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Ac Motor Selection With A Variable Speed Drive(vsd)


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My first question is :

Is it a rule that we have to choose Nominal Voltage of an AC motor one step under supply voltage when that is controlled with a VSD ?


I saw a system with these characters : Nominal voltage of AC motor : 270 VAC , 50 Hz

Main bus voltage : 400 VAC , 50Hz

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Hello Moy


Welcome to the forum.


Most installations would use a motor rated the same as the supply voltage.

If you choose a motor with a lower rated voltage, the current will be hiigher so you will need to use a larger rated VFD. - the rating of the VFD is current based.

The advantage of using a lower rated voltage for the motor is that you can get a higher torque at speeds above the rated speed of the motor.


If the motor is rated at the supply voltage, the voltage applied to the motor will increase with speed up to rated speed and then will be limited to the supply voltage. This means that below rated speed, the motor will operated at rated V/F, but above rated speed the V/F will reduce with speed.

The V/F determins the flux in the gap and this in turn determins the torque available.


Best regards,


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Thanks a lot;


I have to improve a system, a part of that is drives and motor, it has been used in lubrication and ventilation in Gas turbine.


Ac motors has used for pump and fan, I think that the cause of the difference between voltages is Controllability. In this situation motor can be controled better.


I guess another cause is that preventing noise on other control systems.


what do you think?


If I chang level og voltage, what will happen?

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I have another Idea about it, decreasing voltage level help protection systems agains Over Voltage fault,() especially in the Starts time.


what do you think, dear these page visitor? PLZ say your opinion easily.Do not worry about saying.

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