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

https://mediatum.ub.tum.de/doc/1226682/1226682.pdf

[2] https://cdn.vector.com/cms/content/events/2018/vEMOB18/01_In...

It does not matter whether the driver generates AC or chopped DC, when passing through the gap between stator and rotor, any DC component is removed.

To provide the DC component needed by the excitation current, a rectifier is required, or at least a nonlinear impedance, but the latter would have a lower efficiency, due to higher AC components that would cause losses without providing useful torque.

A brushless synchronous motor which relies on an electric excitation current in the rotor (i.e. not on permanent magnets, hysteresis or variable reluctance) cannot generate the required excitation current when only linear circuit elements are present in the rotor.