How can increasing the voltage/frequency ratio of an induction motor result in magnetic saturation?
An induction motor is like a transformer with the motor's stator acting as the primary winding and the rotor the secondary. From this basic transformer equation:



it can be seen that the peak flux density is proportional to the voltage divided by the frequency multiplied by the number of turns and the area of the coil. Once the stator has been designed, the number of turns and the core area is fixed, so the flux density is completely determined by the ratio of stator voltage to frequency, V/f. A given core material (type of steel) can support a limited flux density. When the V/f is increased the flux density increases fairly linearly up to a point. Then further V/f increase results in less increase in flux density such that further increase of V/f increases the stator current and heating without providing much additional flux density. Under that condition, the motor is said to be saturated. Induction motors are generally designed so that the rated V/f is at the point where the rate of increasing flux density vs. V/f has begun to decline significantly.