VFD discussion related to non-inverter rated motors 2

[nightfox1925]

Hello folks,

Is there any thread here that discusses non-inverter rated motors being damaged by VFD's or any technical link that would help me understand how this is so?

Thanks

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[itsmoked]

Well that is kinda all over the place. Newer motors shouldn't have a lot of problems with it. Older motors can have a lot of problems. All motors may have some issues with bearing machining. Skoggsgurra would know on that.

Better would be a specific question. Can you be more specific? What size, voltage, application?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gepman]

There is a lot of information at Allen-Bradley. Go to

http://literature.rockwellautomatio...itle&SortOrder=Asc&passedLangVal=EN - English

which has a lot of white papers on the subject of VFDs. Select "Installation Considerations for IGBT AC Drives" which is on page 2. There is considerable information on the reflected wave phenomena in this paper.

About six years ago I designed the power distribution for a winery bottling line. The bottling line equipment supplier supplied both the VFDs (which were A-B) and the motors (which were Reliance at the time a Rockwell company). For some reason the supplier furnished the cheapest basic Reliance motor which has insulation rated at 1000V. Within two weeks the motors started failing, starting with the furthest motor from the control panel. Within three more weeks all 28 VFD motors failed, the closest motor failed last.

I don't think that anyone supplies motors without 1600V pk to pk insulation nowdays. There still is a maximum motor lead length which I usually get from manufacturer's tables which bypasses any calculations.

This is no endorsement of A-B drives, but I am most familiar with them over the other major brands.

 

[nightfox1925]

Thanks gepman, I will look into it deeper. By the way, is there a chance that insulation failure will occur to non-inverter rated motor even though the VFD is only used as a soft starter?

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[gepman]

I am sure that there are others out there that understand this better than I but the reason that the insulation fails is due to the dv/dt of the voltage waveform of PWM drives (and the reflected wave phenomena) which causes a lot of stress on the insulation. You can slow down the carrier frequency (A-B drives are standard a 4kHz but can be adjusted down to 2kHz (or up to 6, 8, or 10 kHz) but I doubt that this will be enough. Assuming that you have the VFD still running the motor at full speed you still have this dv/dt at all times. If you had a bypass contactor once the motor got to full speed then you would only have the dv/dt during starting which would be a lot less time for the insulation exposed to the stress but eventually you would have problems.

Why don't you just use an electronic soft-start which is cheaper and does not have the dv/dt issues that a PWM drive has?

 

[DickDV]

nightfox1925, your question is highly related to the voltage of your drive/motor system. At 230V, you can use an ordinary Insulation Class B or F motor as long as the motor is not hundreds of feet away from the drive.

At 460V, as a general rule Insulation Class B is not good enough. You need Class F Insulation.

At 575V, Class F is good enough only for the shortest of motor leads.

If you want to be safe, specify motors with a MG1 Part 31 endorsement. With that, at 460V, at 100hp, you can go out to 250 feet without additional apparatus. That number drops to 60 feet at 10hp. You can extrapolate linearly between those two points.

 

[gepman]

DickDV, I thought the insulation class for NEMA motors was a temperature rise rating and not a voltage rating. Is there a good correlation between the temperature rating of the insulation and the voltage rating?

I have attached a few pages from the A-B Powerflex 700 manual on motor lead length. I have followed the manufacturers length recommendations and never run into a problem. Interesting these tables have varied with the different VFD models of the same manufacturer and between manufacturers. I have always assumed that it had something to do with any output inductance, carrier frequency, IGBT type, and other factors that I don't know about. I always use the 1600V rated motors (NEMA MG1 Part 31) as DickDV mentioned. In these pages you can see the difference in allowable lead length with the different voltage ratings of the insulation (and with carrier frequency).

 

[electricuwe]

You should also consider that isolation is not the only concern, bearing damage due to common mode currents is also a concern.

But regardless of that there are many situations where it is possible to run a standard motor from an inverter without risk. The manual of the inverter should provide information on that.

If that is not feasible applying approbiate filters will provide a solution.

 

[Skogsgurra]

This Gambica report

http://www.gambica.org.uk/pdfs/Report2_2nd_ed.pdf

is very complete and contains "all you need to know" about VFD:s and motors.

There is just one little mistake in it; the common mode filters are NOT ferrite rings - they are iron powder toroid cores or wound tape amorph metal cores. Ferrites do not harm, but they do not help much either.

Also, the insulation system letter does not say that a motor can take the steep voltage edges . But most or all insulation systems with special "VFD hardened" insulation also have the higher letters (F and H).

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[gepman]

Skogsgurra

That is a great report and is very useful.

Also to expound a little on what electricuwe stated regarding output filters, the tables that I posted with maximum lead length show how some A-B VFD output filters and terminators affect maximum lead length so nightfox1925 can get an idea if they are feasible for his case. I always forget about these output solutions because I don't like to use them but they are useful if you don't have any other means to fix the situation.

 

[nightfox1925]

Thank you very much for all your valuable inputs. i will go through the literatures that you all provided keeping in mind the information I obtain from all your exchanges of opinions.

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