How does motor slipage affect fan performance with a VFD?

[joshman]

Im a HVAC contractor/designer. I am 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 I am 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? I am not speaking theoretically.

Maybe someone has seen a motor curve of rpm vs Hz?

 

[RainmanAero]

Hi joshman,

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 I am 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? I am not speaking theoretically.

Well, first of all, are you aware that for a VFD you really need to keep the Volts per Hertz (V/Hz) ratio constant in order to keep motor torque constant? So in your example, you will want to change the voltage to 230V when you bring the frequency down to 30 Hz. If you do not, you will change the total torque output, and this will affect your slippage.

Maybe someone has seen a motor curve of rpm vs Hz?

Well, the exact equation (ignoring slippage) also includes the number of poles in the motor: Speed (RPM) = 120*freq/npoles. The following page has a good summary of VFDs and an estimate for slippage, as well as some info on typical means for controlling VFDs:

http://www.joliettech.com/what_is_a_variable_frequency_drive.htm

Did this help? If you like, we can discuss the specifics of the mass flow rate... as this is really a question of the fan's aerodynamic efficiency at varying speeds and fan pitch.

Rainman