harmonics?

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mickeyrench

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We have a 55k vfd 1990's type 75 hp motor 480 v. Used a fluke 337meter and tested the frequncy on the feed going into the vfd was 180hz and the output to the motor was 55 hz. This i don,t understand. Can someone explain it to me? thanks
 
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mickeyrench said:
We have a 55k vfd 1990's type 75 hp motor 480 v. Used a fluke 337meter and tested the frequncy on the feed going into the vfd was 180hz and the output to the motor was 55 hz. This i don,t understand. Can someone explain it to me? thanks
Click to expand...
Yes, harmonics as others have said.
The 180Hz (third harmonic) is a little surprising though. Given the power rating and voltage, I assume that the VFD has a three-phase input. The significant harmonics associated with such a drive are 6n+/1 (i.e. 5th, 7th, 11th, 13th etc.).
If your measured input frequency was based on voltage, then maybe some other piece(s) of equipment are causing the third harmonic voltage distortion.
 
Well just thinking a little vfd we all know create harmonics , you need to filter the vfd or vfds there is a Line Reactor basically three coils on each phase ahead of drive to slow backfeed of trash feeding back into ac input line . The switching of the SCR s chop up the ac input this filters that out not all but most you can purchase these from the vfd manufacture .These are not pf correction caps as you can not use a capacitor on a vfd not a good thing . most important they are as close to the motor drive as can be on ac input only to drive . one more added input your readings on a drive with any meter is mostly strange do to the freq voltage and the current changes dont be alarmed its normal nothing is going to be normal on volts or freq . comments ?take care best to ya
 
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Thanks for the replays, most are very informintive.[ Please excuse the spellings]. Now we know why the vfd is dropping out when starting another motor across the line, that never caused a problem before. We have 2 identicle vfd's and the same one only drops out .

I think we will have to get power quality contractor to address the problem with the 180 hz. Thanks Ohmhead for the great info.
 
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mickeyrench said:
Thanks for the replays, most are very informintive.[ Please excuse the spellings]. Now we know why the vfd is dropping out when starting another motor across the line, that never caused a problem before. We have 2 identicle vfd's and the same one only drops out .

I think we will have to get power quality contractor to address the problem with the 180 hz. Thanks Ohmhead for the great info.
Click to expand...

If only one VFD drops out it, could it be that it has lost one of the three phases? That could explain the source of the third harmonic. Single phase rectification produces third harmonic.
 
Phase loss relay

Phase loss relay

Well most VFD s Ive installed have a phase loss relay which when you drop one phase it drops out the vfd , meaning there factory installed part of the drive and this should be displayed on the lcd screen on the drive . Also they have a over and under voltage over current over freq ect ect --. so that should have been displayed on the drive lcd screen ? comments best to ya
 
ohmhead said:
Well most VFD s Ive installed have a phase loss relay which when you drop one phase it drops out the vfd , meaning there factory installed part of the drive and this should be displayed on the lcd screen on the drive . Also they have a over and under voltage over current over freq ect ect --. so that should have been displayed on the drive lcd screen ? comments best to ya
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Some do. Some don't. Without knowing the inverter type and vintage it is not possible to be categoric on this point. One thing I would add to that is anything recent enough to have a built in LCD display is unlikely to employ SCRs as a control measure in the front end.

But my point was about third harmonic. Three phase VFDs under normal operating conditions don't produce third harmonic to any significant degree. That it was detected implies that it is present either on the supply or possibly as a result of a drive fault.
 
delta pri blocks triplins

delta pri blocks triplins

Well you are correct ,but what can one do that 3rd harmonic is present and its there ,so we install a filter line reactor but there not enough ? drive may have some bad scrs ? install a delta /wye transformer to isolate the harmonics no good we need 480v for motors or maybe delta/delta trans 480v isolation harmonic type copper with double the size kva to block the harmonics . someone told me that the harmonics can be trapped in the primary of a balanced 3 phase delta ? comments ?
 
Does your drive use a PWM waveform?

Does your drive use a PWM waveform?

The PWM (pulse with modulation) waveform can screw with measurements. Is your drive setup for PWM?

Is the other drive giving a different result?
 
VinceS said:
The PWM (pulse with modulation) waveform can screw with measurements. Is your drive setup for PWM?

Is the other drive giving a different result?
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It is usually just the VFD output that is pulse width modulated. The OP mention input frequency measurement which would be derived from the supply.
 
ohmhead said:
Well you are correct ,but what can one do that 3rd harmonic is present and its there
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If the meter reads 180Hz then yes, this would indicate the presence of a third harmonic component. It should not emanate from a VFD with a three-phase input unless the VFD is faulty.
ohmhead said:
so we install a filter line reactor but there not enough ?
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In general, in my experience at least, line reactors don't do a whole lot for lower order harmonics.
ohmhead said:
drive may have some bad scrs ?
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The VFD is unlikely to use SCRs. Most have an uncontrolled 6-pulse rectifier at the front end. It is possible, but unlikely, that one or more has gone open circuit. Or maybe an input phase connection has been lost.
ohmhead said:
install a delta /wye transformer to isolate the harmonics?
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That would prevent third (and triple-n) harmonics getting back into the supply but, as mentioned above and in previous posts, the VFD shouldn't generate such harmonics in the first place.
 
vfd

vfd

Well we are interested in third harmonics odd harmonics why is the 3rd harmonic present at the vfd ? And if there is no S C R s what is used to control the frequency of the drive ? Were interested in learning not questioning your points made just , trying to understand better . I know that there is 6 diodes on the dc power supply but i always have seen SCR s or TRIAC s to change freq of the drive output are these not a triac ? also iam just a electrician not a engineer but dont hold anything back ill let you know when i get lost . comments best to ya
 
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ohmhead said:
Well we are interested in third harmonics odd harmonics why is the 3rd harmonic present at the vfd ?
Click to expand...
We don't know. As I have suggested, it could be a fault with the VFD on the supply as a result of other connected harmonic generating loads.

A slight digression here......
Some years ago we had a project for the upgrade of a water pumping station here in UK*. The project included eight relatively small variable frequency inverters, the three largest being 110kW. Typically for such projects, we had to provide guarantees on total harmonic voltage distortion and compliance the current Electricity Association engineering recommendation. You have to commit to this prior to award of contract so it is based on calculation only. Getting it wrong is not an option. Non-compliance incurs serious financial penalties plus the cost of remedial measures. I included the usual get out of jail statement - "this excludes the effects of any background harmonics already present on the system".

Having been awarded the contract we had to demonstrate compliance. This required 24-hour monitoring before and after installation. Perhaps not surprisingly given that this was in a mainly residential area, the supply voltage distortion was considerably outside compliance limits with none of the VSDs running - mostly because of third harmonic. Many single-phase loads are non-linear resulting in significant third and other triple-n harmonics. Over the years I have seen a gradual degradation of supply quality. I shouldn't grumble - it provides some work for me.


ohmhead said:
And if there is no S C R s what is used to control the frequency of the drive ?
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Usually IGBTs on LV inverters.
Some MV drives us GTOs.

*A further diversion....
The particular project was in St Albans, a city with lots of character and history. It should be a "must visit" for anyone coming to UK.
 
Drive issue

Drive issue

Well thanks for the response , yes today IGBT s but in 1990 thats old so i was thinking scr as they were popular back then. I agree 3rd harmonics are neutral related and straight 3 phase non neutral is high 5th to 7th ect ect , so we are now at a point of saying no the vfd is fine and the problem is in the neutral of the service ? And if thats the case then its not from the vfd . Next we need to look at why the voltage is higher on that vfd if the problem is ahead of the drive why is the voltage higher than normal ? Just wondering why comments best to ya
 
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