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Thread: Running VFDs without Symmetrical Cable

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    Running VFDs without Symmetrical Cable

    Years ago this was how it was done. I need to hear pros, cons, and limitations for a simple scenario involving a 2HP drive on roughly 30 Ft. length of load side cable.
    Back at A&M there were all kinds of earlier generation drives on standard THHN wiring of varying lengths that never appeared to give much trouble. While I have set a few up with the new cable, it would be desirable not to have to re-run this service if possible.
    Microwave Radiation Dangers should be openly discussed

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    Like an assisted living facility, there are a lot of “depends” involved. It depends on how the wire is run, it depends on what carrier frequency you use, it depends on the quality of the VFD, etc etc.

    In previous generations of VFDs, the transistors used were turning on a lot slower than the latest generation (relatively speaking). The faster turn-on time is what made the newer designs smaller-cheaper-faster, but exacerbated the potential for wiring issues to some extent. So the old argument of “ive done it that way for years” is catching a lot of people by surprise. Using “VFD cable” addresses some of the variables, but is not the end-all be-all solution.

    Just FYI though, “symmetrical cable” is a variant within the CFD cable world that generally applies to much larger cables than you are looking at for your small drive, ie 2/0 and up.

    I am a proponent of not using THHN any more on the output side of a VFD, I have seen numerous failures in the last 5 years. THHN is made cheaply for a big market, it was not designed to handle the voltage spikes that can happen in VFD outputs due to reflected waves. The PVC insulation on THHN can have tiny bubbles in it from manufacturing that have no significance to standard 600V circuits, but the potential of 1400 -2000V spikes in the output of a VFD will punch right through eventually. I now recommend using XLP insulation that you will find in XHHW or RHHW wire. It’s not worth going out and ripping out old THHN, but in that has to be replaced, or for new installs, use the better stuff.

    If you are using anything OTHER than steel conduit for running the wires, you need the shielded VFD cable anyway.
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    Many Thanks

    Quote Originally Posted by Jraef View Post
    Like an assisted living facility, there are a lot of “depends” involved. It depends on how the wire is run, it depends on what carrier frequency you use, it depends on the quality of the VFD, etc etc.

    In previous generations of VFDs, the transistors used were turning on a lot slower than the latest generation (relatively speaking). The faster turn-on time is what made the newer designs smaller-cheaper-faster, but exacerbated the potential for wiring issues to some extent. So the old argument of “ive done it that way for years” is catching a lot of people by surprise. Using “VFD cable” addresses some of the variables, but is not the end-all be-all solution.

    Just FYI though, “symmetrical cable” is a variant within the CFD cable world that generally applies to much larger cables than you are looking at for your small drive, ie 2/0 and up.

    I am a proponent of not using THHN any more on the output side of a VFD, I have seen numerous failures in the last 5 years. THHN is made cheaply for a big market, it was not designed to handle the voltage spikes that can happen in VFD outputs due to reflected waves. The PVC insulation on THHN can have tiny bubbles in it from manufacturing that have no significance to standard 600V circuits, but the potential of 1400 -2000V spikes in the output of a VFD will punch right through eventually. I now recommend using XLP insulation that you will find in XHHW or RHHW wire. It’s not worth going out and ripping out old THHN, but in that has to be replaced, or for new installs, use the better stuff.

    If you are using anything OTHER than steel conduit for running the wires, you need the shielded VFD cable anyway.
    Jraef, thanks for this primer. My intuition seemed to also say that THHN could be a problem in this respect. This is not an extremely long run [40 Ft.] and the output wires are the only thing in a 1/2" EMT run to the motor. It would not be that hard to re-run with XHHW. Am I understanding that 600V rated XHHW2 is adequate for the job?
    Microwave Radiation Dangers should be openly discussed

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    Quote Originally Posted by StarCat View Post
    Jraef, thanks for this primer. My intuition seemed to also say that THHN could be a problem in this respect. This is not an extremely long run [40 Ft.] and the output wires are the only thing in a 1/2" EMT run to the motor. It would not be that hard to re-run with XHHW. Am I understanding that 600V rated XHHW2 is adequate for the job?
    For a 40’ run already pulled in steel conduit, I wouldn’t redo it.

    But yes, any XHHW would be better than THHN. I generally recommend the 1000V rated just because there is no difference in price from the suppliers I use, but 600V is fine.
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    Quote Originally Posted by Jraef View Post
    If you are using anything OTHER than steel conduit for running the wires, you need the shielded VFD cable anyway.
    Steel or metallic? (I assume you mean metallic since plastic doesn't shield very well at all.)

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    Quote Originally Posted by zbang View Post
    Steel or metallic? (I assume you mean metallic since plastic doesn't shield very well at all.)
    Steel, as in any ferromagnetic metal, not including some stainless steel alloys, can provide some shielding against magnetic fields as well as electric fields, particularly when the current inside the conduit sums up to zero. Aluminum or copper might not do as well.
    (Note: for definite high attenuation of magnetic fields, even steel does not do a particularly good job, the way it will for electric fields, but there are very expensive high susceptibility alloys (such as mu-metal) which work much better. Nobody makes conduit of those.)022

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    Quote Originally Posted by Jraef View Post
    For a 40’ run already pulled in steel conduit, I wouldn’t redo it.

    But yes, any XHHW would be better than THHN. I generally recommend the 1000V rated just because there is no difference in price from the suppliers I use, but 600V is fine.
    I tend to be obsessive about both reliability and redundancy. 1000V rated it is.
    Thanks for the expert assistance.

    Star
    Microwave Radiation Dangers should be openly discussed

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