Given a three-phase asynchronous AC motor ("induction motor"), I tried figuring out whether lowering speed / torque by lowering the supply voltage should work, similarly to how it works with DC motors. From all that I could learn from Wikipedia, voltage-control is not possible, but one has to control the supply frequency to control the speed.
Here's my understanding of a asynchronous AC motor: the three-phase supply current induces a rotating magnetic field in the stator, the stator's field. The rotor rotates slower than the stator's field (the speed difference being the "slip"). This means that the stator's field moves relative to the rotor, which induces a current in the conductive rotor. This current in the rotor creates a magnetic field (the rotor's field). The rotor's field rotates with the same speed as the stator's field, but phase-shifted. The two phase-shifted fields push away from each other, which creates torque, driving the rotor.
Now, if I lowered the voltage across the stator's coils, nothing should change but the strength of the magnetic fields - right? And that should lower the torque. It would probably also lower the slip (given a fixed torque), but would that be a problem?
A slight hint that such a speed control should be possible is that (according to Wikipedia), larger asynchronous AC motors are started using a "YΔ-Connection" (translated from German, could not find that in English), where the motor is run in a star connection at start, putting 230V (in the European system) across each of the stator coils, and switched to a delta connection afterwards, putting 400V across the coils. Thus, a lower voltage seems to be possible…