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A centrifugal pump requires 21.6 hp to flow 350 gpm at 2800 rpm with a 230/460V, 40 hp, 2-pole (3600 rpm) motor. The VFD available can operate up to a 25 hp motor at 230V, with a maximum output of 68 amps. Would this work, provided the pump flow requires no more than 25 hp? The VFD frequency would be limited to 48Hz to limit the motor speed.

This would be a temporary measure, pending a new 460V power supply and new 40 hp VFD.

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    \$\begingroup\$ The 68 amps only limits the time to reach full speed. THe motor must have sufficient cooling for this to work as it rises in speed if conduction losses would be constant. \$\endgroup\$
    – D.A.S.
    Commented Mar 3, 2019 at 18:24
  • \$\begingroup\$ The pump was designed by FloFab to use the 40 hp at this speed, so they are confident of the cooling capacity. My concern is whether the VFD can supply the needed combination of voltage and amperage, at 47 Hz. A 25 hp motor would be running at 52 Hz to produce 21.6 hp. \$\endgroup\$
    – David Shepherd
    Commented Mar 4, 2019 at 15:18
  • \$\begingroup\$ You would have to program the acceleration rate to max speed to prevent overcurrent, or if possible choose constant power on startup on VFD. Otherwise it is operating near limits but ok. But cycling pumps tend to be rated on RMS power not peak, so I'm not sure how VFD responds to peak current limits or RMS. \$\endgroup\$
    – D.A.S.
    Commented Mar 4, 2019 at 16:57
  • \$\begingroup\$ Here's more relevant info. The VFD is actually a 40 hp model that is derated to 25 hp, because it is acting a single phase to 3 phase converter. \$\endgroup\$
    – David Shepherd
    Commented Mar 12, 2019 at 21:56

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It would likely work, but the motor current for 21.6 Hp is likely to be higher than 21.5/40 X the full load current of the 40 Hp motor. That may cause nuisance over-current tripping of the VFD. If that occurs, the maximum speed may need to be further limited. Using the under-sized VFD is certainly a reasonable thing to try. The VFD should protect itself and the motor. The worst that is likely to happen is nuisance tripping.

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