THD issues when using a VFD

The water association I am with had contractors install a 240 volt, three phase, 10 H.P. submersible pump for our community water system. After connecting it to a VFD, THD measurements on the three legs, with the pump OFF, were 93.5%, 84%, and 90.4%. With the pump ON, the THD measurement on leg 1 was 23.2%, (with no data recorded for the other two legs, sorry). Line voltages were 209, 209, and 211 V.A.C.

The pump has been operating since May 6, 2007, maintaining water pressure between 40 and 60 psi as designed, ramping up and down properly, with no apparent problems.

After reading about THD, I am concerned about the long term effects the THD might have on the motor. Also, could someone comment about the THD values that were recorded (they seem extremely high when the motor was off, and above standards when the motor was on).

FYI, there is a school building connected to the same transformers that are providing power to our pump. To my knowledge, there are no VFDs at the school or any devices that would cause the THD percentages to be so high when our pump is off. The school does receive three phase power to drive A.C. motors for 1-3 H.P. ventilation fans and water pumps using simple on-off relays.

So far, the engineers behind this project have been silent, so I am trying to increase my understanding about THD problems.

Thanks for any help, bstrom1953

possibly an measurement error? at 90% distortion the waveform would look more like a spike then a sinewave.

You may be worrying two much--but maybe not. You did not say how the variable drive was implemented. I presume that the harmonic percentages you give are of voltage. Under load, the harmonic voltage may go down significantly.

Bottom line: You need some one who can understand what is going on. Acting upon casual remarks from people like me without understanding is asking for trouble.

Bill

bstrom1953, I have been dealing a little bit with THD, also in relation to VFDs being used to drive pumps. THD is not an easy thing to measure, and I think it can easily be measured wrong. In the case of getting 90% THD, it might just be wrong, or you are correctly measuring the wrong thing. I've done some testing and I have found that a VFD driving a single pump at full load is likely to produce around 30% THD. At no load, the THD will be higher.

If you have a VFD, you have distortion. If you want to clean up the distortion, this can be done using either line reactors or harmonic filters. Line reactors are a few hundred dollars; harmonic filters are a few thousand dollars.

As a general rule of thumb, if 25% or less of your total load is non- linear (driven by a VFD), a line reactor will be sufficient to clean up the distortion. Example: if you have 4 50 HP pumps, and three of them are across the line starts, and one is VFD driven, you could get away with a line reactor. As far as damaging the motors is concerned, I don't think you have much to worry about.

FYI: You should be measuring current distortion, not voltage distortion. It is the job of the local utility company to make sure that the voltage distortion is within limits. It is your obligation as a customer to ensure that your load is not producing a bunch of current distortion. Your local power company will have specific limits which will be a function of the short circuit current available at your location.

Andy