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Thread: VFD Questions

  1. #1
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    VFD Questions

    I bought a 3ph machine, one is 3HP and I'm considering another that is 10hp.

    So, could basic questions. Do I buy VFD's with the identical HP ratings?

    These are basic machines, without anything digital going on. One is a mortising machine (Felder 250) and the other is a planer. Anything I should be concerned about?

    I'm considering one more machine later on, which is a big big jointer, I'd like to use the 10HP setup for both machines (not at the same time of course), is this possible?

    I can manage with electric but I'm not an electrical engineer so talk slowly and loudly

    Cheers
    Brian
    Bumbling forward into the unknown.

  2. #2
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    IS THIS TOO LOUD..?

    What matters is the amps that a given leg of the VFD is subjected to, what amps a motor draws on that leg, and what amps the manufacturer says the VFD will support.

    Assuming all 240VAC -
    For a 3Hp motor, 3-ph needs 9A, but 1-ph needs ~16A (depending on what motor chart you look at). So, the VFD input has to have components that support 16A, since I assume the whole point of this thread is how to convert 1-ph to 3-ph. Some of the Asian VFDs will say "3Hp, 1-ph or 3ph" - - which means they probably increased the current carrying capacity of at least 2 of the 3 input legs for the VFD.

    If VFD just says "3Hp / 3-ph", I wouldn't use it ... unless you can stand the warranty arguments. In this case I'd jump to a 5Hp VFD, which will support 16A on each phase (as you need). This is the much-bandied-about 'de-rating' of a drive - - you use 5Hp VFD on a 3Hp motor.

    If they don't list the Hp for a drive, multiply the motor's FLA by 1.7 and make sure the VFD's specs can support this.

    Warning - the 10Hp is gonna get painful (...looking thru my 'hobby' filter).
    Last edited by Malcolm McLeod; 11-15-2017 at 4:58 PM.
    Molann an obair an saor.

  3. #3
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    I had to go look up the 10Hp numbers, so replying separately.

    First motor chart I looked at uses 230VAC (it's probably old) with a 10Hp motor needing 28A (3-ph) and 50A (1-ph).

    So to run at this full capacity, a VFD has to have output circuits designed to safely handle 28A, but the input circuits have to deal with 50A (power in = power out). Typically, this means you'd need a 20Hp (=54A) VFD to support the 10Hp input current.

    I haven't shopped for drives this big for home use, but maybe manufacturers have started addressing this market?
    Molann an obair an saor.

  4. #4
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    Thanks Malcolm! Much appreciated and now it makes sense. Woof,,,40 amps for 10hp! Looks like a 15hp covers that and the 3hp will require a 5hp unit. Any issue in oversizing these?

    Replied at the same time, I found 40 amp, but you may be right could be 50 Amps. I may rethink this machine, seems like overkill.
    Bumbling forward into the unknown.

  5. #5
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    Quote Originally Posted by Brian Holcombe View Post
    Thanks Malcolm! Much appreciated and now it makes sense. Woof,,,40 amps for 10hp! Looks like a 15hp covers that and the 3hp will require a 5hp unit. Any issue in oversizing these?

    Replied at the same time, I found 40 amp, but you may be right could be 50 Amps. I may rethink this machine, seems like overkill.
    No issues with over-sizing, excepting cost. You can set the VFD parameters to limit the power applied - - so a 20Hp VFD could safely operate a 3Hp motor, if you program it to do so. (I'd probably go "belt & suspenders" with this big a discrepancy, by putting a 3Hp overload/CB inline between the VFD and motor.) You just have to re-program every time you plug the 10Hp motor into it. (We all have different pain thresholds.)

    Before you splurge on the 5HP VFD, Huanyang lists 3Hp VFDs on the Bay, seemingly rated for 1-ph OR 3-ph. Several SMCers have gone this route...? Just not me. Yet.
    Last edited by Malcolm McLeod; 11-15-2017 at 5:49 PM.
    Molann an obair an saor.

  6. #6
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    Cool, thanks Malcolm! Much appreciated. I think I'll use separate units for each machine.
    Bumbling forward into the unknown.

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    Well, I thought I had this all figured out but apparently not so much. The phase converter must be derated from what I've read for anything above 3hp, which actually seems like for anything 3hp and above and they start getting (as you noted) pretty expensive.

    I'm in the territory where it is actually starting to look less expensive to simply buy a rotary converter for the 3hp motor and dump my plans of a 10hp planer and instead get something smaller and single phase.
    Bumbling forward into the unknown.

  8. #8
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    I bought a CV Max with a 5 hp, 3ph motor to be able to lower the speed when appropriate. I did what the Aussie Max users do and bought a Powtran VDF from China through Alibaba. The VFD can if needed soft start the motor so it draws fewer amps. When I was inquiring with the sales person she checked with their engineers to make sure I got the correct sized unit. It cost, if I remember correctly, a little over $300Can delivered. I suggest you do the same to see what they recommend.

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    Thanks Peter!
    Bumbling forward into the unknown.

  10. #10
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    Were it me I would and did buy a rotary converter and use it to feed a 3 phase panel. That allows you to easily add more machines
    Steve Jenkins, McKinney, TX. 469 742-9694
    Always use the word "impossible" with extreme caution

  11. #11
    +1 on using a rotary phase converter for running 3P equipment at home. They are relatively easy to source, or to build (Google Fitch-William RPC for one), and they can be any size needed up to the limit of your electrical supply. An advantage is that they do not need to be modified or "tuned" for different motors unless you are running a very finicky CNC machine.

    I've attached a PDF of an integrated RPC and 3P breaker panel for your reference. I think I found it on the Practical Machinist website several years ago - their sub forum on electrical issues is a gold mine of information
    Attached Files Attached Files

  12. #12
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    Quote Originally Posted by Brian Backner View Post
    +1 on using a rotary phase converter for running 3P equipment at home. They are relatively easy to source, or to build (Google Fitch-William RPC for one), and they can be any size needed up to the limit of your electrical supply. An advantage is that they do not need to be modified or "tuned" for different motors unless you are running a very finicky CNC machine.

    I've attached a PDF of an integrated RPC and 3P breaker panel for your reference. I think I found it on the Practical Machinist website several years ago - their sub forum on electrical issues is a gold mine of information
    Just to be clear a VFD need not be tuned or modified either. Just as you can connect a fractional-Hp motor to a 50A circuit breaker, you can run a 3Hp motor on a VFD supplied and rated for 20Hp. Everything works fine - - until it doesn't. Then you melt something, let the magic smoke out, or have a 'rapid oxidation event'. To prevent this, you can insert a suitably sized circuit protection device between a (big) VFD and a (small) motor. Or, use the current limiting parameters built in to most VFDs.

    I don't 'RPC', but I would suspect that the same disaster potential exists between a (big) RPC and a (small) motor. If the RPC will adequately supply a 10Hp motor, then it will melt a fractional-Hp motor, unless you use a interposing CB or turn the RPC 'down'? Maybe this 'turning down' is possible, but I've not seen it discussed.

    ...Name your poison.
    Molann an obair an saor.

  13. #13
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    Thanks guys, appreciate the discussion.
    Bumbling forward into the unknown.

  14. #14
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    Quote Originally Posted by Malcolm McLeod View Post
    ... let the magic smoke out, or have a 'rapid oxidation event'.
    Nice! We've used the "let the smoke out" for decades ("electronic circuits run on smoke - they quit working if you let the smoke out") but I've never heard the "rapid oxidation event" description. It does make me wonder if oxidation is necessarily involved since vaporization by electrons can occur in the absence of oxygen but I can still think of uses, perhaps for the secondary effects which often follow an initial event.

    JKJ

  15. #15
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    When I bought my lathe it's 3Hp motor was supplied by a one Hp VFD. It worked fine but I never took heavy cuts. I am in the process of replacing the vfd with a 3hp one and wiring it properly to the existing switches. Many of the 3hp and lower vfds are designed for full output with single phase input. Get to 5hp and more and VFD prices skyrocket. I wonder how much the 762HP VFD used in a Tesla car cost? probably as much as a transmission would
    Bill D

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