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Phase Conversion Only


Jun 16, 2001
St Louis
I'll throw in a wild card, here. The more recent trend is to have 3 phase VFDs with smaller capacitor banks, because three phase has low ripple voltage even when not filtered.

I designed one with a 50 uF film capacitor as the bus capacitor. It worked very well. Of course, the client then revealed (for the first time) that they also wanted a version for single phase input.... for that we had to add an external capacitor bank, involving a delay and extra cost since it was not revealed until the design was completed.

Sucj small capacitors have fairly actual low inrush, and if they are film capacitors, the allowed inrush current of the capacitor can be quite high, so there may be no need for any special protection.

Your Yaskawa may be older, and likely does have a big bank of capacitors.


Dec 19, 2008
JST: While on the topic of capacitors, I have a question...do they deteriorate with age if NOT used (i.e., not powered)? For example, is the main cause of failure, over time, heat cycling or something else (e.g., age related deterioration of internal materials whether powered or not).

This is related to the OP because I've always wondered if it's best to leave electronic equipment containing a large number of capacitors "on" (standby) all the time or turn it off after use. I'm talking specifically about what's best for longevity, not other concerns such as idle power use.