I have several compressors driven by two speed motors. We want to install VFDs on these for better capacity control. The issue is that these are two speed-constant horsepower motors. The first set that we want to do are 25 HP @ 1150 RPM and 25 HP @ 1750 RPM. These are HVAC compressors and they operate in low speed or high speed depending on the condition. The HP is approx 25 HP regardless of the speed (less torque requirement in high speed, greater torque requirement in low speed due to compression ratio)

The first time we started talking about doing this most of the other guys said "We'll just wire the VFD to the high speed winding." If I am thinking about this right and doing my math right the 25 HP motor operating at full load in low speed @ 1150 RPM will satisfy a 114 lb-ft torque requirement and operating at 1750 RPM will only satisfy a 75 lb-ft torque requirement assuming 25 HP at both speeds.

Here's what I told them, I'd like to see what anybody here thinks: If I install a 25 HP VFD on the high speed winding and I attempt to run the motor at a reduced speed (1150 RPM) with the 114 lb-ft torque requirement I will overcurrent the high speed windings, correct?

I figure, the only real practical way I can get away with this is if I get a single speed oversized motor and VFD. If I need 114 lb-ft at approx 1150 RPM and 75 lb-ft at 1750 RPM from a single speed motor then I will need a single speed 1750 RPM 40 HP motor and VFD, correct? A single speed 40 HP 1750 RPM motor at full load will produce 120 lb-ft. If I run this motor at 1150 RPM with my 114 lb-ft torque requirement I will not overcurrent the windings. If I run this motor at 1750 RPM with my 75 lb-ft torque requirement, obviously I will not overcurrent the windings.

I guess the motor wouldn't exactly be "oversized" for this application. In high speed you could consider it oversized but in low speed the motor will be near full load current when driving a 114 lb-ft load at 1150 RPM.

We ended up submitting a request to purchase two 40 HP 1750 RPM motors and two 40 HP VFDs.

Any thoughts? I'm pretty sure this is necessary considering the application and the desired outcome. I'm sure that we could use the existing motor with a 25 HP VFD and use load contactors to switch between windings depending on what speed we want and what our load is but that's getting a little hairy and I don't know if I want to go that route. That would require a little more than "simple" control logic.

Thanks very much for any input.

-Allen