Inverter - V/F constant control of an induction motor

[MissP.25_5]

Hi. I am writing a report on an experiment. The experiment is on power electronics. An investigation of V /f constant control of induction motor with inverter was carried out. A converter and an inverter are connected to an induction motor to change AC to DC and back to AC, as shown in the diagram.
. The output frequency of the inverter is set to 20, 30, 40, 50, 60, 70 Hz and the output voltage, output current and rotation speed (rpm) of the inverter are then recorded. I want to know the reason to the results for f-V relation. At first they're proportional but when it reaches 225 V, the voltage seems to go constant, why is that? My teacher said this was expected to happen and we have to write out the reason. Help, please?

 

[KataKoniK]

Hello,

Thank you for sharing your experiment on power electronics. I believe the phenomenon you observed in your experiment is known as the "saturation voltage" of an induction motor.

In an induction motor, the voltage and frequency of the power supply are directly related to the motor's speed of rotation. As you increase the frequency, the motor rotates faster, and as you decrease the frequency, the motor slows down. This is known as the V/f ratio, where V is the voltage and f is the frequency.

In your experiment, you observed that up to a certain voltage (225 V in your case), the voltage and frequency were directly proportional, as expected. However, beyond that point, the voltage remained constant while the frequency continued to increase. This is because the motor has reached its maximum speed and cannot rotate any faster, even with an increase in frequency.

This happens because of the saturation effect in the motor's magnetic core. As the frequency increases, the magnetic field in the core also increases, which in turn increases the motor's speed. However, at a certain point, the magnetic core becomes saturated, meaning it cannot hold any more magnetic flux. This results in the motor's speed reaching its maximum limit, and the voltage remaining constant.

In summary, the reason for the saturation voltage in your experiment is due to the induction motor's magnetic core becoming saturated, which limits the motor's speed and causes the voltage to remain constant.

I hope this helps in understanding your experiment results. Please let me know if you have any further questions.