Triplen harmonics in star-connected 3-phase system?

[powersys]

I've a 3-phase PM brushless motor. It's operated as a BLDC drive. The output of one of the phase current sensors is connected to Agilent Dynamic Signal Analyzer 35670A to obtain the FFT of the phase current. I find out that there are triplen harmonics (their amplitudes are not very high compared to other non-triplen odd harmonics, e.g. 5th, 7th, 11th, 13th, etc) in the FFT spectrum. If I'm not mistaken, for a star-connected 3-phase system, triplen harmonics should not appear because neutral (star point) is left open (i.e. not connected to ground). So, any idea why triplen harmonics are observed in the phase current in my system?

Thanks.

 

[powersys]

Hello... could anyone pls advise? Thanks.

 

[FvM]

It's rather simple. The 3rd harmonics is only zero, if the individual output currents are exactly symmetric and have 120°
phase shift. Both conditions are not neccessarily met in your system.

 

[powersys]

Hi FvM, you always shed light on my problem and help me to move forward... I'm really appreciate! To further enhance my understanding, can I say as follows:

If a 3-phase Y-connected system (with open neutral, such as the motor in my case) is UNBALANCED, triplen harmonics will exist in phase current.

Thanks.

 

[FvM]

With unsymmetric currents, I meant a non 50 % duty cycle of the driving waveform. But I think, unbalanced magnitudes can also achieve 3rd harmonic components.