looking into a VFD / motor installation, on the output of the VFD to the motor is approx. 200' away and is NEMA MG-1 inverter duty rated with 1600V insulation, looking at two technologies to put on the output of the drive either a reactor or dv/dt filter to help with the potential voltage reflection issues, looking to utilize VFD 1000V rated cable, but am curious what opinions are on the reactor vs dv/dt filter and pros and cons, I have been doing some research and appears the filter is the better solution but would like to hear from others on this.
VFD output reactors or filters?
- Thread starter Mike01
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Often the VFD chosen will have an associated filter. With that length of run, the filter likely needs to be at the motor.
Puting in a reactor will reduce the dv/dt.
Given that the motor is inverter duty rated I would have thought a filter unnecessary.
Most of the inverter manufacturers I have dealt with don't normally recommend either until you have cable runs of over 100 metres which is a bit over 300 feet.
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I agree with Besoeker. We also use inverter duty rated motors and provide optional filters or reactors to be installed at the input to our packages, not the drive output.
Merry Christmas,
Mark
skeshesh
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Besoeker would you mind elaborating on that a bit? What is the technical concern with inverter duty?
Please forgive me if I'm repeating stuff you already know. But, since I don't know what you might already know....
The basic circuit arrangement of most variable frequency inverters these days is a rectifier bridge to convert the AC supply to DC and then an inverter bridge to switch it back to AC at the required power frequency.
As you are probably aware, the output voltage from most inverters is a train of square wave pulses at many times the power frequency, typically a few kHz, and with a pulse width modulated pattern. Even though the motor gets a string of square wave pulses, its inductance means that the current can't change quickly and the result is usually a pretty good sine wave current.
But, and there had to be one, inverters generally use IGBTs as the switching devices to produce the train of pulses. The IGBT switching rate is very fast. It takes a fraction of a microsecond to turn on. As a result, the motor winding can get hit with a voltage rise of several thousand volts per microsecond - the dv/dt issue. This induces capacitive currents that would not normally be seen with the very much lower dv/dt of a "clean" sine wave voltage. (i=Cdv/dt).
I sometimes get involved in motor failure problems with inverters.
Here's a bit from one of my reports where motors were routinely failing after just a few weeks operation.
"Conclusion
The insulation failures have the appearance of partial discharge failures such as would be created by the very high electric field intensity caused by the very fast dv/dt. The recorded dv/dt is as high as any we have measured at or close to the motor terminals.
........
In summary, the installation of dv/dt filters (chokes) to reduce the rate of change of voltage applied to the motor is highly recommended."
In the event, chokes were fitted and the problem went away.
skeshesh
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No problem. Firstly I was the one who asked for elaboration. Secondly I find that repetition works well to solidify concepts, at least for myself.
I think I may have misread your post which read: "Most of the inverter manufacturers I have dealt with don't normally recommend either until you have cable runs of over 100 metres which is a bit over 300 feet."
I took this statement to mean that there will be an adverse effect on the inverter duty machine, whereas I think what you are saying is that it's simply not required.
To my knowledge, and I'm by no means an expert on this, the inverter duty machines use a special grade magnet wire as well as a class F or higher insulation to minimize the voltage spikes caused by high frequency switching used in power converters. I don't see how adding what is basically an LC filter would have an adverse effect.
Well, not from a technical point of view.
Commercially is another matter.
skeshesh
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Can't argue with that!
Yes. I could have phrased it better.
I'm not a motor designer so I can't claim to have expertise in that field either.
My expertise, such as I have, is in the area of power electronics and applications of that. To that extent, you can't treat the power electronics, and the driven machine in isolation. And sometimes it takes the problem projects to put you on a steep learning curve.
A few failures of newly installed 800 kW drives motors tends to get a bit of attention from all parties involved. Meetings about who will pay can quickly degenerate into the blame game.
But we learn from such experiences.
In most cases I install a reactor on the input to the drive to midigate harmonics from getting in the power system. As for you motor application a reactor would be my choice on the output to reduce the reflective wave to the motor. Sometimes you use a filter and reactor but this is on very long wire runs.
robbietan
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- Antipolo City
I would put chokes on both the line side and the load side of the VFD. the chokes at the line side will limit harmonics from the VFD from going to the electrical system and the chokes at the load side will protect the motor form the harmonics of the VFD
Getting the cue on the above-mentioned fact, that the wave pulses are modulated pulses at a few kHz, is it possible for you to do away with output filters by lowering the wave carrier frequency? We did it once but I guess different installs require different solutions!
Yes, horses for courses.
Reducing the carrier frequency will reduce the switching losses in the inverter IGBTs. The effects on the motor are a bit mixed. The fast switching edges of the individual pulses are still there as is the potentially damaging high rate of change of voltage, dv/dt. The potential damage is degradation of the motor winding insulation and it is cumulative with the number of pulses. I've seen some failures that happened over a few weeks and others over a few months. If the failure is going to occur, reducing the carrier frequency won't prevent it happening - it will just be longer before it does.
A down side to reducing the carrier frequency is that it will increase the ripple current in the motor windings with an attendant increase in losses. It may also make audible noise more evident. For the sort of work we do, noisy pumping stations and paper mills, this would probably go unnoticed.
The choke/line reactor on the line side of the drive will do very little to reduce harmonics transmitted from the drive back into the power system. A 3% reactor will reduce voltage THD from approx 43% to 39% - this is such a slight reduction that it's not worth the money.
The reason to install reactors on the line side of the drive is to minimize the number of AC line transients that make it through the rectifier and turn into DC bus transients. Large magnitude DC transients cause DC bus overvoltage or undervoltage alarms and trips. DC link reactors will also help with the problem, but depending on the design of the power system a DC link reactor may not be enough.
At my plant we have a utility 138 kV substation onsite with a 138 kV capacitor that is switched daily. Some of the cheaper drives with missing or undersized DC link reactors will trip on bus overvoltage when the capacitor is switched in. A line reactor, higher class drive with a properly sized DC link choke, and possibly some drive programming changes will solve the problem.
The VFD doesn't produce harmonics on the motor terminals in the traditional sense. In a purely technical sense any non-sinusoidal waveform can be declared to contain harmonics and mathematically modeled as such, but this is a technicality.
The load side of a VFD will experience reflected waveforms, meaning the chopped square wave output waveform of the VFD is reflected back and forth on the motor leads between the motor and the drive. This is caused by the impedance mismatch where the cable meets the first few turns of the motor winding. The reflected waveform means that the cable will have twice the VFD DC bus voltage on it, approximately 1500V. This is why standard THHN in conduit will fail prematurely if it experiences reflected waves. A high voltage somewhat lower than 1500V will appear across the first few turns in the motor, which is why 1600V or higher phase insulation is required on motors used for VFD duty.
A reactor either at the drive or at the motor will knock spread out the edges of the square waves, increasing the rise time, and lowering the reflected wave voltage stress on the cable and on the motor winding. Reactors are generally applied on the load side of the VFD, not at the motor, but there is nothing preventing the use of a reactor at the motor.
Normally the device used at motor terminals is a resistor and capacitor network called a snubber that is tuned to have the same high frequency impedance as the cable. When a cable or transmission line terminates into its characteristic impedance reflected waves are minimized.
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