Basically fans work on the basic principles of electromagnetic induction and the relevant forces are fundamentally determined by the current through the coils. Therefore the answer to your query is current.
Okay, but you should clarify more about the motor (Type of it). Squirrel Cage Induction Motor - Slip Ring Induction Motor or Shaded Pole Induction Motor, which one?
Yes, Basically you are right but there is a bit difference on motors!. If you consider the following formula for an electric motor, then you can change speed with the following components.
S.F and P. S(slip) related to torque and related to Voltage of motor( see equivalent circuit of it)
Thus we can change the speed by Voltage and Frequency.
Sorry for my bad English.