How to Select Capacitor Sizes for Balancing 3 Phase Rotary Converter Voltages?

Thanks in advance for the help.  I'm have an issue with my CNC lathe I purchased using a phase converter with 120V service.  The tech asked me to check my line voltages across L1-L2,   L-2 - L3, and L1-L3.   I was originally way out of spec, but by reducing the capacitors I was able to get much closer, but still not as close as the tech wants it to be.   (1%)    I've got final voltages (after running through a transformer) of 206.6, 202.4, and 200.2   across terminals.   Prior to the transformer I have voltages of 236,237, and 232.    The unit comes with a series of   50 mfd  250V capacitors, three on each line, or a total of 9.   After quite a bit of trial and error and by removing 2 on line one, 1 on line 2, and 2 on line three (generated line)  I was able to balance the system this far.   I'd like to get it closer yet, but can't figure out  exactly what size capacitor(s) I need.   I'd like to bring lines 2 and 3 up approximately 4 volts to get better balance across all three phases.   The 50 MFD it comes with  is either  to much of a step up on L2,L3  (or to far a step down on line 1).   What size do I need to purchase?   I don't know how to do the calculation so I'm hoping someone can help me out and I'm not buying 30 different capacitors and doing trial and error for the next two weeks.    I'd like to add two smaller capacitors to lines 2 and 3 and then I would be balanced and still in overall correct required Voltage for the CNC lathe.   Thanks again in advance for your help!   

To a first approximation, the voltage shift due to connecting a capacitor, should be proportional to the capacitance. For example, if 50 uF shifts the voltage 5 V, then 20 uF would shift the voltage 2 V and 10 uF would shift the voltage 1 V. Are the capacitors connected line-to-line or line-to-neutral? From your description, I expect line-to-neutral.

When purchasing capacitors, be certain you are getting capacitors intended for this use! Many capacitors intended for electronics applications have adequate voltage ratings, but cannot handle the AC current your application requires.

Hi Bob-I've never heard of a requirement for the phases to be balanced within 1%.  In fact, most 3-phase systems will accept a wide range of voltage, from around 200 volts to over 240 volts between phases. We've got 3-phase power here at the shop and when I checked the meters a few minutes ago we had 206, 209, and 216 volts. In an industrial power distribution system there are usually single-phase loads on each of the three phases.  As these loads are switched on and off, the voltage on each phase varies slightly. What kind of problem are you having with the lathe? Is there a variable frequency drive involved?  What is the acceptable range of input voltage given on the lathe's spec sheet? Are you measuring the voltage while the lathe is running?-Rick 

Rick, Thanks for response.  Sorry for delayed reply.  I'm dealing with a tech from Fanuc.  This is a 208 volt machine so it can run from 200-220 volts.   I had to also purchase a step down transformer to get it to the correct power.   I spent a considerable amount of time putting different capacitors in and out of the system.  I was finally able to achieve 207.9, 209.2, and 210.4.    I'm going to purchase one other capacitor in an attempt to get it slightly closer.   (I need a 35 and only have a 30).  The convertor came with a bank of 50's.     It was a lot harder than I thought.  It would seem straightforward enough, but adding to any leg would also add or take away from another leg and screw up the balance.   I figured out it is because of how the capacitors are wired into the legs.  So even if I only changed a capacitor on leg three (generated leg) it would somehow effect when I measured across 1 and 2.     It was way more confusing than it seemed it should have been.    Adding to the confusion, if you were to measure to ground, leg 1 only had about 78 volts and no matter how you switched capacitors around that leg never changed more than 3 volts.   I know you don't measure to ground with three phase, but I was trying to figure out which leg needed more or less voltage.   The other two legs I was able to obtain large voltage swings with capacitor changes.I called the tech back once I got it this close and he still wasn't exactly thrilled.  It was "OK"  but I could continue to have problems with the rest of the machine due to the voltage not being within 2 volts between legs.  That is the tolerance for some reason.    The fact I have a phase converter is even worse as it can change voltage with the incoming service.