Regarding Induction Motor Flux

It's a bit difficult to follow your question, likely something getting lost in translation there. But it appears you are questioning something you read somewhere about how motor flux can be said to decrease as speed increases. Is that correct? If so, and without knowing the context of the statement, I will hazard a guess.

Think about HOW you can change the speed of an induction motor; you can either change the number of poles, or you can change the frequency. The statement that you read most likely was in relation to changing the frequency, a common thing now by using a VFD.

A common issue that comes up when using a VFD is that most VFDs can put out a much higher frequency than the incoming line frequency, therefore higher than the motor DESIGN frequency. For example let's say you have a motor designed for 400V 50Hz (in deference to my assumption that you are not in North America). If you want to change the speed of that motor, you must change the frequency. But not JUST the frequency, you must also change the voltage at the same time if you want to maintain torque. This is because to get full torque from the motor, you must maintain the flux density of the original design, which can be represented as a ratio of voltage and frequency, or the "V/Hz ratio". So your motor has a V/Hz ratio of 8:1, and if you can maintain that ratio, +- 10% in typical motor designs, you can get rated torque from it because you are at the designed flux density.

Where this becomes problematic is when you go OVER the designed speed of the motor. So if you want to run your motor at 150% speed, you will need for the frequency to be 75Hz. But you STILL only have 400V available, so the V/Hz ratio is reduced to 400/75=5.33:1. Therefore your flux density has dropped by 33%, so you are LOSING torque as you rise above base speed. Many people have a difficult time understanding why their machines stop working at higher than base speed, so those of us in the VFD world must explain it. Over and over and over it seems. You most likely read something that was attempting to do just that.

Hoped that helped you.