Motor insulation Class and VSD
Motor insulation Class and VSD
(OP)
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
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





RE: Motor insulation Class and VSD
Can you run a B class insulated motor on a VSD? Yes, but it won't last very long. You need one rated for varibale speed drives.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Motor insulation Class and VSD
Most will likely run fine with a VFD. But there are really a whole bunch of variables.
1) How was the actual motor assembled? Well or shoddy?
2) How long is the cable between the VFD and the motor? The longer the worse.
3) How hot does the motor run? The hotter the worse.
4) Which brand VFD?
5) Which brand Motor?
6) How close to the motor's nameplate hp is it run?
The list goes on and on.
Then there are remediations like power reactors in the motor leads that carve off all those spikely corners Turbine mentioned.
It comes down to often people use what they have. If you need variable speed and he loss of a motor isn't going to break the bank you can just try it. The motor may last years or it may not. If you want to 'help' add a reactor to the motor leads and keep the leads as short as possible.
If you're buying new motors reach for better class,(tougher), motors.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Motor insulation Class and VSD
RE: Motor insulation Class and VSD
The voltage spike issue is separate. IF you are running these at 480V, it will definitely be an issue that must be dealt with. But you can get very effective load filters that can be easily added downstream from the VFD that will protect standard motors for probably longer than anyone cares about in an HVAC system. Some VFDs even have filters built-in to them anyway, especially some of the more common "HVAC" drives, because the retrofit market for older existing motors is huge. That's the first thing I would check in to. The next thing to check is if the motor mfr used the same "spike resistant" magnet wire anyway. Some of the bigger motor mfrs are doing that now, because they buy it in enough bulk that it makes economic sense for them to not have to inventory different styles of wire. "Inverter Duty" motors have that type of wire, and incidentally they also only tend to use Class F just because again, it wouldn't be economically feasible to have special wire in lower insulation class when the difference in cost is probably insignificant. Inverter Duty motors have other benefits that would have made them a good idea, but not so much that I would tear out perfectly good working motors, especially new ones.
If your system is 240V, chances are you may not even need to be concerned about it unless you have some extreme distance from the VFD to the motor, i.e. over 150 feet (that's an arbitrary number). I know people who say this isn't true and the spikes created on 240V systems can damage motors as well, but my experience is that this problem was never noticed until it started to become a problem on 400V class systems, then the research came out to claim it happens on 240V as well. That may be theoretically true, but how is it then that so many millions of 240V motors survived for so long until then? I think it's a bit of a red herring promoted by magnet wire and filter manufacturers. Adding filters will not hurt ad there are other good reasons to do so, but if you are in a tight spot, you may consider "going commando" with those motors.
Either way, I see no reason to go to the expense of a prophylactic change of the motors. If they are 480V and need filters, just add them. If they are 240V, see how long they last and THEN replace them with better versions.
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RE: Motor insulation Class and VSD
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If we learn from our mistakes I'm getting a great education!
RE: Motor insulation Class and VSD
Second, as jraef mentions, this question is all about voltage. At 240V, don't worry about it. At 400V, you might have a problem if the motor leads are exceptionally long. At 480V, you have a problem unless the motor is over 100hp and wired right next to the VFD. At 575V, you for sure have a problem and a motor lead filter will be needed to get even a week or two service from the motor.
Third, motor lead reactors and dv/dt filters are inexpensive and make good insurance if you are at all in doubt about your situation.
Fourth, definitely buy replacement Class F motors when the present Class B machines fail if you are operating at 400 or more volts. The increase in cost is minimal and you can sleep nites again.
RE: Motor insulation Class and VSD
Are they actually Class B insulated, or Class F insulated with a Class B rise? I haven't seen a new LV motor with Class B insulation for ages; maybe I'm looking in the wrong places.
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If we learn from our mistakes I'm getting a great education!
RE: Motor insulation Class and VSD
If that is true, a lot of Class B motors would be suitable for VFD usage and would provide a less-expensive option to motor buyers, especially in HVAC.
Is there some way to discover this information beyond simply looking at the nameplate?
In my classes, I've always advised my students to buy MG1P31 motors on 480V service, even for fans and pumps. That was primarily because there is a lack of reliable specs for any other terminology such as "inverter ready" and "inverter duty". Maybe there is another option.
RE: Motor insulation Class and VSD
All our rewinds are done using Class F insulated wire, mainly because anything else would be a 'special' - and thus more expensive - even if was to an inferior spec. We don't have any Class H machines, so it it's never been an issue.
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If we learn from our mistakes I'm getting a great education!
RE: Motor insulation Class and VSD
Anyone know for sure?
RE: Motor insulation Class and VSD
In olden days, the designer's mandate was that the motors should last their life-time. Now, they need to just last the warranty period. Which of course, keeps us, the dirty winders, happy.
As for the OP's question on VSD and winding insulation, most of the VSD motors fail due to interturn shorts because of high dv/dt. The turn insulation in LV motors is a very thin layer of varnish enamel. Both class B and original class F enamels could not withstand this high dv/dt. I have rewound many class F motors, which failed quickly when the clients ran them on VFD's. The wire-coating enamel for the VFD duty is a very complex (and expensive) chemical, which just happens to have a high thermal class index (typically class H) and hence the myth class F insulated motors are automatically rated for VFD duty. Both class B (prior to seventies) and class F rated normal motors are more likely to fail on VFD duty.
My suggestion would be to run the present motors on VFD and upon their failure, rewind them to VFD duty. (Hey, we, rewinders, are an endangered species, and need to be preserved.
The problem is not so serious with MV motors with pre-formed coils since they have sufficient thickness of inter-turn insulation to survive the VFD duty. Of course, these MV motors could be derated for other reasons like higher core-loss and higher conductor eddy current loss due to harmonics.
Muthu
www.edison.co.in
RE: Motor insulation Class and VSD
RE: Motor insulation Class and VSD
Muthu
www.edison.co.in
RE: Motor insulation Class and VSD
Here in Australia and from my experience in the UK, Class F insulation is the norm but can really only be used when referred to the temperature rise classification. The 'norm' being Temp rise Class B.
This means (temp rise B), that a temperature rise of 80 degC in an ambient of 40degC is accepted.
Class F temperature rise means a temperature rise of 100degC in an ambient of 40degC is the limit.
Class H temp rise is the highest and this is a temp rise of 125 degC in an ambient of 40degC.
So, if your ambient is lower, you have more headroom.
A (relatively) new standard in Europe is EN 60085:2008 and this determines Electrical insulation- Thermal evaluation and designation.
However, let's get realistic and look at your industry: HVAC. This is where the lowest cost components is the norm, so you take your (electrical) life in your own hands unless you are prepared to spend a little more and get a longer life.
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. Above this and you will always need some form of filtering out from the VFD.
RE: Motor insulation Class and VSD
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RE: Motor insulation Class and VSD
The reason is that, as long as the voltage front is being built up (not reached its maximum value), the reflected voltage at the motor terminals "steps on" a lower voltage than the maximum and the combined voltage (incoming plus reflected) will not reach its maximum possible value.
It is when the cable length corresponds to rise-time times wave velocity (usually 60 - 70 percent of speed of light) that you get the maximum voltage rise. It is usually something like 1.8 - 1.9 of the DC link voltage.
When cable gets longer, the voltage front gets dispersed and therefore does not cause the same high voltage. That means that voltage stress decreases when you get above a certain cable length. I think that would be an interesting fact to add to the thread.
http://www.rema.uk.com/pdfs/vsd1.pdf page 4 has a graph (MOTOR PULSE WITHSTAND CHARACTERISTIC CURVES
PEAK VOLTAGE/RISE TIME) showing this for a set of risetime, peak voltage, cable lengths and other parameters. Good read!
Gunnar Englund
www.gke.org
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RE: Motor insulation Class and VSD
I'm not that familiar with vfd's, but I was under the impression that for repetitive pulses from vfd (as opposed to single pulse from switching surge), there could be resonant effects that increase the peak voltage far more than twice the source voltage. Is it an incorrect impression? (I could be wrong).
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RE: Motor insulation Class and VSD
It would be very unfortunate to have resonance at the output of a PWM VFD. It can happen if undamped motor reactors resonate with cable capacitance and VFD carrier frequency. That would certainly produce a large overvoltage, but it is a vary rare situation and is easily avoided if normal design rules are employed. I have put together a collection of wavforms from different jobs in the attached pdf. I hope that recordings and comments are helpful.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Motor insulation Class and VSD
Yours
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor insulation Class and VSD
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.
RE: Motor insulation Class and VSD
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.
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.
RE: Motor insulation Class and VSD
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