cairoliu said:
Not really.
Nowadays some EV motors still use a pair of slip rings, though many power transistors are used to switch current direction, so as to obtain a complicated multiple phase AC power supply to stator, not rotor.
Of course, two cases are exceptional: one is permanent magnet motor, another let field coil fixed on stator.
Motors that use a commutator don't use semiconductors, for exactly the reason that the commutator does the job of semiconductors!
Those that use semiconductors all have no coils on rotor, they are either asynchronous induction type motors or synchronous type, the last ones have permanent magnets within rotor.
One of the reasons why electric cars don't use slip ring or commuator style motors is because in order to avoid the need for a gearbox, the motor is attached either directly to each wheel or to the axle (rear or front) via a differential gear, this means that the motor has to cover the whole speed range of the car with it's RPM which can get quite high when your driving say at 150 MPH. In Tesla's IIRC the motor rotor was going above 10k RPM easily.
Having a commutator at such speeds would mean it would wear out fast.
Back when commutator motors were used on almost all rail locomotives and trains this wasn't a problem because those motors were so big (the size of a smaller fridge) that they had a huge torque therefore their gear ratio wasn't as high as in electric cars therefore they turned at much lower RPM and having a commutator wasn't that big of a deal.
Besides wearing out and graphite dust the only real thing that determines whether a commutator style motor can be used is the rotor RPM, because at low RPM the commutator copper slot surface speed is not that high and the wear is minimal, while going to higher speed it becomes a problem.
One other important aspect is that at higher surface speeds, the commutator can start to arc over, this happens when the copper slots are moving past the brushes so fast that the disengaged slots with their inductive "kickback" manage to draw an arc that goes all around the commutator surface (especially if the commutator is worn and dirty with graphite dust between the slots) eventually the brushes simply short circuit themselves and burn out the commutator..
Watch this video, it shows exactly what I talk about.