I think AC motor advantages over DC has been discussed a few times, but hopefully will my understanding help.
I know a little about large DC motors, this what I know about these motors: DC motors used (like in forklifts, series wound) are ideal for non continuous (over)loads. Plenty of starting torque, relative low maintenance, best is that their are relatively cheap. Controlling the speed of a DC motor is the simple but elegant. Varying the (avarage) voltage, varies the rotor speed. Varying the current varies the torque. This can be done with a chopper circuit. I won't go into detail now how chopper circuits work.
Inside the DC motor an electro-mechanical solution called the commutator, provides the field rotation/phasing needed to keep the rotor spinning after the first step is made.
Detail:
Soft carbon brushes press against the commutator contacts to conduct the electricity which magnetize the rotor coils during each step. Mechanical abrasion & arcing of the carbon brushes against the copper commutator wears both components. Mechanical abrasion is nearly irrelevant compared to a second source of wear, electrical arcing.
Electric arcing due to commutator contact impurities & magnetic energy stored in the rotor during each step represent them selves as a high voltage kick-back. I wont go into detail way they kick back now.
AC motors wear only at their bearings. (Not to confuse with ageing, a motor also ages, insulators age due to dielectric stress, time & heat) No serviceable wearing components such as brushes are incorporated in AC motors. Watercooled & Oil cooled design allow them to be used at much higher continuous power levels than size & mass compared DC counterparts.
AC motors used in the industry are made in various Nema Design classes. A,B,C,D, possibly even more today. Each class actually describes how the motor behaves from start, 0 RPM to max RPM. The reason they made classes is that AC motors can be designed to respond/react specific to load-changes. The higher the class name (D), the higher the available starting torque. There is a catch, higher starting torque designs trade motor slip for torque. This results less torque when the engine speed is near the rotational field speed. This draw back of high-torque motors can be 'ironed' out by using a variable frequency drive, upping the frequency forces torque to be produced at elevated speeds.
Detail:
AC motors, and brushless DC motors do not have commutators. That means something else will have to do the phase-rotation for the motor. In the industry, the 3-Phase mains power provides all the required commutation. An AC Motor driver/inverter/vfd, or motor controller is required to make the motor turn at various speeds from a DC source. AC motor inverters are technically 3 chopper circuits linked together to create 3-phase AC. The luxury with an advance motor inverter is that complete control of the motor is at your finger tips. Programmable start/spin-up behaviour, custom torque curves, regenerative braking. etc.
Down side is that ac motor drivers are still quite expensive above certain power ratings. Safety: if an AC motor inverter blows up and shorts the DC-Battery to the motor, it is NOT possible to generate a motor run-away situation where the vehicle speeds out of control.
//Steven