Linear and Non linear

[gar]

230602-1627 EDT

Jraef:

I disagree with what you described.

An ideal semiconductor diode has a reverse impedance of infinity, and a forward impedance of 0 ohms.

A real world semiconductor diode has a reverse impedance very close to infinity up to some breakdown voltage. So it is quite similar to the ideal diode.

In the forward direction a diode has a small voltage drop that depends on the magnitude of forward current. In a useful working range. This is around 1 V for a silicon diode. Too much forward current will cause excessive diode heating and failure. The size of the semiconductor chip and its thermal conductivity, and heat sinking will determine current rating.

If the load on the diode is a pure resistance, then the output voltage across the load resistance will be close to a 1/2 sine wave.

If the load is a parallel combination of a resistor and capacitor, then depending upon the component values of the load you will get some stored energy on the capacitor ( means a voltage on the capacitor at the start of the next cycle ), and capacitor voltage will not drop to 0. The diode forward drop is still in the 1 V range, and on the next cycle the diode won't start to conduct until the source voltage exceeds the capacitor voltage by the diode drop. Thus, forward conduction is less than a half cycle. Make the capacitor big enough, and forward conduction will occur for a short time, and voltage ripple on the capacitor will be very small.

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[ron]

With all this in mind, it would seem that with LED lighting, VFD driven mechanical equipment, and computers, that most commercial buildings should be considering their neutral to be a current carrying conductor on a typical basis, since a major portion of the load consists of nonlinear loads.

310.15(E) Neutral Conductor.
Neutral conductors shall be considered current carrying in accordance with any of the following:
(3) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be consider current-carrying conductor.

 

[synchro]

With all this in mind, it would seem that with LED lighting, VFD driven mechanical equipment, and computers, that most commercial buildings should be considering their neutral to be a current carrying conductor on a typical basis, since a major portion of the load consists of nonlinear loads.

Of course, this will only be an issue with L-N loads that are nonlinear and not for L-L or 3-wire delta loads.

 

[jim dungar]

Of course, this will only be an issue with L-N loads that are nonlinear and not for L-L or 3-wire delta loads.

And for those situations where the harmonics add instead of subtract.

My personal experience has been harmonics adding on the neutral is an extremely rare event and cannot be predicted.

 

[ron]

Of course, this will only be an issue with L-N loads that are nonlinear and not for L-L or 3-wire delta loads.

Correct, my point was whether the neutral is a CCC or not. It is hard to think of L-N loads that are not nonlinear.

 

[wwhitney]

Correct, my point was whether the neutral is a CCC or not. It is hard to think of L-N loads that are not nonlinear.

But the phrase "the major portion of the load" could be taken to refer to the characteristics of the loading and the extent to which it is non-linear, rather than just classifying each load as linear or non-linear.

Cheers, Wayne

 

[Joethemechanic]

This is all so much fun. Pretty soon we are all going to be carrying oscilloscopes as part of our standard gear.