Capacitor differences - motor start caps vs regular electrolytic

Thread Starter


Joined Oct 10, 2014
I'm looking at a way to run a 3 phase motor on single phase and from a video I've seen it looks possible by adding caps inline with the phases. I'm running 240v so I need caps that can handle that. I've taken apart many air compressors and the start cap on them it HUGE in physical size compared to comparable farad sized caps I pull off of boards.

From the video it looks like I will need 160-200 uf to run the motor which would be no problem with the aluminum electrolytic caps I have pulled out of power supplies, UPS backups and such. I have plenty of 450v caps that are ~200UF.

But I have to think that there is a difference in the way these caps work if there is such a difference in size.


Joined Feb 8, 2018
You would require capacitors that are non-polar and can handle a lot of AC current through them. Ordinary electrolytic capacitors are polar, which means that they are only suitable for circuits where one terminal is always positive with respect to the other, even if the voltage isn't constant. Good ones can handle high "ripple current" (constant partial charge and discharge) but if you were to use them in your application they would very loudly and violently announce their displeasure. They are normally designed so they will "vent" if the pressure in them builds, but if you were to use them for AC they would probably blow apart.

There are non-polar electrolytic capacitors available, but they generally aren't especially high performance types. Where high current AC must be handled, plastic film types are usually most suitable, but they are large and quite expensive in high values. There are many different plastic film types - different plastics, different "winding" designs, types with metalized plastic and types with separate plastic dielectric and metal foil.