??? Induction motors depend on frequency not voltage for final rpm?
I don't quite see how you would do that with a RPC?
The third phase is created from the 240v 1ph 60Hz.
It would be interesting to put a tach on the motor and a scope on the 3 phase?
Is this a 4 pole or 2 pole motor, if 2 pole, you could be exceeding the rpm rating of the motor if somehow running high.