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Running VFD from step-up transformer

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  • #16
    Originally posted by rdfeil View Post
    Another thing to keep in mind is the actual ratings of the converter. I looked around a bit and one VERY important specification was the "duty cycle" or how long it can supply the power before overheat or failure. Many converters I saw online were only rated for 30 minutes at full rating and the continuous rating was around 50%. In any case, one very telling specification actually has nothing to do with electricity. It is the weight of the unit. In basic converters it takes IRON to make a high power transformer. After many years in the electric control industry, I have NEVER seen a 20 pound transformer that can provide 2000 watts for any length of time. .........!

    The amount of "iron" is actually a measurement of the wire size....... (current capacity/powercapacity) You might not think so, but it is.

    In a transformer, a small core takes many turns of wire to build up the inductance to operate across the line at a particular voltage successfully. And, since the core is small, to get a lot of turns, the wires have to be small.

    A larger core (heavier, too) means two things.... one, it takes fewer turns of wire to get a usable inductance, AND since the core is bigger, there is more winding space, so the fewer turns can be quite a bit larger wire... lower resistance, and capable of operating indefinitely at the same current a smaller, lighter transformer can only stand for a short while, due to heating up.

    That smaller core and short duty cycle at full power are typical of a transformer not intended for long term use at full power.

    That said, you may not NEED high power for a long time, unless you are hogging off 4140PH making production parts, or the like. You might never strain the capabilities of that transformer. From my experience, 95% of the time I am not running any machine in my shop hard enough to get into trouble that way. But once in a while, I am.

    I got a limited duty 3 phase motor for the Logan, enclosed motor, no fan, etc.. I was solemnly "warned" by several folks here that I'd hate it, that I'd have trouble with it shutting off, etc. It's been on the machine a decade or more, and I have had it click the overheat one time. (it has undoubtedly come close several other times) If I put any sort of fan on it I bet that would never happen at all.

    Keep eye on ball.
    Hashim Khan


    • #17
      If you're looking for 2000 watts and want to stay within say 10% of rated voltage, you'll need probably 40 lbs worth of transformer or more- and that's figuring on a torroidal one, not the 'old' E and I core transformer. I've got one rated at 1000 watts, and it weighs over 30 lbs.
      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


      • #18
        As a reference point, a 1 kVA open frame isolation transformer from Hammond weighs 23.6 pounds. If you connect it as an autotransformer, it will handle 2 kVA. The transformer under consideration does not specify if it has an isolated output, so it is conceivable that a 16 pound voltage converter might handle 2000 watts, especially if the rating is intermittent (30 minutes ON, 30 minutes OFF). The true continuous power is about 70% of that rating, or 1400 watts. Overload duty cycles are based on the square of the amount of overload, so 2x (2800 watts) would be a duty cycle of 1/2^2 or 25%, typically maybe 15 minutes on and 45 minutes OFF. Most transformers can be overloaded up to 10x continuous, but duty cycle would be only 1%, and perhaps 1.2 seconds ON and 2 minutes OFF.

        [edit] A 1000 VA toroidal step-up autotransformer weighs only 7.1 pounds. So a 14 pound toroid should handle 2000 watts.
        Last edited by PStechPaul; 09-04-2020, 12:04 AM.
        Paul , P S Technology, Inc. and MrTibbs
        USA Maryland 21030


        • #19
          Originally posted by no704 View Post
          Do you have 2 or more separate 110 circuits? Jump them together
          I wont admit to doing that previously, but i wont deny doing it as well. That said, running 2 extension cords into a box from across the house between 2 circuits out of phase kinda gives me the heebie jeebies, so id like to find something a little safer


          • #20
            Electric clothes dryer less than 50' feet away? Use that circuit with a suitable cable.