Neutral Question

[joeyww12000]

I have been reading and trying to learn more about neutral current in general. My question is what is the easiest way to calculate neutral current? I have read that in a balanced 3 phase system the neutral current would be 0 for loads such as incandescent lighting and approximately that of the line current for loads such as high pressure sodium or flourescents. What makes the neutral current different in these scenerios?

 

[gar]

080607-0632 EST

joey:

Calculate
i = I (sin (t+0) + sin (t+120) +sin (t+240) )
for any value of t from 0 to 360 and you will find the value is zero.

With a resistive load and a sine wave voltage applied to the resistor the current will be a sine wave. Thus, with a resistive load and equal currents in each leg the neutral current will be zero.

Graphically draw a current waveform consisting of rectangular pulses 10 degrees wide located at the peaks of a sine wave. Next copy these pulses displaced by 120 deg, and again at 240 deg. Add these three sets of pulses and you will not get zero.

Certain types of harmonic content in your current waveform will not cancel in the neutral.

.

 

[nakulak]

I have been reading and trying to learn more about neutral current in general. My question is what is the easiest way to calculate neutral current? I have read that in a balanced 3 phase system the neutral current would be 0 for loads such as incandescent lighting and approximately that of the line current for loads such as high pressure sodium or flourescents. What makes the neutral current different in these scenerios?

as previous poster stated, the incandescent loads are purely resistive, therefore the waveforms cancel on the neutral.

the loads with ballasts, however, contain inductive loads. The inductive loads can generate waveforms which do not cancel, and therefore create current on the neutral (unbalanced).