High 3rd harmonic on the neutral

Chris Pearson:

. . .

Now introduce a harmonic at 150 Hz i.e. 3 times the fundamental. Because they are now going three times as fast, the vectors are not 120 degrees out of phase with each other, but in phase so they add up.


. . .

Very succinctly put, Chris! I was trying to think of a way to put this over simply but you've beaten me to it.


Over the past several years I have seen in IET journals references to rectifiers, switch-mode supplies, thyristors, etc. and the effect that the wave chopping of these ever-increasing devices will have on the power supply. Effectively, these chopped current waveforms are a series of harmonics. The third harmonic is particularly troublesome because of the way in which the waveforms add up and augment one another on the neutral. Higher harmonics can also add, but they are less of an issue because they are much smaller.


I think these harmonics are becoming a part of life, though in this instance they are larger than I would expect. In the case of this installation, a proper analysis would require to display the current waveforms of each of the phases on an oscilloscope and assess the extent of the harmonic distortion.

Chris Pearson:

. . .

Now introduce a harmonic at 150 Hz i.e. 3 times the fundamental. Because they are now going three times as fast, the vectors are not 120 degrees out of phase with each other, but in phase so they add up.


. . .

Very succinctly put, Chris! I was trying to think of a way to put this over simply but you've beaten me to it.


Over the past several years I have seen in IET journals references to rectifiers, switch-mode supplies, thyristors, etc. and the effect that the wave chopping of these ever-increasing devices will have on the power supply. Effectively, these chopped current waveforms are a series of harmonics. The third harmonic is particularly troublesome because of the way in which the waveforms add up and augment one another on the neutral. Higher harmonics can also add, but they are less of an issue because they are much smaller.


I think these harmonics are becoming a part of life, though in this instance they are larger than I would expect. In the case of this installation, a proper analysis would require to display the current waveforms of each of the phases on an oscilloscope and assess the extent of the harmonic distortion.