[pbmonster]

> Also, the slip rings capable of passing high current to an electromagnet rotor are expensive and less robust than permanent magnets.

The new designs are all inductively coupled, i.e. they have a rotating transformer coil built into the shaft. No brushes, no slip rings.

[Workaccount2]

Coupling without using a core (or using an air core) is gonna hit efficiency pretty badly though.

[pbmonster]

Unfortunately, I can only find material from the marketing department on that issue. But if I understand correctly, both coils have a ferrite core (and yoke), but there is a small air gap where the rotating side is separated from the stationary side.