Scaling RPC balance cap's 240 -> 415v?

Would like to run this past Those Who May Know...

I'd like to add balance cap's to my RPC, but I'd like a ball-park figure to start from:

On 240v the rule-of-toe run cap values in uF suggested are 4 - 5 times HP P1 to P2, 10 to 15 times P2 to P3, starter 40 - 100 times HP.(Figures taken from the Waterfront-Woods site).

Running from 415v rather than 240, the motor voltage is root3 times, so the current for the same power in is divided by root3. This suggests to me that the motor impedance and thus capacitor impedance want to be 3 times the value on 240v, so the capacitor needs to be 1/3rd the value required for 240v balancing - am I completely wrong or would this be a good starting point when selecting the caps?

If this works out, for a 7.5HP idler on 415v I'm looking at starting at 10uF and 25uF for balance, working up from there; minimum 200uF for the starting cap (currently 300 uF and starts pretty quickly).

I'll leave power factor correction until everything else's sorted, and aim for a fair bit less than 100% correction as would be shown by a minimum on input current

Does this sound reasonable, am I barking up several wrong trees?


Thanks in advance,
Dave H. (the other one)

correct.

Something that will help you understand this is to look at the energy stored in the capacitor. Its proportional to the voltage squared.

1/3rd uF at at 415v is the same energy stored as 1uF at 240v

Dave H,

Have a look at these threads, to answer your questions.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/rfc-supply-voltage-vs-caps-102736/

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/capacitor-types-values-101554/

That should get you in the ballpark.
SAF Ω