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  1. #1
    viper is offline Titanium
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    Default non-inverter rated motors on a VFD

    So for a general use application that would be in use 1-3 hrs a week, how does a non- inverter rated motor hold up? We have always used dual voltage motors and ran at the lower voltage for extra insulation protection but not sure if this reduces life drastically?? One of our old mill ran with a non-inverter motor from the OEM. That was a Tree machine.

  2. #2
    rklopp's Avatar
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    Every one of the four machine tools I've run off VFDs in my home shop have had non-inverter duty motors. The one on the Bridgport M head was probably 50 years old. I've never had an issue, and I do not baby my machines. I've also run several non-inverter-duty motors off VFDs at work, with true three-phase input. Again, never a problem. I wouldn't worry about it unless you were trying to drive a 200-HP motor from 1928.

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    I run three machines with VFDs with no problems. They all have POMs (Plain Old Motors) that were built before VFDs were invented.

    The main concern is heat. The motor can over-heat at either end of the speed range, for a variety of reasons.

    I would suggest using a remote-reading infrared thermometer or a hard-wired sensor with a remote display to monitor the motor temperature when operating at normal 60 Hz line frequency and moderate load. Use that as a baseline.

    Monitor the temperature in use with the VFD, and discontinue operation if it exceeds your baseline value. This should be safe under all conditions.

    - Leigh

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    how does a non- inverter rated motor hold up?
    At 240 volts, they seem to run forever ... as long as you address the heat issue that Leigh mentioned. Most every newer motor is wound with insulation rated for 600v, so even the high voltage spikes are within that range.

    ----------------
    Barry Milton

  5. #5
    DaveKamp is offline Titanium
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    Default What RK and Leigh said...

    Vipe- I'll say what RK and Leigh said.

    From a purist-scientist standpoint, there's all sorts of high-frequency EMF voltage multiplication/insulation breakdown nightmare consequence ramifications, but in the true scheme of things, you'll be hard-pressed to ever get to that.

    EVERY machine I have running on a VFD... has a standard motor.

    Leigh's point about heat is really the foremost issue- when you have a motor with internal cooling fan, that fan moves a certain amount of air across the motor. When you slow the fan down, the fan's output is not necessarily linear... but one can guarantee that it's moving LESS air than at full speed. The motor, however, may be generating a whole lotta power, hence, there'll be plenty of waste heat to shed.

    My favorite solution (and a common one, too) is to yank the mechanical fan, and install a constant-speed 'muffin' fan on the shroud... that way, regardless of motor speed, the airflow will be continuous, and plentiful. The motor will be happy at 100rpm, or 6,000rpm.

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    Quote Originally Posted by DaveKamp View Post
    My favorite solution (and a common one, too) is to yank the mechanical fan, and install a constant-speed 'muffin' fan on the shroud... that way, regardless of motor speed, the airflow will be continuous, and plentiful. The motor will be happy at 100rpm, or 6,000rpm.
    I do the same thing. Shoulda mentioned it.

    I use large fans made by Orion, designed for cooling electronic equipment. www.orionfans.com (the website sucks )
    They're round, 6.7" OD with a 6" dia blade, 235 cfm, cast aluminum housing with ball bearings, available for 110 or 220 volts, $38.

    Part numbers are:
    OA172AN-11-1TB for 110 volts
    OA172AN-22-1TB for 220 volts

    And they're NOT made in china.

    Orion fans are sold nationwide by Mouser Electronics www.mouser.com which is my electronics distributor of choice. There are many other national and local distributors.

    - Leigh

  7. #7
    Pierce Butler is offline Hot Rolled
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    Of all the studies that I could find, none of them showed a increase in failures for 230 volt legacy motors powered by a VFD. One rather large study actually showed fewer failures for the VFD powered legacy motors.

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    Quote Originally Posted by Pierce Butler View Post
    One rather large study actually showed fewer failures for the VFD powered legacy motors.
    Hi Pierce,

    I'm inclined to believe that's because the VFD isolates the motor from voltage spikes on the AC supply lines.

    In an industrial area those spikes can be quite high in both voltage and energy, causing damage to the motor insulation.

    - Leigh

  9. #9
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    The DC bus in the drive on a 230V feed is only 340Volts - not enough to damage the insulation, especially when the longest motor leads are typically less than 25 feet and not likely to cause ringing.

    The problem is when you run a 480V derived DC bus at 680Volts and then run motor cables 100 feet long - you get voltage ringing to 1500 Volts which causes insulation break down in the windings. I guarantee you that in these conditions, the motor will fry unless it has Inverter Spike Resistant Wire which is rated at 2000 Volts minimum.

    Most "POM" have 600 - 1000 volt rated insulation, even the 240V motors - that is why you typically never see inverter caused motor insulation failures in older motors.

    If you have any concerns about this - simply buy a load reactor and put it between the drive and the motor, this will knock down the harmonics / ringing / voltage spikes and the motor will run cooler.

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    Now that's what I'd call a cooling fan

    Thanks for those part numbers, Leigh. The Orion fans look much more robust than the others I've seen.

    ---------------
    Barry Milton

  11. #11
    viper is offline Titanium
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    Thanks for the replies guys and I agree totally with the constant speed fan. All my mill motors have that and I take that as a huge concern for motors that can be fully loaded at lower rpm. I think any motor that runs on a VFD should do that mod. Just makes sense.


    We will move ahead with a non-inverter motor since everyone has confirmed my findings. Thanks

  12. #12
    jim rozen is offline Diamond
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    A simple muffin fan can be fitted to most motor end bells via the long screws that
    hold the motor together. A (large) picture:

    http://www.metalworking.com/dropbox/SB3.jpg

    Jim

  13. #13
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    JST
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    Old motors hold up because when they were made, doing the insulating right was considered important, and the people doing the work were good at it. The windings were tied and varnished , and the standards were higher.

    These days, they figure they can hardly afford to slap it together any which way, let alone be careful, due to chinese competition.

    Doing the between-phase insulation right is very, very important for longevity of the motor in VFD service.

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