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operation of induction motor on lower frequency than its rated

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i`m having a 3ph induction motor star connected with rated voltage = 380 v and rated frequency =50 hz ......

i`m trying to control its speed by keeping the supply volyage constant=380v and decreasing the applied frequency to= 30 hz


an engineer told me that i must also decrease the applied votlage by the ratio =380/50 so as to keep the flux constant , and to avoid the increase of the flux that may casue overheating to the motor .......


i just want to ask :how could the high flux in the air gap or in the iron core of the stator or the rotor could cause over heating on the induction motor


please answer me as quick as possible.....



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Hello maxona


Welcome to the forum.


Increasing the flux in the iron has two affects.

1] increaseing the hysteresis losses in the iron. (heating the iron)

2] Changing the inductance.

As the flux in the iron increases, the inductance changes. Above a particular amount of flux, there is a reduction in inductance as the flux is increased.

The effect of the reduction in inductance for a voltage driven circuit, is that the impedance reduces resulting in more curent flow and more flux. This in turn further reduces the inductance, increases the flux and increases the hysteresis loss etc etc.....


Best regards,

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Hello mazona


Saturation of the iron wil cause a significant increase in the iron loss of the motor. This will increase the heating of the iron and can lead to motor failure.


Typically, at rated load, the motor is designed such that the iron loss and copper loss are of the same order of magnitude. The copper loss is often in the order of 50% higher than the iron loss, but may be equal. If the iron loss is increased significantly, this will result in a significant increase in the total losses of the motor. For example, if the iron loss was doubled, the motor could over heat under open shaft (no load) conditions.


Best regards

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