In my real life as an electrical engineer designing power supplies, I learned that you can't just hook in a second unsynchronized AC source to a circuit and expect everything to be peachy. Frequency, phase, peak voltage, waveform shape (and therefore RMS voltage) -- all of these things are important if you're going to have multiple AC sources. Yes, the math becomes much simpler if all of those parameters (except for voltages, of course) are identical, but conditioning the sources to achieve that state is far from trivial. That gets back to my earlier comment about how in the real world, a trip through DC is often required.
If we were simply adding another DC supply in parallel with an existing DC supply, things would be much simpler. Assuming that the voltages of the supplies were identical, assuming that the resistance of the wires between the supplies, and assuming that the supplies remained linear for the expected load, then sure, just hook the new one in, disconnect the old one, and call it a day. The big difficulty is that, as you mentioned, the machines are designed to consume AC. I suppose that one could crack open the case and start hooking into the DC lines after the power supply, but the number of required voltages would make that rather difficult.
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In my real life as an electrical engineer designing power supplies, I learned that you can't just hook in a second unsynchronized AC source to a circuit and expect everything to be peachy. Frequency, phase, peak voltage, waveform shape (and therefore RMS voltage) -- all of these things are important if you're going to have multiple AC sources. Yes, the math becomes much simpler if all of those parameters (except for voltages, of course) are identical, but conditioning the sources to achieve that state is far from trivial. That gets back to my earlier comment about how in the real world, a trip through DC is often required.
(Ref: http://books.google.com/books?id=lrN8wLFL3R4C&lpg=PA55&#... )
If we were simply adding another DC supply in parallel with an existing DC supply, things would be much simpler. Assuming that the voltages of the supplies were identical, assuming that the resistance of the wires between the supplies, and assuming that the supplies remained linear for the expected load, then sure, just hook the new one in, disconnect the old one, and call it a day. The big difficulty is that, as you mentioned, the machines are designed to consume AC. I suppose that one could crack open the case and start hooking into the DC lines after the power supply, but the number of required voltages would make that rather difficult.