I do not believe in that in existing situations. My company converted over a hundred plain jane NEMA motors that were originally installed in the 1970's to VFD'S without replacing a single motor. Fast forward 15 years later and only a few old motors burnt out during that time. We always purchased inverter rated motors for replacements. Always ran the THHN/THWN wire feeding motors on drives in steel conduit ( EMT & steel greenfield ) and ran a copper ground wire to motors.
VFD used instead of multi speed winding
- Thread starter bpk
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petersonra
Senior Member
- Location
- Northern illinois
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- engineer
I would not even worry all that much about this issue. There are millions of old motors people have put VFDs on that are still in service. The motor is eventually going to fail anyway, so when it fails it can be replaced with an inverter rated motor. In the meantime, it will likely last a long time.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
That’s how I roll with it. You already own that old motor and unless your down time cost is extreme, you might as well use it and replace it with a better one when it fails.
Given the supply chain issues right now though, I would suggest ordering that replacement and keeping it on the shelf.
The anecdotes on old motors surviving for years has some problems though.
First off, if discussing 230V or less, that’s one reason. The voltage stresses created by VFD use at 230V are usually still way below the insulation level used on the motor windings, ESPECIALLY if it was a 230/460V motor.
Second, older VFDs used even 15years ago had a different type of transistor system that operated at a much lower switching speed, so they were less inclined to cause these voltage spikes unless the distances were long, as in over 100ft from drive to motor. So it was entirety possible to “have a whole plant full of old motors on VFDs” and never see a lot of motor failures. But modern VFDs that are designed to be smaller/cheaper/faster use transistors that switch on and off 100 times faster than the older generations, and it is the steep rise time of the pulses that helps to make this problem worse with shorter lead lengths now. So people often pull out a 20 year old VFD and replace it with a new one, then the motor fails within weeks.
Have to disagree. The large hospital/research center/ ambulatory care building had well over 500 drives. Halve of them replaced starters that were installed in the 1970's with plain Jane NEMA motors. Had at least 100 drives that were over 15 years old and at least halve if them had over 100,000 run hours. Very few motors burnt out. All of these drives used the IGBT'S to supply varying frequency & voltage to motors. Believe they had a 5,000 switching frequency. We performed PM'S 1 to 4 times a year on each drive and did a quick check on motors. Used an IF camera on all connections & components. Important to blow out drives at least twice a year and clean filters. I replaced old obsolete drives that I could no longer get replacement boards for from 3 to 200 HP with never taking out a motor within 6 months of upgrade. Luckily we had a great drive company that only charged around $200 to perform drive start ups. I would always pick the techs brain because he only worked on drives for over 25 years and nothing stumped him. This start up extended the free parts & labor out to 3 years. We had some 75 HP cooling tower motors where the drive was over 200' away and in 10 years running never had to replace a motor. All of the drives used metal conduit ( EMT ) , steel greenfield or sealtite conduit with steel jacket and a copper ground wire to motors.
Understand what inverter duty is.
According to drive manufacturer, sales, etc.: pure profit.
According to NEMA MG-1 part 31: I’ve attached a link to the entire document but let me break it down for you. An inverter duty rated motor is different from a general purpose motor in three ways. First, the insulation is rated for voltage surges up to 3.1 times the AC voltage instead of the usual 2.25 times rating. This rating ONLY MATTERS if you install the motor with more than 200-250 feet of cable from the inverter (reflected waves) or less if you have multiple motors fed from a single conduit. The fact of the matter is that the premium price you pay for the inverter duty rating to run lengths of 200-500 feet vastly exceeds the price of a dv/dt filter. Plus it is typically a non stocked motif so if the customer needs to replace it, the lead time will be high. Many motors by the way are sold with a 1700+ V surge rating as standard because the increased insulation cost is no impact over keeping inventory of different rated wire. Rewind shops just use class H, 1750 V wire for everything. Only some manufacturers pinch pennies like that.
The second difference with inverter duty motors is that it has a speed range instead of a single speed. This is important if you run deeply into the low end. A general purpose 1.15 service factor motor has a rated speed range of 2:1. Inverter duty motors with a constant torque range of about 3:1 are out there but the way they get there is by taking a motor 3 times larger and derating it. On variable torque duty a general purpose motor can already do 10:1. Derating can push this to 100:1. Any more and the motor comes with a separately powered fan instead of an integral fan. The same thermal limitations apply to both motors.
Now what I’ve found is that almost all manufacturers offer blowers as an option. In some cases it is cheaper than the inverter duty motor, other times it isn’t. You can get one installed after the fact. It is more expensive to have a motor shop do it but the delivery times are greatly improved. I’ve seen lots of home made things with muffin fans strapped to a motor. It can work if you know what you are doing but if you want a performance guarantee it is best to buy a fan and duct work intended for the application.
The third difference is that if the shaft voltage exceeds 300 mV, the spec calls for an insulated bearing. This particular shaft current (one of 4 types) is a known issue going back as far as 1910. It has nothing to do with shaft currents created by VFD common mode voltage and the NEMA standard specifically states that it doesn’t address this one, so there is nothing special at all about inverter duty motors in this regard. All manufacturers automatically install an insulated bearing (technically a sleeve) in motors starting at around 1000-1500 HP for this reason. So this isn’t a “feature” of inverter duty motors. It’s a “get out of jail free card” for VFD manufacturers.
So when you buy an “inverter duty” motor you are buying a special name plate, upgraded insulation that most manufacturers now offer as standard, and possibly an external blower that costs at most a couple hundred dollars, in exchange for a 25-50% price premium. With a 15% or better markup that’s a huge amount of profit for something that is just a name plate change. Take it from a motor industry “insider”.
You are both right, sort of.
A motor has a drastically different impedance compared to the cable. This causes reflections. On the first reflection you can reach up to 200% of the DC bus voltage. With longer cables you can increase this even further. The ratio of the impedances at the motor terminals determines the maximum reflection you will get (per pulse). Cable impedance is fairly consistent regardless of the type of cable used. The only major difference is whether or not it is magnetically shielded. This means surrounded by steel, whether it is armor or conduit or “VD cable”. No magnetic (aluminum) shielding makes little difference. Motor impedance decreases with size up to 200 HP. After that point we are into multiple conductors per phase and the effect tends to be that impedance is relatively constant. So the result is that YMMV.
Now remember we quote voltages in an AC sine wave as RMS. The actual peak to peak voltage is 145% higher. At 480 V it is 696 V. This is the voltage seen by the motor from the pulsed output of a VFD connected by short cables (under 10 feet). The minimum impulse rating according to NEMA MG-1 for years was 1000 V but I have never seen one surge that bad. I can only assume those were back in the mica asphalt (pre 1970) days. So based on rise time, a 100 ohm surge impedance (THHN in PVC) and a rise time that assumes no impedances in the IPM, plus a 528 VAC input you can get to an 80 foot minimum distance. But as soon as we plug in a more realistic 480 V (or less) and a more realistic 1300 V surge now the motor can tolerate up to (1300/480/1.45) or 187% of VDCbus. That’s awfully close to 200% and places the maximum distance out to 200-400 feet. Now on a new install when I walk down the distance I will warn you that you are in the “caution zone”. You might not have problems but there are no guarantees. Plenty if customers have done this with no issues. Others have problems post installation so we install a filter after the fact. It is no different than attempting to predict shaft currents.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Bottom line, luck is not a valid design strategy.
sound like a pretty cool career Garbo!
I was very lucky in working in some fascinating jobs & locations. Worked in a house in Elfreys Alley that is the oldest block of homes in the country, Worked in a 300 year old Quaker meeting house, electrified a hundred year old bell in an old church, worked in a couple of large slaughterhouses, largest candy company of boxed candy and work in machine & tool & due shops and injection molding plant. Even got to play with wires while in Viet Nam ( telephone & power ). Have to thank my electrician dad for letting me wire up our American Flyer train sets and lights in plasticville buildings starting around ten years old and taking me to be his helper on moonlighting jobs. I was lucky that when I got laid off once on a Friday that by Monday I had several weeks of work lined up. Same thing when my first company went out of business and only giving us 3 hours notice.
At the large hospital / research center that I retired from we replaced plain Jane NEMA single & two speed motors that were installed back in the 1970's and never had problems. In mechanical rooms where they would replace 12 to 16 AHU motors they would specify energy saving replacement motors but seldom asked for drive rated motors. All of our 500 plus VFD'S ( from 2 to 1,750 HP ) had steel EMT conduit and a copper ground wire from drive to motors. Never wasted money on expensive shielded drive cable. Used plain Jane type THHN/THWN building wire on all 480 volt drives.
I firmly believe that you can use existing plain Jane NEMA frame motors that are not inverter rated with out any problems. I rather spend money on a line reactor. At the large hospital that I retired from we had dozens of motors that were installed back in the 1970's then installed VFD'S. Most were on AHU supply & return fans & pumps from 5 to 125 HP. Very seldom had a motor burnt out. Some of the project managers would only specify an energy saving motor over an inverter rated motor when they were changing from starters to drives. On a single or just a few change overs to drives they would not touch the twenty plus years old motors.
ptonsparky
Tom
- Occupation
- EC - retired
I would just warn the owners that their old motors may not like the VFD and be aware. Run it until it dies. Mission critical would suggest having spare parts on hand.
We had some that failed within a few weeks - incorrectly wound randomly wound machines. That cost them a pretty penny. Four 760 kW units.
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