If I'm reading his first post correctly, it works in every speed except the highest speed of 1800 rpm at the spindle. So the motor is fine, the rpc works, its just the rpc bogs down and trips the 50A breaker when he starts it with 1800 rpm spindle speed. He used #8 wire to feed rpc, when mfr recommends #6 minimum, will upgrading wire size fix that problem, I don't know, but it seems easy enough to check.......I don't agree with all that. Unless the drive train in that picture is a lot of work to turn.
Remove the belt or put the rig in neutral and see if it turns.
If it turns then connect the belt. If it stalls then I agree about RPC size and a few caps.
But this guy has 10Hp to 5Hp variance and that should be enough.