Im a HVAC contractor/designer. Im looking to install a fan unit with a VFD. This is 3 phase, 60HZ, 460 volts, 40HP AC motor. Im trying to create a "system curve" like a fan curve that would show the cfms at various frequency settings being changed by the VFD. I can reiterate or integrate my ductwork calculations for multiple rpms of a fan curve. And I know that RPM theoretically varies linearly with frequency, but im wondering how much or much less, motor slipage will occur at lower frequencies. you see my question is: If 60hz=1750 RPM does 30Hz really = 875 RPM? Im not speaking theoretically. Maybe someone has seen a motor curve of rpm vs Hz?