AC motors, other than universal motors, are constant-speed electric machines, with the possibility of re-connecting the stator coils of an induction motor, to double the number of poles and thus reduce speed by half, where the stators can be constructed with two independent windings, calculated for the number of poles desired. https://www.mrosupply.com/couplings-collars/elastomer-couplings/116850_011297_dodge/ This achieved by means of reversible poles and with small number of connections vary the synchronous speed of the motor.
With winding rotor induction motors it is possible to achieve any speed from zero to approximately the synchronism speed, by changing a simple resistance attached to the rotor winding, and that does not imply heating of the rotor, since losses in resistance are out of the engine.
Each of these windings may be connected so as to allow two speeds, in the ratio of 2: 1, thus achieving four independent synchronous speeds, however, they can not accommodate any intermediate speeds.
The regulation of the speed of the AC motors allows to achieve in the axis a speed that can go from zero to twice the synchronous speed, by the known system of the rotor with switch, by means of offsets of the brushes.