FanGuy
New User
- Location
- Pennsylvania
- Occupation
- Industrial Sales Rep
I thought I knew the answer to this question, but I have had some instances recently where my understanding has been challenged, and I feel like I cannot find a concise statement anywhere online that confirms what I had learned.
Do I need to select an "oversized" motor (in terms of horsepower) for my centrifugal fan if I intend to operate it at less than nominal speed on a VFD?
In my current application, because the customer wants the fan in direct drive, and the fan speed needs to be at a non-nominal motor speed to be at the right operating point, I am looking at running a 3600 RPM motor at just 2200 RPM, and at that 2200 RPM speed, the fan will require 176 BHP. The way I would typically size the motor would be to de-rate its horsepower by the ratio of the VFD turndown. That is:
Required Nameplated Motor Horsepower ≥ 176 x (3600/2200)
Required Nameplated Motor Horsepower ≥ 288
So I would need a 300 HP 3600 RPM motor, which, turned down to 2200 RPM, would be capable of delivering 183.3 HP.
Is this correct? I can't use an inverter-duty 200 HP 3600 RPM motor, right? Does it matter what my actual torque requirement is (i.e. the WR2 of the fan wheel)?
Do I need to select an "oversized" motor (in terms of horsepower) for my centrifugal fan if I intend to operate it at less than nominal speed on a VFD?
In my current application, because the customer wants the fan in direct drive, and the fan speed needs to be at a non-nominal motor speed to be at the right operating point, I am looking at running a 3600 RPM motor at just 2200 RPM, and at that 2200 RPM speed, the fan will require 176 BHP. The way I would typically size the motor would be to de-rate its horsepower by the ratio of the VFD turndown. That is:
Required Nameplated Motor Horsepower ≥ 176 x (3600/2200)
Required Nameplated Motor Horsepower ≥ 288
So I would need a 300 HP 3600 RPM motor, which, turned down to 2200 RPM, would be capable of delivering 183.3 HP.
Is this correct? I can't use an inverter-duty 200 HP 3600 RPM motor, right? Does it matter what my actual torque requirement is (i.e. the WR2 of the fan wheel)?