IBM 4331 Mainframe 3 phase converter

Jon Elson elson at
Thu May 4 10:54:30 CDT 2017

On 05/04/2017 09:36 AM, Guy Sotomayor Jr via cctalk wrote:
> Does anyone know how to convert KVa to 208v 3-phase currents and/or HP?  IBM’s docs all specify power in KVa and most of the 3 phase converters (other than what Bob posted) seem to specify HP.  If I’m going to spend serious coin on a phase converter (I think I found it online for ~$3k…ouch!), I want to make sure it’s sized properly.
Line current times voltage give you KVA per line.  Then, 
multiply by 1.732 to get total KVA.  KW / .7457 give HP, not 
counting losses or power factor.  You don't actually need a 
3-phase source, you only need to generate one new phase
(although if the 4331, etc. want 208, you probably don't 
want to feed it 240 V).  But, you DO need a true sine wave 
source, and VFDs do not produce sine waves, they put out 400 
V PWM waveforms that look fine to a motor, but not good at 
all to electronic loads.

It is possible that a hefty used VFD with an appropriate 
filter would turn it into a nice sine wave at lower cost.  
For multiple KVA loads, that might be a more economical way 
to go, if you don't mind creating such a filter.  (Probably 
be good to put a transformer between the VFD/filter and the 
computers, so the line could be balanced around neutral, too.)


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