apologies Peter and to all who contributed. Thanks Praveen for reminding us of this thread. The problem at the time became academic, as the power company upgraded the mains, the comps all ran which i already suspected they would as there was no winding resistance imbalance. I'll get some workshop time in the next 2 weeks and try all the options and formulas you have provided on a variety of compressors to see which works. and this time i won't leave it for 2 years before i return to the thread.