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?
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.
My concern would be back-feeding.
I am guessing the reason the two transformers have separate neutrals is because they are going to be plugged into different sockets.
If that is the case then you have to consider what happens if one of the transformers is unplugged. Hazardous voltages are likely to feed through the plugged in transformer, through the 240V load and then back to the plug of the unplugged transformer.
Your circuit is good. The only thing you should pay attention to is in-phase secondary voltages (as @Jack Creasey said) to make them be added (and produce 240V output), not substracted (and produce 0V output).