I suspect this ZF motor is like what is described in this paper [1]. An air gapped transformer is used to transmit power to the rotor and diodes rectify it. See Figure 1.
This [2] appears to give a good summary of PSM vs ASM vs EESM and introduces iEESM (i is for inductive excitation instead of brushes, sounds like the ZF motor) at slide 14.
[1] Design of Inductive Power Transmission into the Rotor of an Externally Excited Synchronous Machine