Motor insulation Class and VSD 3

VSDs require a heavier insulation on thier motors due to the waveforms they generate. They repeatedly pulse the motor at full voltage to acheive a sinusiodal average signal. In reality if you watch it on an oscilloscope you'll see it is variable width square wave pulses. This puts significantly higher stress on the insulation and will cause it to fail early.

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.

whatapainintheass; It's very hard to answer. It's a bit of a crap-shoot. I have seen many class B motors run full lives under a VFD.

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 -

itsmoked has a good answer. I would emphasize his point that you should consider keeping the class B motors in service. Since they have no resale or scrap value, why not run them until they fail, whenever that may be (days,weeks,months,years?).

Insulation "Class" has to do with temperature rise, not voltage spikes. VFD controlled motors should have higher VOLTAGE ratings on the winding insulation, but that is NOT reflected in the class rating. Class F is a really good idea, but not an absolute. If for instance your pumps are in a well ventilated space, Class B would probably be fine. Class B means it can operate in a continuous environment of 40C and allow the winding temperature to rise by another 80C. If they are not, i.e. they are in a hot mechanical room, you need to evaluate the ambient situation and the extra heating effects the VFD operation will have on the motors. When a motor is slowed down with a VFD, any cooling fan (if part of it) slows down as well. At really low speeds, i.e. below about 40%, motor fans can become completely ineffective. But HVAC pump applications rarely if ever call for speeds that slow, because the pumps cease to work as well. You can also just add separately powered cooling fans to the motors that do not vary speed with the motor speed if the pumps will run at slow speeds for long periods. This is done all the time.

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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These days you will be hard pressed to find a class B machine, unless it is a *really* big one, because class F insulation allows manufacturers to run the core closer to saturation and put more current through a winding of a given cross section, both of which cause the machine to run hotter. This is presumably a retro-fit application on existing motors?


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If we learn from our mistakes I'm getting a great education!

First, brand new Class B motors are still very common.

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.

Hi Dick,

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!

I see your point, ScottyUK. I'm just going by the nameplate. As to what wire is actually used internally, I simply don't know.

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.

I'll have to let the rewinders answer that question! Of course, if they let such information loose into the public domain then they won't be able to charge a premium.

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!

I've known that, here in the US, rewound motors would generally have Class F wire. But, for new Class B motors, I wonder.

Anyone know for sure?

I will let you in on the rewinders' secret. Class F has been de facto insulation for over 30 years with OEM's. Only difference, they name-plated them till the nineties as class B in order to ensure a long life as a factor of safety. But ever since the bean counters started running these OEM's, factor of safety be damned and so the newer motors are rated class F, which is basically pushing the envelop of a particular frame.

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. . Class H has been de facto insulation in my shop for LV motors for nearly 20 years now.

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

OK, I've got what you said, edison123. One last open question then: Assuming a brand new off-the-shelf motor, would the expected life of a nameplated Class B motor be any different from a nameplated Class F motor, neither of them inverter rated or MG1P31?

DickDV - I doubt if you can get a new class B motor now-a-days, but if you find one, go for it. It would last longer but possibly be a frame size bigger.


Muthu

I was always of the opinion that you couldn't talk about insulation class without talking about temperature rise limits.
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.

I was always of the opinion that you couldn't talk about insulation class without talking about temperature rise limits.

Click to expand...

True if the subject if thermal margin. Here the subject is turn insulation's ability to withstand high dv/dt from vfd. The thermal class is just being used as a proxy to discuss the typical material types used.

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It has been mentioned in this thread that insulation stress increases with cable length. That is true, up to a certain limit. The stress is then reduced as cable length increases.

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.

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
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A factor of close to two sounds like the number we can come up with for a single pulse, reflected by an impedance mismatch at the end.

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

Thank you for the information Gunnar.
Yours
Bill

Bill
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