Charles' comment is appropriate, but to simplify and give you some background:
AC machines are designed to operate at a certain flux level. Beyond that the machine can saturate which can cause damage, high currents, overtemperature, fuse/breaker trips, etc.
So to step back, consider just an inductor across the AC line. Let's say it's in the US where the frequency is 60Hz. That inductor will have an impedance proportional to the line frequency, L*jw.
Now if you were to drop the frequency in half, to 30Hz that inductor would drop to half the impedance and draw twice the current. Unless it saturates.
Since an AC induction machine (for example) is designed to operate at a particular frequency, the core utilization is designed for proper operating flux at that frequency.
Now like in the case of the inductor if I drop the frequency in half, the inductance drops in half and if I keep the same voltage I would have twice the flux. Far more than the design target of the machine.
So in order to keep the machine operating near its design target flux I have to make the voltage/frequency ratio constant. (220V/60Hz or 110V/30Hz for example) That's called scalar or V/f or volts/Hz control.
It kind of breaks down at low speed due to resistances involved, a vector control scheme is preferred.