What's new
What's new

DC Start Capacitor

Feb 7, 2024
Mead Washington
My 15 HP rotary phase converter quit after 15 or 20 years. I built it when I was younger and cant remember why I did what I did. Anyway i have a large 1900 MF dc capacitor as a start capacitor. Through everything I have read they say it will not work BUT it has worked for years. It also looks like it is a little difficult to test these capacitors. I haven't tested the run capacitors yet but I am curious about the DC cap as a starting cap. Thanks In advance
DC? Picture?

An actual polarized electrolytic capacitor would not work for that. Probably it is not quite what you think.

There are some purpose made testers for those, or you can try a digital meter that has a capacitor tester. Some meters don't seem to do well testing those, others do fine.
This is the capacitor that has been in there for years. I replaced it yesterday with regular start capacitors. Then found that the windings are shorted in the motor. I double checked my meter on another 3 phase motor and got readings that I would expect from a good motor.


  • FullSizeR.jpg
    634.6 KB · Views: 18
That's 2 of us..... Especially with a shorted motor, which would be a serious stress for even a proper motor start part.

A standard motor start capacitor is generally two of the regular electrolytic portions like that, connected in series opposite to each other, so that the current that is "reverse polarity" for one, is "correct polarity" for the other, whichever direction the current flows. In that way they are made"non-polar".

Without the two parts, reverse polarity on the capacitor is usually a short.

Only thing I can figure is that being a high voltage part, the oxide insulating layer is thick, and a half cycle of the mains cannot damage it much before the next half-cycle "undoes the damage".

It's definitely the first one I have heard of to withstand AC without popping it's cookies.
Thanks Everyone!! This is about the only thing has gone right. BUT I did just stop by a buddies place and he gave me another 15 hp 1725 motor that looked to be in very good shape. He also had a 15 /11HP two speed motor that got my attention for possible future projects.