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Can a step up transformer designed to step up from 110v to 220v be used to step up from 230v to 460v? (I'm thinking one of those cheap Chinese auto transformers from ebay, in the 1kva-2kva range)

I expect it will tolerate a small over voltage, such as 10% or 20%, but could it do double (or more), or will it overheat pretty much instantly?

The application is to step up single phase to input into a motor VFD to get the most out of it and drive a motor which can't be re-wired from Y to delta to run at 230v. I'm aware that this will only work with a single phase input VFD, or a three phase VFD de-rated appropriately.

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    \$\begingroup\$ You are asking if a cheap (your description) component can be used at twice its rated voltage. Additionally, the voltages involved are in the lethal range. I don't think anyone on this site is going to recommend that this is OK. \$\endgroup\$ – Barry Oct 8 '19 at 2:02
  • \$\begingroup\$ A higher voltage will not cause overheating of the transformer (at least not in the first place). But the isolation might not be able to withstand the higher voltage and you get a short circuit. THIS might cause overheating. \$\endgroup\$ – jusaca Oct 8 '19 at 6:12
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    \$\begingroup\$ "I expect it will tolerate a small over voltage, such as 10% or 20% ..." Without a datasheet you can expect very little. The primary will saturate on a higher voltage than it was designed for. For a "cheap" transformer I would expect that it's already on the edge at nominal voltage. \$\endgroup\$ – Transistor Oct 8 '19 at 6:20
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    \$\begingroup\$ Take two 110V --> 220V transformers, wire the primaries and secondaries of these transformers in series, and you have 220V->440V. \$\endgroup\$ – TEMLIB Oct 8 '19 at 6:28
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Only for about half a mains cycle.
ie NO.

Even running a 60 Hz mains transformer on 50 Hz causes it to run hot (ask me how I know) due to increased magnetising current.

Power transformers are designed to use the core iron well (except i very special cases) and magnetising current is arranged to flux the core to the point on the BH curve where the core is starting to saturate and go into a non linear mode where current increases faster than voltage applied or than flux increase.

Doubling the voltage will drive any normal transformer deep deep deep into saturation, massively increase the current with no positive effects whatsoever, and destroy the transformer almost instantly.

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  • \$\begingroup\$ Thanks for your feedback, It sounds like I need to use 2x 230v-230v transformers with the inputs in parallel and the outputs in series, or two 115-230v step up transformers with both the inputs and outputs in series. But for either of these suggestions to work they will need to be isolating transformers and I will need to ensure that the phasing is correct. does this sound reasonable to people? At this stage this might be more of a thought exercise, as i'm probably just going to have transformer wound for the task. \$\endgroup\$ – user233614 Oct 8 '19 at 21:57
  • \$\begingroup\$ @user233614 As you note - if used "sensibly" auto transformer WILL NOT suffice . \$\endgroup\$ – Russell McMahon Oct 9 '19 at 1:15
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You will cause the transformer core to saturate and the windings will overheat and the transformer will be destroyed if you try this.

No, it will not work. Not with a cheap transformer, not with a high quality transformer.

You can go down in voltage (from rated), but not up, certainly not 2:1 (for given mains frequency).

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I tried that trick in years past. One worked, one arced. Answer, don't do it.

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  • \$\begingroup\$ @Robert, Don't you think you could put more effort into this post? Maybe as to why it arced and why this shouldn't be done? \$\endgroup\$ – Voltage Spike Oct 8 '19 at 19:21
  • \$\begingroup\$ I don't really know a whole lot more. There was arcing I could see and hear (as I remember). It convinced me that running a transformer above its voltage rating was not safe or reliable. \$\endgroup\$ – Robert Endl Oct 8 '19 at 19:38

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