Avoiding all problems - frequency inverter installations 3

[Skogsgurra]

This is not an easy one. I am trying to compile a check-list for VFD installations in domestic buildings. And I am asking for comments on all the issues that come up. Do you have experience or knowledge about these items, then you are very welcome to share with us. Here they are:

1 Mains current distortion: Reactors? Active front end? PFC circuits? Other means?

2 Mains HF pollution: Filter? Isolation transformer? Other means?

3 IT grid and EMC filters: Configuration? Where to connect "ground"? How does the filter affect the insulation level? Other issues?

4 RCDs (earth fault breakers): What setting? Can they be used at all? Getting around nuisance trips.

5 Motor cable reflected waves: Motor reactors? dU/dt filters? Sine filters? Varistors? Other means?

6 Motor insulation problems: Motor reactors? dU/dt filters? Sine filters? Varistors? Extra insulation? Other means?

7 Motor bearing problems: Shaft grounding? Common mode filters? Insulated bearings? Hybrid bearings? Other means?

8 Load bearing problems: Insulated coupling? Equipotential bonding? Shaft grounding? Other means?

9 Bypass switch: Interlock. Shielded cable. Shield connection? Between which units to connect the switch? Other comments to that?

10 Safety switch (to isolate motor when working on it): Interlock. Design for 50/60 Hz or for low (almost DC) frequencies?

11 Derating at higher switching frequencies: Long cables and derating? Long cables and reduced switching frequency? Derating or reducing switching frequency when output filters are used?

12 Any other issue not mentioned here?

To avoid lots of posts recommending the manufacturer's manuals and recommendations, I need to mention that I have studied most of them (ABB, Schneider, Baldor, Siemens, SEW, AB...). I now turn to you, the users, to get some field input and "how to" comments. Your contributions are highly valued.

 

[dpc]

In the past we have seen problems with incoming feeder sizing, especially when an isolation transformer is involved. Some designers size the feeder based on the motor hp instead of the drive input kVA requirements. And if there is an isolation transformer, the upstream breaker must be sized to handle the transformer, and the feeder must be sized per the breaker rating (in the US at least). Not really a drive issue, but related, I guess.

Another issue is undersized drives when supplying low speed motors. Drives are often specified and quoted based on horsepower of the motor, rather than the drive output capability. We have seen drives that were too small for the motor when a low speed induction motor is used, due to higher current requirements.

RF interference from motor leads comes to mind as well.

But you've got a good list started, certainly.

 

[Skogsgurra]

And you added some very valid points, certainly. Thanks.

 

[DickDV]

Good grief, skogsgurra, that's a big enough list, isn't it?

Seriously, speed reference noise (shield, vibrating speed pot, etc) comes to mind.

Also, common reference conflicts when inputs are not isolated.

But, as I said above, the list is really long enough already! Shucks, you might be in a position to put some VFD manufacturers out of business! You wouldn't want that now, would you?

 

[Skogsgurra]

Well, DickDV... No.

But It is a pity that such a good energy saving device is such an EMI generator. Not to mention all the other problems.

I do not want to put VFD manufacturers out of business. But I think that it is about time for some straight talk about the problems. If we keep asking for it, we might even get VFDs with better EMC. That is what I hope for.

 

[GordS]

Also:
1. Expect greater sensitivity of ground fault detection devices on the same bus as the VFD. We had chronic ground fault tripping of our 600 V EHT monitoring system( tripping at > 30 mA ground fault...when the 600V drive on the same MCC as the EHT was ON, we would see up to 100 mA "groundfault"). The motor feeders were 3C Teck. Tried EMI filters - negligible change. Vendor tried to design a sine wave filter, filter burned up in 2 days. Finally moved the VFD starter to another distribution transformer and problem is gone.
2. Expect more issues if applying VFD's onto generators such as instability and generator damage if one isn't careful about the generator capabilities.
3. VFD's voltage sag ride-thru and recovery capabilities may be issues depending on the Process.
4. Hazardous area approvals of motors fed from a VFD need extra consideation.
5. VFD's are like computers - buy the time you have installed it, there is a new version out and technical support on the old version as well as spare parts start to decline.
6. Be aware of additional large budget maintenance items such as replacement of electrolytic capacitors. This cost needs to factor into the economics to justify a VFD.
7. There is a need to consider mechanical resonances at higher horsepower ( torsional analysis)
8. There is a need to consider sizing drives for charging current on low HP applications.

 

[Skogsgurra]

Thanks GordS,

I read that with great interest and some nods of acknowledge.

 

[thinker]

I would suggest on your p.1 when talking about mains distortions to consider not only current distortions but also a voltage distortions which are much more significant for PWM based front end converters.