non Linear loads

[kwired]

It's a safe bet that it's single phase.

99% of single family residences only get a single phase service here. It takes a lot of money and cajoling to get a utility to run 3 phase out to a residential service, often including you having to pay the full cost for them to run the third wire from the nearest 3 phase connection point, which can be miles away.

On a farm though there can be high leg delta systems supplying the farm - three phase is run to structures that need three phase, like a feed mill or grain storage/handling area. If there is a larger water system there may be a three phase well. If there is a house on the property it still normally just gets 120/240 supply run to it but originates at the high leg delta supply and not a separate supply, same for other outbuildings with minimal power requirements.

 

[Smart $]

They are only "additive on the neutral", when the harmonic multiplier of the fundamental frequency is divisible by the number of phases. So your fifth harmonics are not additive on the neutral, but your even harmonics in split phase, and your multiple of 3 harmonics in 3-phase, are.

:thumbsup:

A matter of physics often overlooked due to published works covering 3Ø only. The question is, do these power supplies generate these additive harmonics?

 

[gar]

170810-1246 EDT

If you take two identical waveforms, invert one relative to the other, and add these together, then no matter what the harmonic content is the result is zero.

.

 

[GoldDigger]

170810-1246 EDT

If you take two identical waveforms, invert one relative to the other, and add these together, then no matter what the harmonic content is the result is zero.

.

Fair enough. But what we are arguing is that if there are line to neutral loads there is no guarantee that the phase relationship between fundamental and harmonic will be the same on both legs. That invalidates your premise that they are inverses of each other.
If the loads are in fact symmetric, then your point is valid.

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[Besoeker]

Fair enough. But what we are arguing is that if there are line to neutral loads there is no guarantee that the phase relationship between fundamental and harmonic will be the same on both legs. That invalidates your premise that they are inverses of each other.
If the loads are in fact symmetric, then your point is valid.

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From my post #17:

If the computers are all the same and are distributed equally on each half with a common neutral back to the supply then you might reasonably assume that, for the non-linear loads on each half, their input current waveforms would the same magnitude but in antiphase and cancel even if they are not sinusoidal

Dennis mentions that they are computers. I think it was in his opening post. Maybe 50 or so of them. My response, including the caveat, was based on that.

 

[GoldDigger]

From my post #17:

Dennis mentions that they are computers. I think it was in his opening post. Maybe 50 or so of them. My response, including the caveat, was based on that.

FWIW identical does not imply symmetric.
One example is the half wave rectifier supply. There will be a DC component in the neutral. (Not a "harmonic", of course.)

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[Besoeker]

FWIW identical does not imply symmetric.
One example is the half wave rectifier supply. There will be a DC component in the neutral. (Not a "harmonic", of course.)

Think it will be AC if the 120-0-120 are antiphase.

 

[GoldDigger]

Think it will be AC if the 120-0-120 are antiphase.

Think again. If a half-wave rectifier is used, with the neutral as, for example, the negative side of the DC output, then the direction of the current in the neutral will be the same from both legs, just 180 degrees out of phase.

 

[Besoeker]

Think again. If a half-wave rectifier is used, with the neutral as, for example, the negative side of the DC output, then the direction of the current in the neutral will be the same from both legs, just 180 degrees out of phase.

Brain fade. It happens at my age.
But I think we are drifting off the original topic.

 

[gar]

170810-1500 EDT

GoldDigger:

Identical means identical. That does not include a time shift.

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[retirede]

Brain fade. It happens at my age.
But I think we are drifting off the original topic.

Drifting off topic provides for some the most entertaining reading on this forum!

 

[Besoeker]

Drifting off topic provides for some the most entertaining reading on this forum!

I've been censured for doing so more than once.

 

[Besoeker]

Think again. If a half-wave rectifier is used, with the neutral as, for example, the negative side of the DC output, then the direction of the current in the neutral will be the same from both legs, just 180 degrees out of phase.

Depends on the polarity of the diodes. With equal loads on each half, you could bypass the neutral altogether.
But we are discussing PC supplies and it isn't likely that their PSUs will be half wave rectifiers.

 

[Smart $]

Depends on the polarity of the diodes. With equal loads on each half, you could bypass the neutral altogether.
But we are discussing PC supplies and it isn't likely that their PSUs will be half wave rectifiers.

But with switch mode power supplies, don't they have like controlled rectification passing only current at various points in the cycle then shutting off at zero crossing. If when they pass current is not equal in magnitude with the other line's load current at that instant, then the 'average' neutral current is greater than the 'average' current of each line.

 

[Besoeker]

But with switch mode power supplies, don't they have like controlled rectification passing only current at various points in the cycle then shutting off at zero crossing. If when they pass current is not equal in magnitude with the other line's load current at that instant, then the 'average' neutral current is greater than the 'average' current of each line.

Most I have dealt with have full wave rectification.
I readily accept that I am not an expert in that field.

 

[GoldDigger]

This whole variation was proposed more as a theoretical oddity contradicting overstated generalizations than as a practical problem.