I have a question regarding Single Pair Ethernet, is it possible to carry power with two wires?
For example, controlling and powering a motor 12V 0.5/1A
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Possible, yes. Ethernet is an AC-coupled signaling method, meaning there is spare bandwidth below the signal, where DC could pass.
The basic method to do this is called a bias tee. A balanced type is required, as the data is balanced.
This is different from the transformers used in multi-pair PoE, where the DC is balanced within each pair (currents go the same direction in each wire within a pair); cheaper (or even identical, to non-PoE that is) transformers can be used. (The CMCs must be arranged differently however.) A bias tee requires inductors rated for DC current flow, with enough [inductive] value not to disturb the data.
I would not suggest constructing one yourself from this answer alone, as the DC needs to be well filtered to avoid coupling interference into the data path -- or destructive transients, as the nominal signal level is only a couple volts. Additional common mode chokes will likely be necessary to resolve data and DC path impedances to meet EMC.
Get a copy of the appropriate standards and follow them closely, and/or use components designed specifically for the relevant standards.