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  1. #1
    Join Date
    Dec 2007
    Location
    Connecticut
    Posts
    880

    VFD For 2 Speed Motor

    I have a customer with 2 speed motors on their cooling towers. They are 30HP 3PH 460V. They would like to put VFD's on the fans and would like to reuse the existing motors. Can we just use the high speed windings and abandon the low speed? I would love to sell him some new motors, but they just don't have the money for both. Thanks in advance for any help.

  2. #2
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    It's best to use inverter-rated motors in VFD applications. They have beefed-up insulation to withstand the voltage spikes resulting from high-speed switching. Standard motors will certainly run on a VFD, but you run the risk of shortened motor life due to insulation breakdown because of the switching transients.

  3. #3
    Join Date
    Jul 2010
    Location
    Az
    Posts
    73
    Motor VFD tip sheet.pdf This might be of help for your question.

  4. #4
    Join Date
    Dec 2007
    Location
    Connecticut
    Posts
    880
    Thanks for the info guys.

  5. #5
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    you also need to verify the bearings are rated for low rpm applications.

  6. #6
    Join Date
    Mar 2013
    Location
    Central IL
    Posts
    103
    Quote Originally Posted by waregl82 View Post
    you also need to verify the bearings are rated for low rpm applications.
    Most bearings are not rated for these low rpms. grease does not get distributed and bearings will burn up in short time.

  7. #7
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Quote Originally Posted by Abrnth3 View Post
    Most bearings are not rated for these low rpms. grease does not get distributed and bearings will burn up in short time.
    You know, this is the second time I've heard this on this forum. I know that sleeve bearings do indeed have a minimum speed necessary to develop the hydrodynamic oil wedge that prevents metal-metal contact, but I have never heard of a minimum speed for ball bearings - and as far as I know all inverter-rated (and all motors inverter or not, above a HP or so) have ball bearings.

    I looked up Timken's engineering guide for ball bearings and could find only one indirect reference to low speed. Here's a link:

    http://www.timken.com/en-us/products...%20Section.pdf

    Here's the reference (on page A165):

    BearingSpeed.jpg


    According to this graph ball bearings - no matter what the lubrication - have no issues down to zero RPM. If anyone has info on minimum speed for ball bearings I would like to see it - I would definitely like to learn something new on this.

    Now, if your load has sleeve bearings then low speed operation is definitely a concern.

  8. #8
    Join Date
    Nov 2008
    Location
    Europe, Hungary
    Posts
    17
    My opinion, You can use it at hte high speed (I have use it few times), but the lowest freqency is must be at least 15-20Hz, because otherwise the motor can burn (not the bearings). I have see once a motor which has been burned in 5Hz - because the cooling of the motor was not enought.

  9. #9
    Join Date
    Mar 2010
    Location
    Tidewater Virginia
    Posts
    64
    I've converted several VAV AHUs from inlet guide vane to VFD, reusing the original motors (all 1.15 SF but sized to keep current at or below FLA) and gotten more than 10 years on motors with at least 5-10 years or more of original service before the conversion. The biggest failure I've seen has been "washboarding" of the bearing races related to high voltage discharge through the bearings. I always suggest shaft grounding on a VFD driven motor to give the induced voltage somewhere to go instead of through the bearings. So far, I've not seen failures related to bearing lube failures, even at low speed, but have had two fail after conversion because a contractor turned down the minimum speed below 25% (15Hz input to motor), which cooked the winding insulation. I can't think of any problems created by abandoning the low speed windings right off, but I've attached a couple of links below for more reading which might help. Good luck.

    http://www.evapco.eu/sites/evapco.eu...ncy-Drives.pdf
    http://www.usmotors.com/Service-Supp...otorsVFDs.ashx
    http://www.industry.usa.siemens.com/...tion-guide.pdf

  10. #10
    Join Date
    Mar 2010
    Location
    Tidewater Virginia
    Posts
    64
    Makes me wonder about the split oil bushings operating as mid shaft bearings on the Evapco multiwheel towers. I've seen several with VFDs and their operating speed averages about 900 to 1000 RPMs at the shaft. How do they maintain enough oil wedge to keep clearance or are they just built that loose to begin with?

  11. #11
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    We just converted two old (late '70's) air handlers to VFD from inlet vanes, the first of several we will be converting at a hospital. We spec'd new inverter-duty motors and required the grounding kits as well.

  12. #12
    Join Date
    Mar 2013
    Location
    Central IL
    Posts
    103
    We have been converting a lot of our systems over to VFDs. Always setting min hz above 12, usually 15hz. Due tomotor burn out, especially on air over motors. No burn outs yet and several have been running for 3-4 years.
    Last edited by Abrnth3; 03-16-2013 at 11:22 PM. Reason: add info

  13. #13
    Join Date
    Feb 2013
    Location
    Phoenix, AZ
    Posts
    198
    Quote Originally Posted by Abrnth3 View Post
    Always setting min hz above 12, usually 15hz.
    Me too. The OP's application was a cooling tower - just run the speed down to 15 Hz and on further reduction of temp just cycle the fan off, done that many times with good results.

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