Inverters with non-inverter rated motors

Just curious-
What experience have you had with inverters
using non-inverter rated motors?
I know the difference between the two, but what are your experiences- good or bad?

We have lot's of inverters on standard motors - including some quite old ones - that work just fine. Inverter duty motors essentially have a higher class of insulation to resist the transient voltage spikes you get from a PWM (Pulse Width Modulated) VFD waveform.

If the length of the power leads to the motors are short this isn't a problem. If the leads are long, (varies with power and VFD type, but generally > 50 feet) then you may need to consider a line reactor or reflective wave trap to mitigate the harmonics and transients. Generally speaking Class F or Class H motor insulation will not cause problems.

You of course also need to make sure that a constant torque load is not running so slow that motor cooling is inadequate. Usually 50% or faster is OK. Inverter duty motors have that same restriction too.

Unfortunately, the term "inverter duty" is part of the snake oil that flows abundantly in the drive-motor field. Each manufacturer defines it differently with some on the low end calling any motor with Class F insulation "inverter duty" or maybe "inverter ready". On the high end are those manufacturers that will only put that title on motors that have either an auxiliary blower or are rated full torque down to stall by virtue of oversized framing or a TENV enclosure.

A couple of things can be said without qualification, tho. First, inverter duty ratings as far as insulation is concerned are only a factor in 380VAC and higher applications. Second, in those higher voltage applications, an insulation class A or B motor is NEVER good enough. Under the best of conditions, motor life is measured in days and weeks, not months and years. Class F insulation, at the least, is required.

Further, heat management issues in motors are really a separate consideration from insulation integrity issues. There are many applications where Open Drip Protected or Total Enclosed Fan Cooled motors are plenty good enough even at low speeds. You must understand how the load uses torque over the speed range and how that compares to the manufacturers' motor heat dissapation curves to be confident you've got it right without over-specifying.

And don't forget about ambient temperatures. A motor operating in a cold storage warehouse will likely be able to work harder than the same motor located next to a furnace or other heat source.

Unfortunately, the subject of long motor lead lengths, reflected waves, etc. is being muddied by more snake oil. If you talk to reactor manufacturers, any motor with leads over 50 feet in length is sure to fail while some drive manufacturers state clearly in their manuals that at least 250 feet lead length is OK without the addition of extra hardware. My own experience is that problems with reflected waves and especially electrical noise (another ugly subject full of snake oil and false claims) are highly unpredictable. I've seen problems on short leads and no problems on long leads. I hate to admit it but, most of the time, I'm baffled! I try to write my quotations in such a way that I can work as a partner with my customer to solve any problems that come up, rather than as an antagonist. That way, I can quote the simplest system that will likely do the job and, if there are problems, we'll deal with them as efficiently as reasonable people can. Usually, but not always, that works out quite well.

It looks like we are on the same page, Dick. If JRW is looking ofr a short answer, it is, as always, "It depends".

As long as you brought up snake oil, I would like to add my pet peeve, IEEE-519 compliance. This spec for "clean power" VFDs relates only to the line side of the VFD, not the load (motor) side. An 18 pulse "clean power" VFD won't inherrently help your motor life.

And even on the line side, I can buy a lot of filtering equipment for my PLCs and PCs for the cost of an 18-pulse VFD!

I must be one of the lucky ones, I have installed over 100 inverters on a whole range of motors (0.2kW up to 150kW) some up to 20 years old.
I have only destroyed 1 75kW motor, although 1 75kW motor is more than enough.
We used to have more problems with older Inverters, but the newer ones are much more inteligent and offer much more in terms of motor protection.

In most of my applications the inverters have increased motor and machine life rather than reducing it.

Barry

To Barry Humphries. Lucky, maybe, but Class F insulation has been around a long time. I've used old motors often on inverters, as well, with good results. But, I'll bet you looked carefully at the nameplate to be sure it was up to par before using it. And, of course, 380V isn't quite as demanding as 460V and certainly not as demanding as 575V like our friends in Canada have to deal with.

To Tom Jenkins. Sound to me like the same page too. You are certainly right about IEEE 519 compliance. Anyone familiar with this current and voltage distortion specification will know that compliance is determined by the complete power supply network, from distribution transformer to all the loads on that service. To state that a particular inverter is IEEE 519 compliant is completely baseless, or "snake oil", as you said. And this business about 12 pulse and 18 pulse! Sigh!! There are filters out there that cost less than the upcharge for these special inverters, waste less energy, and produce cleaner power on a basic 6 pulse drive (MTE's Matrix filter is one of these filters) and almost no-one ever specifies them. Most A & E firms are clueless about IEEE 519 and how to acheive compliance.

It's not that IEEE 519 is an undesireable or useless spec. Quite the opposite. But getting the performance and being able to verify it--that's the tricky part. And your comment about protecting the small sensitive loads with filters, transformers, etc.......I've often wondered why that route isn't taken rather than clean up the 150hp system with very expensive filters, transformers, etc. Of course, this assumes that the supply transformer can handle the harmonics. If not, you really don't get a choice. You have to fix the problem at the source = big money!!

Barry, new drives use IGBT transistors and these are a cause for motor failure over long cable rungs >50m. They fire on and off much quicker than previous, older types. On commisioning of my site we lost 25 motors with a blown first winding turn within three months. These motors were on VSDs. We had 250 odd VSDs so we had 10% failure rate. The reflective wave caused by the IGBTs overshooting their "firing point" ( I think thats how to describe it ) was being magnefied over the long cable runs due to "refective wave" and was measured at the motor terminals at being 1400V in some cases. This is why the first turn was always blowing. We added some extra filter to troublesome units and it fixed the problem.

It took some months to work out the prob, but was quite a interesting job. We blamed the motor manufactor and switched companies believing it was a build problem but when we discovered the real prob despite the embarrassment we did inform them.

Still dont quite understand it all though........Not much out there on info on the subject.

TimeFluxCap. You are certainly right that older drives, those with Darlington transistor and SCR outputs were much easier on motor insulation. With those you might even get away with Insulation Class B and not get premature failures. I had forgotten that.

I'm curious how long the leads were on the failed motors you mentioned?

Guy's

Let's discuss the term "OLD MOTOR". Just because a motor is old, does not mean that it is an old motor. If the motor has ever been rewound then it is not an old motor. What most of you may be seeing is and old motor may have been rewound one or more times. Usually when this is done the motor shop will install a higher grade of insulation in the windings than the motor originally had, or is stated on the nameplate. If this has been done then you no longer have an "OLD MOTOR", you have an old motor housing with new higher rated motor windings and bearings.

This may be why some of you are stating that you have installed VFD's on "OLD MOTORS" with out experiencing any problems. I can assure you if you install a VFD on an old motor, which has never been rewound that has less than class F insulation you will be replacing that old motor, it’s just a matter of time.

Yes, there is some snake oil in regards to reactors and some motor manufacturers definition of inverter duty motors, but makes no mistake you should an inverter duty motor to get the maximum life out of your installation.

Mike

It is true what you say about old motors, when most of our motors are sent away for re-winds they tend to come back better than new.

All the inverters that we install have Class A filters as standard, the bigger ones will also have chokes. The longest Cable run between motor and inverter is about 25 meters. So I guess its as much about good practice of installation as drive quality.

Mike's comments about rewound motors almost always being rewound to Class F insulation standards or better is certainly correct. That's partly what I was referring to when I said to study the motor nameplate to be sure you have what you need. If the motor's been rewound by a responsible shop, normally the nameplate will be restamped or a new nameplate will be installed.

Barry, I would not normally expect motors on 460 volts with 25 meter (80 feet) leads to fail if they are Insulation Class F and certainly not if they meet MG1 Part 31. But, as I think about this, there are inverters out there with 15kHz and 18kHz carriers that might well cause problems on these lead lengths. I avoid these high carrier units. I've never seen any benefit to going above 8kHz and I'll only go to 8kHz for audible noise reduction. Given a choice I'll stay at 4kHz or below and use a high performance sensorless vector system like ABB's DTC to get good slow speed control.

I don't know what a Class A filter is but, if you are referring to input lead filters or a DC bus choke, these have no effect on the output waveform to the motor and would be of no help. They are for cleaning up the harmonics on the supply leads. There are various types of reactors, wave traps, and dv/dt filters that are designed for the motor leads and, if you are using any of these, then the improvement in motor life is understandable.

Dick you asked

I'm curious how long the leads were on the failed motors you mentioned?

Click to expand...

Do you mean the stripped tails of cable at the motor terminals or the length the motor leads from the windings?

Sorry TFC. My question on lead length referred to the length of wire leads to connect the output of the inverter to the motor terminals. Or, stated another way, what was the electrical distance between the inverter and the motor. Barry has responded above that the distance was 25 meters.

Some runs of cable were up to 150m. Most were over 50m. The problem units were the ones with runs over 50m.

Another thing that some motor mfg'rs include in their specs when they state that a motor is "inverter duty" is better bearings and a stronger rotor construction...not to plug any names, but when I worked in the glass industry, we used some motors that were rated at 200 htz, which as I remember was way above what our drives could supply at that time...ie, dummy resistant for those cases where someone wanted to realy jump up the speed w/ out too much effort. I remember our PE testing a standard mtr, and one of these super - duper motors on a blower...lucky no one was hurt when the impeller let go...something to think about if you concider jumping the htz up, is the load rated for the extra rpm.

I have installed quite a few VFD's...one large system had 67 VFD's driving blowers up to 200 ft from the panel, and a similar system w/ the VFD 's at each mtr...never lost one on either system, but just a few weeks ago, we put two small 1hp drives on a roller conveyor, only 10 ft from drive to motor, and have blown both motors ...

David

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