The flux produced in the core can be calculated with the formula as given below.
The EMF in the stator/phase(Eb)= -n* Rate of change of flux
Flux = Eb/ 4.44 f*n ----------(1)
Where Eb is the back EMF/phase produced in the stator, f-frequency, and n- Number of turns in the stator coil
Thus, the flux in the stator core is proportional to the ratio of V/f. If speed is increased by increasing frequency, the voltage has to be increased in the same proportion with the increase in frequency to maintain the constant flux in the core in order to avoid the overheating of the core. For constant torque applications, the V/f ratio is maintained constant by PWM inverter. The voltage and the frequency get varied simultaneously to maintain the constant flux in the stator core of the motor.