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Harmonic for Motor

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Thanks Mark for his prompt reply regarding harmonics. (I should have posted the topic under motor section)


I attached a harmonic graph measured from a 3phase 1.5kw motor. Apparently, 5th harmonic is the highest in terms of %Vn and %In.


Is this a bad sign??


Keen to know more...


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Hello klangwcng


Unfortunatley, iron is not as linear as we would like and if you increase the flux in the iron too much, it goes very non linear as it reaches saturation. It suspect that you are running close to saturation on the crest of the waveform and this is introducing the 5th harmonic in the current. This will appear to be highest when the motor is operating open shaft. As you increase the load on the motor, you will find that the ampltude of the 5th harmonic will stay the same, but the fundemental component will increase, droping the THD of the current. The iron curves affect the magnetising current component.


If you reduce the terminal voltage, you will find that the 5th harmonic will rapidly drop.

Small motors operate closer to saturation than larger motors and so the effect is higher on the small motors.

I do not expect that this is a problem.


Best regards,

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