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Is there such a device (e.g. a VFD) that can take a 6 kW 3-phase service (240 V 10 A per phase) and output a single phase that will allow me to use a search light that needs about 14 A?

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    \$\begingroup\$ At what voltage? \$\endgroup\$
    – MOSFET
    Commented May 21 at 4:13
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    \$\begingroup\$ If you want us to tell you what to buy, that's off topic for here. If to design it - well, there's a motor generator set, a rotary converter (though they usually make 3 phases out of 1), if the load is consistent enough some phase-shifting Ls and Cs might work, and then a full-blown (or half-blown) VFD will certainly work. The best answer is probably to get a 3 phase version of your load, or a larger single phase service. Your question is missing some context of what the load is, and what your other constraints like time, money, location, safety, whether you're amateur or pro, etc etc \$\endgroup\$
    – Neil_UK
    Commented May 21 at 5:47
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    \$\begingroup\$ A honking three phase DC supply + a 3.4 kW single phase inverter. You are looking at a few thousand €. Try to find a different solution, limit the current to 10 A, use slow fuses or split your load. \$\endgroup\$
    – winny
    Commented May 21 at 6:40
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    \$\begingroup\$ Yes, drive a 3-phase motor and turn a single phase generator... A solution seen in either direction... \$\endgroup\$
    – Solar Mike
    Commented May 21 at 7:06

2 Answers 2

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While you can use a rotating machine to do the conversion as Solar Mike suggests, a three winding transformer will get you there, too:
the sum of two phases is the inverse of the third.

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  • \$\begingroup\$ A rotating converter with a decently sized flywheel would act as a continuous load. Would a three-winding transformer act as a continuous load, or would it draw more power during the peaks of the output wave and less at the zero crossings? \$\endgroup\$
    – supercat
    Commented May 21 at 16:19
  • \$\begingroup\$ @supercat: Two of the windings fed symmetrically, the third would be a sine, in phase or opposite phase with the "non-feeding line". Not quite sure what you want to ask with would [this transformer] draw more power during the peaks of the output wave and less at the zero crossings? - magnetisation current should max about the zero crossing, a (transformed) resistive load would of course draw more power during the peaks of the output wave. \$\endgroup\$
    – greybeard
    Commented May 21 at 18:29
  • \$\begingroup\$ According to a video I saw, a three-phase generator with balanced loads will impose a relatively uniform torque load on whatever is driving it, while a single-phase generator will impose a lot of torque during some parts of its rotation but very little torque in others. My question was whether a transformer would be able to receive and store energy from a three-phase supply during parts of the output cycle where demand was low or zero, and then deliver that power to the output when demand was high. \$\endgroup\$
    – supercat
    Commented May 21 at 18:40
  • \$\begingroup\$ @supercat a regular transformer ideally does not store energy. I'm confident on the primary side the instantaneous power is quite as "swelling" as in the load. This is different with solutions that store more power than put through in a single cycle, be it reservoir capacitor, rotor, or ferroresonant transformer. \$\endgroup\$
    – greybeard
    Commented May 21 at 19:30
  • \$\begingroup\$ I wonder if a three-phase motor attached to a heavy flywheel might work well in an application like this to even out the load, without need for a separate motor and generator? Instead of requiring that all of the energy be converted to mechanical energy and then back to electricity, only the portion of the energy being stored and returned would need to be converted. \$\endgroup\$
    – supercat
    Commented May 21 at 19:42
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A 3.4 kW, 415 V to 240 V step-down transformer may be used.

enter image description here

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  • \$\begingroup\$ +1 for simplicity. \$\endgroup\$
    – greybeard
    Commented May 21 at 10:38
  • \$\begingroup\$ Thank you, greybeard! \$\endgroup\$
    – vu2nan
    Commented May 21 at 10:43

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