> such motors require frequent maintenance for changing the brushes.
"Frequent" is all relative.
The Renault Zoe, 10y ago, was already using a synchronous engine with wired rotor. And most were going over 150kkm without any issues nor brush changes.
> because the electrical currents that circulate through the rotor windings must generate heat
Currently stator heat in wired synchronous engine is less a problem than in SynRMs with permanent magnets.
Most neodymium based permanent magnets start to be irreversibly damaged id they heat up beyond 100°C. That's currently why Tesla has such a good cooling system in their engine.
Wired rotor are bunch of copper coil, as such they are much more resistant to temperature gradients.
150kkm - wouldn't that be 150 Mm?
Yes, you are correct. As the SI brochure states: "Compound prefix symbols, i.e. prefix symbols formed by the juxtaposition of two or more prefix symbols, are not permitted."
Unfortunately, almost no one uses SI units and/or prefixes correctly.
Sure, also 1 mAU and 0.5 light seconds, but treating km as a base unit and prefixing it with another k isn't too uncommon a misuse.