Electronic – combine two separate 120V isolation transformers, tie the secondaries in series, but not tie the primaries

powertransformer

I want to connect two separate 120V (7.5KVA) isolation transformers, tie the secondaries, but not the primaries. The input is 120V for each primary, with the goal to have 240/120V on the output. I guess you can call the primaries of the two transformers as floating and/or isolated primaries. I tried searching all of the world wide web for this type of application, but it seems very uncommon. Typically, the primaries are connected right at the transformers. The logic I had was that the transformers are technically connected at the neutrals upstream at a terminal block, but that distance is about 50 feet. To clarify my intention, I drew what the circuit would like below. I want to know if this is possible, what should I do to make this works, and what issues should I look out for. Also, how would the output look if the 2nd transformer below had Line 2 and the Neutral switched on the primary?

I did see issues online about non-matching polarity, insufficent insulation in the transformers, ease of mis-wiring, and others.
Two 120V Isolation Transformers Secondary Tied Primary Isolated

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Three options - two of which actually work. Note the dot convention on the coils.

  • (a) is the same as your schematic (but lower primary is inverted to keep phase convention the same). The primaries are in anti-phase so the output is zero (as pointed out in the comments).
  • (b) is the corrected version of (a).
  • (c) is an alternate working solution. Since the primary currents are the same what comes out the bottom of XFMR3 must go in the top of XFMR4 so there is no neutral current and the neutral connections can be omitted as shown on XFMRs 5 and 6.
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