Troubleshooting 3 phase pump with VFD

jonr

Re: Troubleshooting 3 phase pump with VFD

My understand is that you should put some inductive filters on the output of a VFD to remove some of the harmonics.

See http://en.wikipedia.org/wiki/Variable-frequency_drive#Application_considerations

BB.

Re: Troubleshooting 3 phase pump with VFD

I am not sure I would add output filtering unless the VFD mfg. recommended it.

Filtering has a lot of side effects--With a VFD (or even MSW inverter), these side effects can be a major concern about the overall efficiency and stability of the VFD/Motor (or AC Inverter and loads).

A motor needs a pure sine wave (usually 5% or less total harmonic distortion). The higher harmonic sine waves (one way of analyzing the problem) are not at the fundamental frequency for rotation--And do nothing but "waste energy" and/or create circulating currents (again losses/waste heat) in the motor (and transformer) internals.

There is no good way of converting the higher harmonic energy into fundamental frequencies. Filtering either changes the phase relationship (just moves harmonic power "around") or converts the harmonic energy into heat (again, wasted energy).

Do you have more information on how/what is would be recommended in terms of filtering, and what the advantages/disadvantages may be?

-Bill

jonr

Re: Troubleshooting 3 phase pump with VFD

Yes, the idea of the filter is to remove the harmonics and they do go into heat. But better this than sending them to the motor where they will also go into heat and possibly damage the motor (heat, motor bearing currents, over-voltages due to line resonance, etc).

Evidently another source of motor damage from using a VFD is if it spins too slow. This reduces cooling.

Some 3 phase motors explicitly say they are "inverter rated". And they list a minimum speed.

I haven't looked into filter details.

edit: looks like 100' is a typical maximum wire length without filters. See
http://www.vfds.com/dv-dt. Mitsubishi’s soft PWM (or any PWM VFD) sounds interesting for motors not intended for VFD use.