380 and 440 volt capacitors

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petersonra said:
Single phase motors need a start capacitor to get the phase shift necessary to start the motor. A 3 phase motor does not need it because the phases are already 120 degrees apart.
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I know, but I'm seeing HVAC caps rated less than the line voltage.
 
mbrooke said:
PSC
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Ok, there's a ton of info out there about it. You have one cap, it's the "start" and "run" cap. The PSC is a dual coil with common.

The caps on a PSC motor typically will run higher V across the cap than line voltage. Yikes, so look out.

Here's my PSC HVAC 240v/60Hz/1ph condenser running:
8.8A through Start coil (which means in/out of the cap @60Hz)
325v(rms) across the cap
19.2A Line

For a 480v(nom,rms) PSC you need a cap that will be rated nom/rms higher than that by about 135-150%. But it all depends of motor design.

see https://news.ewmfg.com/blog/comparing-shaded-pole-psc-and-ec-motors
 
LarryFine said:
Because it's still in series with motor windings, so is not subject to the full supply voltage.
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Unless, of course, the winding and the capacitor come close to a resonant condition, in which case the voltage across both winding and capacitor can be a large multiple of the line voltage. Fortunately this does not happen when the correct capacitance is used.
 
mbrooke said:
Here are some examples:





There is a cap on all these motors.
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The descriptions there say "need cap" and on the left there's a small pic of "this item". The cap they link to seems wrong to me.

Go look at other sites that sell PSC motors, all of the 208-230v 1ph 60Hz PSC's show a 370v or better cap. The PSC will likely see 240v line, and the 370v(nom) cap is more than 1.55x higher than line.

GoldDigger said:
Unless, of course, the winding and the capacitor come close to a resonant condition, in which case the voltage across both winding and capacitor can be a large multiple of the line voltage. Fortunately this does not happen when the correct capacitance is used.
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It's never ideal, and the cap will always see higher than line. The MFD + inductor will dictate how the voltage builds.

We can dump this into Spice to see what it looks like.

A 480v 1ph PSC will likely have a 750v(nom) or better cap. And if it's a big HP motor the cap is gonna be beefy to handle the run amps.
 
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Besoeker3 said:
Electrolytic ???
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Back to back series electrolytics in the same package are commonly referred to as AC electrolytics. I would expect life to be lower than similar construction electrolytics used in a DC biased situation.

Or maybe I have it wrong?
There is a special type of electrolytic capacitors for AC use, which is designed to withstand reverse polarisation. This type is called the non-polarized or NP type.

Read more http://www.capacitorguide.com/electrolytic-capacitor/
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it looks like you can either make an NP electrolytic by putting two discrete polarized electrolytics in one package (with or without a bleeder network to equalize voltages) or you can construct a three layer rather than two layer electrode wrapping to provide two electrolytic junctions in series throughout the spiral. (wrap would be anode material-cathode material-anode material, for example.)
 
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GoldDigger said:
Back to back series electrolytics in the same package are commonly referred to as AC electrolytics. I would expect life to be lower than similar construction electrolytics used in a DC biased situation.
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Oh well............another experience..........
 
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