Yeah... the whole thing is a bit confusing. My understanding is that in a DC motor speed is controlled by voltage. In an AC motor it's frequency.
By definition, all EV motors have aspects of both DC and AC. DC because the battery only gives DC. AC because magnetic fields must oscillate to create motion. It's the nature of how it's done that's the issue on hand.
It used to be simpler. There used to be two types of motor:
- Brushed DC, using a permanent magnet, fed by a DC power supply (the brushes form part of a commutator that reverses the path that the supplied DC current flows through the windings as the motor rotates, oscillate the magnetic fields in time with the rotor's position)
- AC induction, with no permanent magnet, fed by an AC power supply (fixed frequency, voltage, current), which reverses the current flow through the windings to oscillate the magnetic fields at the rate of the supplied AC.
So you had one that took DC, and one that took AC. Easy enough, they're just named after their supply. For an EV of the day, like the early Baker Electrics, was there any question as to which you'd use? Brushed DC of course, because you had a DC power supply. You didn't have tiny, powerful DC/AC converters back then! AC induction motors were for things hooked up to the grid, which you couldn't do with DC motors. Nice easy distinction.
Of course, today we have powerful control electronics that can convert between DC and AC with ease while varying voltage, current, and frequency. We also have more motor options - brushless "DC", reluctance, synchronous, doubly-fed, etc. To reiterate, in an EV, all of their power is originally DC, and for all motors, power in their windings must be AC. And for modern EVs, they're all going through control electronics that take the DC and create AC in some form or another, with a lot of waveform sculpting. So calling some "DC" and others "AC" is really a misnomer. People still tend to use the old nomenclature, however, of permanent magnet motors being "DC" and others being "AC", even though that's not an actual distinction between the motor types.
