom of a low value is the over-current trip operating as the pump is turned on. New ones measure about 15uF and when they need changing they are down to about 4uF.

That price is a rip off - buy new ones yourself for a fraction of the price.

Brian.
 
Presume 45/7.5 uF is a film capacitor, that's at least what I see in various offers of this popular type. If not partially burned out due to massive overvoltage events or overheated due to inappropriate mounting, why should it degrade? I would rather fear wear of the starter relay.

I remember bipolar electrolytic being used as motor start capacitors in some cases, they are a candidate for limited lifetime.
 

Run caps should be rated for actual use - they are usually self healing - yes they can degrade - a simple uF measurement will tell you by how much - I would expect 10 years of life in benign conditions, i.e. not to hot ambient and not too many mains strikes ...
 

Just wondering whether there's any particular brands / lines
known for being extra durable, in this segment?
 

I wonder if its inrush related, because they are switching on/off every day?
Anyway, we had a run cap on our compressor on our thermal chamber constantly blowing.

But yes, if you can get to it yourself, i would just put something in there. I would guess that eg Panasonic or Siemens do good ones.

I think these run caps do get big current surges now and again...and since they dont have wet electrolytic, they arent well robust against this. AYK, "wet electrolytic" is the master of handling enormous surge currents.

Maybe your/our electrician accidentally puts starter caps in there....?...these, AYK, arent as durable as the run caps.

What about making up a board of X2 caps in pllel, to get the value...then just wire that in instead?...sounds more durable.

And what about putting a "transient catcher" in there as well as the run cap......just a big bank of electrolytics behind a diode bridge, with a slow discharge resistor...this 'lytic bank will soak up the transient, instead of the run cap...so make it last longer.
 
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