Motor insulation Class and VSD 3

Hello all

I have been told that motors need to be insulation class F for variable speed drives. I have several motors with insulation class B.

does anyone know why I need insulation class F?
has anyone put a VSD on an insulation class B motor and lived to tell about it?
what would happen if I put the VSD on a class B? over heat? burn up? last more than 1 year or 1 day? all questions that are difficult to answer.

The motors are for an HVAC application(5 pumps, 3 at 7.5 hp and one at 20 hp) they were missed as part of an energy project and I am trying to figure out the best way to handle at least expense(of course)

I had this on the HVAC forum and it was suggested that I put it here also. did not even know this forum existed but it looks good and applicable to HVAC applications. Will have to put this on my list to check regularily.

Thank you

 

[RC20]

One thing that is missing from the discussion is the need for insulated bearings to go with the VFD as well as shaft grouding.

Our experience is that bearing damage via ball arching manifest itself quickly (inside of 3 months) or may take longer (one year) but it happens. The balls and races get pit damage, and the grease deteriorates with the arching. Take you choice of what causes the failure first.

To date the only satisfactory answer has been ceramic hybrid bearings. Grounding does not always work, despite claims to the contrary

A truly Inverter duty motor would have: insulated bearings, grounding brush system and the correct insulation.

 

[LionelHutz]

What Mutha posted is pretty much correct from what I've seen. I see the rewinders who fix the motors so they just "work" being the ones that are staying busy.

The biggest problem I've seen is that the manufacturers are sloppy when winding motors. They have sometimes have the lead ends from coils of different phases touching each other. This is where the good rewinders are keeping their business - they rewind with care to the coil installation and they use lots of phase to phase insulation in the end turns (among other things) so the insulation does not have stress points and will last.

Overall, I'd say it's a total crap shoot if the motors will last or not. Overall though, I'd say the larger the motor the more likely you'll be OK and the single digit hp motors you have definately aren't large.

Another thing to touch on - you say installing these VFDs on pumps is part of an energy savings project. Well, make sure you will actually save energy because with many pumps you really don't save much or anything installing VFDs.

Motor manufacturers like Siemens, ABB and WEG all now produce high specification motors with windings capable of withstanding the rigours of PWM spikes and pulses under 500Vac supplies.

FYI, from what I have seen and can tell from their literature, WEG uses the same wire all the time no matter what type of motor. If you order a MG31 rated motor WEG will drill 2 holes and rivet a tag onto a standard motor. I'd suspect the others in your list employ similar practices.

 

[electricpete]

Thanks Gunnar. I would guess the top example you posted has a "resonant" frequency of around 4 cycles in 10 microsecond = 400khz, far above any likely switching frequency. I imagine there are plenty of variables that go into determining that resonant frequency. A topic for another time maybe. Thanks for the response.

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