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Thread: VFD Power Output Question

  1. #1
    Join Date
    Sep 2006
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    Appleton, Wi
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    VFD Power Output Question

    Are the motor feeds required to be in separate conduits from the VFD to the motor? I always thought that was the case to prevent cross coupling from one motor to the other. But I was recently working at a food processing facility and noticed from the drive cabinet to the machine on the plant floor, they typically had three motors sharing one conduit. They did that for all the motor feeds. These were not shielded motor cables either, just standard THHN wires. I thought that was a no-no? I looked at the installation manual for a Powerflex 525 VFD, but couldn't find where the drive output MUST be in separate conduits, although they mention shielded motor cables. Maybe I missed it.

    Are multiple drive motor feeds allowed to share conduits on newer drives or is that still a no-no?

    Any input appreciated. Thanks.

  2. #2
    Join Date
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    There is no code restriction on it if that's what you are looking for. But it IS a bad idea for sure. Cross coupling will take place because the frequencies and phase angles are all randomly different so there is no cancellation effect between circuits. I have seen motor and drive damage resulting from this on several occasions. Does that mean it's always bad? No, luck steps in once in a while, but luck is not a valid design principal.

    Read page 81 of this document:
    http://literature.rockwellautomation...n001_-en-p.pdf
    Last edited by Jraef; 08-06-17 at 02:29 PM.
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  3. #3
    Join Date
    Nov 2015
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    Texas
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    I worked 8 years in a meat packing plant and I've seen up to 5 VFDs share the same conduit for the whole 8 years with water/fat/grease/mold dripping out of the conduit. It was done that way because there is limited time to get the system wired and running. Lost production time is expensive so all to often every shortcut is taken. In a perfect world ... but meat packing plants are closer to hell than heaven.

  4. #4
    Join Date
    Dec 2012
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    Quote Originally Posted by Jraef View Post
    No, luck steps in once in a while, but luck is not a valid design principal.

    Read page 81 of this document:
    http://literature.rockwellautomation...n001_-en-p.pdf
    Great quote!

    That said we have a system wired like that. 3 drives all small about 1 hp in one conduit. Never any problems last 5 years. These days we don't use conduit
    and wire, it's all shielded tray cable.

  5. #5
    Join Date
    May 2017
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    SCV Ca, USA
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    69
    Quote Originally Posted by Jraef View Post
    There is no code restriction on it if that's what you are looking for. But it IS a bad idea for sure. Cross coupling will take place because the frequencies and phase angles are all randomly different so there is no cancellation effect between circuits. I have seen motor and drive damage resulting from this on several occasions. Does that mean it's always bad? No, luck steps in once in a while, but luck is not a valid design principal.

    Read page 81 of this document:
    http://literature.rockwellautomation...n001_-en-p.pdf
    Electrical engineering is not based on fortune teller's crystal ball to determine if ones idea will work with assistance from Lady luck.

    Pulsed width modulated variable frequency drives had come along way since I got involved on this technology almost forty years ago. Even back then we didn't have to rework motor feeds during retrofit. We did revamps to motors that drive machines that require subtle speed regulation in mfg super thin paper products.

    Cross coupling or cross talk is not big a big issue in vfd-fed drives unlike audio equipment. In a 100 HP drive, cross coupling (if ever there is a significant one) will hardly be noticed.
    Should there be anomaly in the incoming power to the VFD there is a mechanism to cancel out extraneous influences to alter phase angle or frequency instability by using varactors.

    Since most VFD installs are in close proximity with the motor being served, mingling of wires is a hardly common occurrence.

    Sometimes old wives' tales are pervasive.

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