Startup capacitor keeps blowing

I agree it's starting to sound like a run cap. I have seen mostly oblong or round run caps. These run caps are super stout; assuming you are using the right voltage, you should be able to put (L) on one side of the cap and (N) on the other side and plug it into a wall and it should go all day like that - a single series impedance. So being in a circuit with a motor could only be less impedance than by itself, so less likelihood of blowing up. As I said, that is assuming the proper voltage; but you said something about 208V. I would investigate that. I'm not aware of any method a motor could employ to boost voltage.
 
A good video about start and run caps:

[video=youtube_share;OMd9QkinXz4]http://youtu.be/OMd9QkinXz4[/video]
 
I'm not aware of any method a motor could employ to boost voltage.
The winding inductance and the capacitance could be series-resonant at 60Hz, so more than line voltage could appear across the capacitor.

The video could be better. He didn't explain the difference between starting capacitors, which are nonpolar electrolytics, and run capacitors which use mylar or polypropylene film and metal foil or metal deposited on the film. Also the oil is not necessarily there for cooling, but is to prevent voids at the foil edges which would allow corona to form at high voltages. Corona creates ozone which attacks the film.

Start capacitors have significant impedance, which creates heat when they carry current. They need time to cool off between uses. Jogging a motor could, it seems to me, overheat them pretty quickly.
 
Agree with Alan- also sounds like a good excuse to install a variable speed motor! Best thing I ever did for
my lathe..
Mark S
 
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