40.96A isn't likely to make a typical properly functioning 40A breaker
trip soon enough to be of concern

If the breaker is functioning properly, and the breaker is tripping, you
have an overcurrent condition. Running a 10HP motor on a 10HP rotary
phase converter is pushing it. I would replace the breaker and see if
that resolves the problem. You may want to upsize the cable a few sizes
to minimize voltage drop and therefore current draw. Otherwise IMHO you
are looking at upgrading the circuit (breaker and wiring) to handle the
required current.