VFD OVERLOAD

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hawkeye23

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We have 5 vfd rated for 75 hp each that control 5 sump pumps rated 97 amps  [nameplate]. Found each motor did not have their veristers wired into the circuit so the vfd does a calculation to determin the vfd should be shut down anytime it ran at 60hz for any length of time . At full speed the current reading was always 104 amps. or more confirmed by two amp meters. These motors are rated for 460 900 rpm , Looks like the installer who install these units left the veristers unwired on purpose.  Any idea why the vfd's would pull more then the motors flc ?  This problem only started with these new vfd's. Thanks for any help .
 
hawkeye23 said:
We have 5 vfd rated for 75 hp each that control 5 sump pumps rated 97 amps  [nameplate]. Found each motor did not have their veristers wired into the circuit so the vfd does a calculation to determin the vfd should be shut down anytime it ran at 60hz for any length of time . At full speed the current reading was always 104 amps. or more confirmed by two amp meters. These motors are rated for 460 900 rpm , Looks like the installer who install these units left the veristers unwired on purpose.  Any idea why the vfd's would pull more then the motors flc ?  This problem only started with these new vfd's. Thanks for any help .
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Bunch of information that is not too usefull without knowing the ASD(VFD) model and settings. I presume that the 'varistor' is what you presume to be for the setting of the speed, but if the drive is not programmed so then it does not matter if it is connected or not.

Make sure that all the motor parameters are correctly entered into the drive and that no other funky parameters are adjusted, such as torque boost, slip compensation, etc. and if it is a straightforward centrifugal pump application the V/F curve is set for VT application. Make sure that the motors are inverter rated. Sounds like the motors are fully loaded and the difference could be attributed to the same motor running at 60Hz with or without an inverter in place. (A motor always uses more current on a non-sinusoidal waveform.)

It could be that the original setup called for inverter bypass when the motor - based on demand - reached full speed, a common older design philosophy involving lots of extra contactor. Nowadays one would install a single ASD and would let it swing up-and-down and switch the constant speed motors in/out based on demand.

You say the problem started with these new ASD's. Did you have the readings from the old ASD's?
 
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