Run capacitors: why electrolytic can not be used

[sdancer75]

From wiki : Run capacitors are designed for continuous duty while the motor is powered, which is why electrolytic capacitors are avoided, and low-loss polymer capacitors are used.

I am still don't understand, why electrolytic must be avoided....It's because they last less before they fail or there are also other restrictions?

Regards

[RoGeorge]

Motors are usually driven by AC, which means the polarity of the wires alternates between +/- then -/+ on the same pair of wires.

Electrolytic capacitors are usually polarized capacitors, they does not support reverse voltage. 

[sdancer75]

Yes, didn't though of that, so I missed it at all 

But how about non polarized electrolytic capacitors like start capacitors? Does this will work?

[AVGresponding]

Not quite sure why you think electros aren't used for this, they certainly are. They are simply a non-polarised type.

It's not as common as it used to be though, these days inverters are so cheap, most industrial motors use them, and most of those inverters have integrated filters, but some don't.
Depending on the size and ratings, you might still find them there.

[Gyro]

Motors are usually driven by AC, which means the polarity of the wires alternates between +/- then -/+ on the same pair of wires.

Electrolytic capacitors are usually polarized capacitors, they does not support reverse voltage. 

 

Yes, didn't though of that, so I missed it at all 

But how about non polarized electrolytic capacitors like start capacitors? Does this will work?

Motor START capacitors are bipolar (non-polarized) electrolytics as they are typically too high in value to comfortably use film capacitors (cheaper too for the same value). They have high loss however, which would shorten their life and make them overheat in continuous use - they are designed for minimum case size possible for practical reasons.

Motor RUN capacitors are (normally Polypropylene) Film types because their losses are much lower, allowing them to run continuously at low temperature and achieve maximum life and motor efficiency. Motor run capacitor values are significantly lower than start ones, making the film ones practical at a sensible case size.


P.S. Hopefully obvious, but the motor Start capacitor is taken out of circuit by a centrifugal switch as soon as the motor reaches operating speed. Normally a couple of seconds max.