Electric motors tend to be a compromise. Size, weight and efficiency are three considerations but there is also the choice of whether to use permanent magnets. The issue with permanent magnets is the most efficient ones use rare earth materials. Tesla has developed a permanent magnet motor that does not use rare earths (which we don’t know much about).
If you use coils on the rotor you have to power them. This is normally done using brushes and slip rings. This is something that can wear out long before any other service is needed on the motor but they also require space which will increase the length of the rotor and, thus, the size of the motor.
ZF has come up with a motor design where the power for the rotor coil is inductively coupled rather than requiring a “wired” connection. A New Atlas article explains what is going on. It includes a short video on motor operation.
