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I have one auto transformer that we used to convert 3-phase 380V to 3-phase 220V. Now I need to convert 3-phase 220V to 380V. How to use the same transformer for 3-phase 380V output? Please explain the procedure.

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    If it worked as 380V to 220V, it will also work 220V to 380V if you simply swap the input and output without changing the winding connections and do not exceed the max allowed power. For more detailed answer a photo of the system, proper schematic and a detailed list of specs for the transformer are needed. Before using it even one second get a proper electrician to check the system and to measure the voltages. It's far too easy to get electrocuted or burned and worse: to offer the same to another person. – user287001 Dec 9 '17 at 9:19
  • @Alikhan: Welcome to EE.SE. Please edit your question title. All questions on this site are electrical questions. You should also delete "as soon as possible". Someone will answer if your question is good, well written (yours has spelling mistakes and no punctuation), if they are interested and have time. Pushing us will not help. – Transistor Dec 9 '17 at 10:28
  • Also, we're not a single "sir"; we're a lot of different people here, and quite a few of us are "ladies". That really doesn't matter here; we don't want you to address us or to say "please explain" or to say "thanks". These are meaningless phrases that just distract from the question. Omit them. And omit that "as soon as possible", too, that is typically perceived as very rude. – Marcus Müller Dec 9 '17 at 11:35
  • My question regarding auto transformer – Ali khan Dec 9 '17 at 11:46
  • @user287001 variable transformers are different than fixed. It is easy to step down but stepping up is limited, to learn more Google variac. – Ed Beal Dec 9 '17 at 16:51
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If this is a fixed multi taped transformer reversing the input and output will change it from a step up to step down (I an used to auto transformers as variable not multi tapped) if variable Google variacs the connections will Need to be changed stepping down is simple the entire coil is normally used and a wiper provides voltage at the position, stepping up is a little diferent. Taps at approx 10 or 20% in are connected to the line voltage providing a higher output voltage . added: after doing a quick look at 3 different variacs all 3 were only able to tap up 10% of the line voltage so you probably would only be able to reach approx 250 v in a step up mode.

  • I think the OP is asking about a fixed autotransformer, not a Variac. Simple fixed step-up/step-down autotranformers area available. – Peter Bennett Dec 9 '17 at 17:36
  • If it was fixed why call it an autotransformer? – Ed Beal Dec 9 '17 at 23:55
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    An single phase autotransformer has only one winding, with the higher voltage over the full winding, and a tap at some point for the lower voltage. All variable transformers (Variac (TM)) that I've seen are autotransformers, but not all autotransformers are variable. The three-phase transformer the OP mentioned would effectively three single phase transofrmers. – Peter Bennett Dec 10 '17 at 0:04
  • All the standard 3 phase transformers I work on are multi tap for buck boost some as small as 10kva but most 25-75kva electricians call them transformers I have some on my farm 5&10kva to send power to remote locations with smaller feeder wire using the "taps" to have the correct voltage at the load. – Ed Beal Dec 10 '17 at 1:22
  • @EdBeal "Auto" like one, singular, self from Greek (as in autoerotica), not auto like automatic (as in autopilot). So autotransformer means "one winding". That applies to both fixed and variacs. – Harper Dec 10 '18 at 21:31

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