Putting some motors on VFD's. I know the VFD input wiring and supply OCP should be sized for the input current rating of the VFD. I expected that current rating to be a little higher than the motor rating, due to the inefficiency of the VFD, and due to some voltage drop across the VFD line reactor.
The VFD cut sheets list the exact opposite. The VFD input current is less than the motor current.
Example; 25HP motor - NEC rated current at 460V is 34 amps. VFD with a 3% line reactor is about 28.8 amps, with a 5% reactor its 27.9 amps.
Another one: 60 HP motor - rated current 77 amps. VFD with 3% = 67.4 amps, VFD with 5% is 66.9 amps.
Why would the VFD have a lower rated input current than the motor its supposed to drive? I thought it might be because the motor is rated at 460V, and the VFD is 480 volts. But that's only a 5% difference in voltage, and I'm seeing a 10% + drop in current for the VFD.
The VFD cut sheets list the exact opposite. The VFD input current is less than the motor current.
Example; 25HP motor - NEC rated current at 460V is 34 amps. VFD with a 3% line reactor is about 28.8 amps, with a 5% reactor its 27.9 amps.
Another one: 60 HP motor - rated current 77 amps. VFD with 3% = 67.4 amps, VFD with 5% is 66.9 amps.
Why would the VFD have a lower rated input current than the motor its supposed to drive? I thought it might be because the motor is rated at 460V, and the VFD is 480 volts. But that's only a 5% difference in voltage, and I'm seeing a 10% + drop in current for the VFD.