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Thread: single/three phase VFD

  1. #1
    Join Date
    Jun 2002

    Question single/three phase VFD

    As a newbe, I'm confused about most things. The confusion of the moment is use of VFD's to convert single phase to triple phase. After reading previous threads and articles on the internet, I have come to the following conclusions.

    1. ANY VFD can be driven w/single phase power by selectively placing two juice wires (rather than the normal three) on the input side.
    2. With single phase input, the VFD will still deliver triple phase output, albeit at half the rated amperage.
    3. Because of the halfed amperage output when using single phase input, the VFD must be rated at double the power rating when running on single phase input (e.g., use a 10 HP VFD with single phase input to run a 5 HP triple phase motor).

    Did I get it right? If so, how do you know which two terminals to use on the input side of the VFD. Will I fry the VFD if I choose the wrong two inputs? HELP!

    PS: Almost forgot, are VFD's confined to a certain power range like static phase converters? As an example, can I use a 50 HP VFD to run a 1/2 HP motor?

    [This message has been edited by tonydacrow (edited 07-18-2002).]

  2. #2



    You have most of it correct. One point, tho: you should make sure that the VFD is designed to accept single-phase input power, rather than assuming that it can be made to work. Most of the newer VFDs will work on single-phase input power, but that wasn't always the case. You might indeed fry an older model that isn't desined for single-phase. Further, if it IS designed to take single-phase input, the documentation will show you how it is to be hooked up.

    The rating of a VFD is a maximum, and can usually be over-rated a fair amount without ill effects. The limiting factor for most people on over-rating the VFD is the cost. You might run a 1/2 hp motor on a 3 to 5 hp rated VFD, but a 50 is a bit much. I'd sell the 50 and use the difference for more tooling, etc!

    Good luck.

    Rich Kuzmack

    Pi = 355/113 . . . to
    <85 parts per billion
    Rich Kuzmack

    Pi = 355/113 . . . to less
    than 85 parts per billion!

  3. #3


    Never assume anything when it comes to equipment wired to AC lines. If you are unsure of the equipments specs find out first or get an Licensed Electrician to wire it up for you.

    From a safety standpoint and from the view of your insurance company - this is the best course of action. Any reversing must be done from the VFD - DO NOT USE A DRUM SWITCH between the VFD and the motor.

    Rich is right - no point in over doing it. If you have a 3HP motor get a VFD rated for that. And yes, many modern units are derated 50% on single phase id est:a 3HP VFD runs a 1.5HP on single phase supply. Do not presume that - find out for sure!

    Safety first.

    [This message has been edited by Thrud (edited 07-18-2002).]

  4. #4
    Join Date
    Apr 2001


    I think you have it more or less correct, but I'd strongly suggest you either get a VFD designed for single-phase input or, as Indexer says, get one that is specifically rated for both.

    When I was in the market for one, one place I looked told me I could do as you suggest -- run a VFD designed for 3-phase input on single phase, and de-rate it. While that was probably true, I decided to skip all the kuldging around and get one specifically designed for single-phase input. There's no real reason not to, if that's what you need. There are plenty available, if not at one dealer then at another.
    Try to make a living, not a killing. -- Utah Phillips
    Don't believe everything you know. -- Bumper sticker
    Everybody is ignorant, only on different subjects. -- Will Rogers
    There are lots of people who mistake their imagination for their memory. - Josh Billings
    Law of Logical Argument - Anything is possible if you don't know what you are talking about.

  5. #5
    Join Date
    Jun 2002


    The 50 HP to 1/2 HP was a bit of an overstatement. The thrust of the question was to find-out if I could run various motors (5 HP, 3 HP, 1 HP) on a 10 HP VFD w/ single phase input. Thank you all for your information! I will indeed attempt to get the spec.s on any VFD before I purchase/hook it up. I was just hoping that there was a universal solution to all of this. A shame that there isn't. Any way, you guys are the best. Thanks!

  6. #6
    Join Date
    Apr 2001


    This site, provided by someone here, gave a short course in VFD's when I was in the market and made chosing the correct drive pretty easy. The Hitachi drive that I now use has been beautifully effective in supplying my lathe and to wire it was relatively simple using supplied diagrams which even looked like the terminals on the drive.

  7. #7



    I agree with most of the previous comments, but here is my response to your questions.

    1. Only some drives are capapble of single phase input. If the manufacturer doesn't say it will work, it won't. This is particularly true of newer model drives.

    2. Depends on the drive. These things work by converting (rectifying)incoming AC power into DC then chopping the DC up into AC at whatever frequency is required. Drives are rated by their output. If a drive that has been designed for single phase input and 3 phase output is rated to deliver 10 AMPS, that is what it will deliver. The input would be close to 20Amps.

    3. The conversion is actually a bit less than 2. For a given voltage, the single phase input will need to be 1.73 times the 3 phase amperage required by the motor + some for drive inefficiency. You should use the manufacturer's data rather than just doubling the horsepower.

    4. You cannot run very small motors with very large drives. The explaination for this one is really complicated, but suffice it to say that if you run much below 60% of a drive's rated output it will be unable to effectively regulate speed. Of course, the opposite direction is worse - you just can't run a big motor with a small drive under normal circumstances.


  8. #8
    Join Date
    Jun 2002


    Just got some good info on a VFD I thought I’d share with you all.

    I found it difficult to find a VFD that would run on 240 volt, single-phase input and provide a 240 volt, triple-phase output at larger amperages (~ 5 HP output). (All the VFD’s I found are not rated for single-phase input above 3HP.)

    Just got off the line w/ Mitsubishi tech dpt. Their 10 HP Vector VFD (Mitsubishi Part Number: FR-V220E-7.5K(-01)) can be run on single phase input if you derate to ~5HP output and do a few other things. The info on how to do this is not in the manual because you must bypass all the fancy things this unit will do and run in “open loop” (no encoding) mode. Turns out that they have a few too many of these things and are selling them at about the same price as simple VFD’s.

    If anyone else is interested I’ll post the instructions given to me on how to run w/ single-phase input.

    BTW, Alan and KenS, Thank you both for the info. It was EXTREEMLY helpful!

    [This message has been edited by tonydacrow (edited 07-23-2002).]

  9. #9
    Join Date
    Apr 2002
    Central Iowa


    Just want to add a bit to the VFD discussion... most machine tool motors, will rarely load the motor up to the rating.
    Most of the time I'd bet that a typical milling machine or lathe only uses a third or even half the rated HP. And if they did it would be only for a very short time.
    Most home machinists are lucky to get in a couple hours per day (week?) of actual use.
    For the home shop, if the drive has a built in overload you can program it to trip or limit at almost any point. I've got mine set low enough that the drive will shut down a long time before the motor would have a chance of burning out. B.G.
    Retired - Journeyman Refrigeration Pipefitter - Master Electrician - Amateur Machinist

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