Answer :
To reduce magnetic flux density, it is necessary to reduce the voltage to a motor when the frequency is reduced. This will resist lowering of inductive reactance and increase of rotor current
When we decrease voltage applied to the motor. The current in stator is winding as well as the magnetic flux density will decrease along with decrease in induced voltage.
The voltage induced over rotor's conductor is defined here
,[tex]V_{R[/tex] = B.l.u
We can see things in other way also. Changing frequency has effect on both the motor speed and the strength of the magnetic field. With the lowering of frequency motor speed becomes slow.
With reduction in voltage, magnetic field decreases and magnetic flux becomes constant. This keeps torque production stable, regardless of frequency.
To know more about frequency here
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