In-Rush Current Correction

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Dr Buzzard
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Joined: Tue Mar 16, 2021 7:37 am

In-Rush Current Correction

Post by Dr Buzzard »

Hi -

Mark's identification of the power transformer primary winding as a source of in-rush current is incorrect. In fact, the situation is just the opposite (sorry Mark).

Old electronics 'saw': ELI the ICE man. L-Inductance, C-Capacitance, E-voltage, I-current.

Voltage always leads current in an inductor (the opposite is true for a capacitor).

As the first trickle of current flows, the magnetic lines of force in an inductor expand and cut the other winding inducing a reluctance of the opposite polarity. The inductor (transformer) resists current flow. This is further enhanced by the negative reluctance of the secondary winding. The inductor does everything in its power to stop the flow of current - until finally the magnetic lines of force stop expanding, and then current flows with almost no reactance.

This is why inductors are put in series in a power supply while capacitors are put in parellel.

I still like the idea of using thermistors though :)

Cheers -
audiofanman
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Location: Suffolk County - Long Island, NY

Re: In-Rush Current Correction

Post by audiofanman »

But..I thought that briefly the primary looked like it had very low resistance, not quite a dead short but darn near.
And that as the magnetic field was growing (from current, not voltage) but still not strong enough to affect anything because the
change was only from a low current and low voltage to more of both. (Still on the up swing, and not near the peak)
This is what I thought was akin to low impedance.
Does an inductor affect current flow when the current is in steady state (DC) ?
My understanding is not so much. (Now that I have an o=scope and other toys, may be I will see if I can see my self)
I know it makes an electromagnet, but imagine its DCR doesnt change much (A DC low to no inductance)
Again, only my understanding, that its the change in a changing magnetic field that affects the current...
Again, im trying to hash out what I learned, which might not be 100%, let me know..
Say 120V 60hz, just applied to the primary, say while just coming positive in voltage, on a primary which has no collapsing field.
That only after the current produced the field (After the positive peak), would be affected by the voltage reduction in the cycle.

So, as I just learned from another fantastic and sometimes hilarious you tuber... What came first the chicken or egg ?

The rooster dang nabbit... And yes, this was "cleaned up a lot"
What I listen with.... Elsinore's, Yaquin MC-13S / EL34 - MS-300B, My pile of schitt :lol: Mani, Loki, Modi Multibit. Audio-Technica AT-LPW40WN/AT-VM95SH/H, R PiCore/LMS + Digi
jaymar
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Re: In-Rush Current Correction

Post by jaymar »

An earlier thread here on the forum appeared to present evidence that power transformers can have significant inrush current. https://www.blueglow.club/viewtopic.php?f=22&t=27
Hugo
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Re: In-Rush Current Correction

Post by Hugo »

The in-rush does not come from the inductance of the primary winding but from the load on the secondary winding. You have cold filaments that are in near short and if you have solid state rectifiers you will have the in-rush of the power supply capacitors.
jaymar
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Re: In-Rush Current Correction

Post by jaymar »

High inrush currents occur in isolation and autotransformers with nothing connected to the secondary.

Reposting what I wrote in the previously linked thread:
I've occasionally noticed a brief current surge when I plug in my 1 kVA toroidal isolation transformer.

Just went to Wikipedia and found an explanation in the article about inrush current.
https://en.wikipedia.org/wiki/Inrush_current
When a transformer is first energized, a transient current up to 10 to 15 times larger than the rated transformer current can flow for several cycles. Toroidal transformers, using less copper for the same power handling, can have up to 60 times inrush to running current. Worst-case inrush happens when the primary winding is connected at an instant around the zero crossing of the primary voltage (which for a pure inductance would be the current maximum in the AC cycle) and if the polarity of the voltage half-cycle has the same polarity as the remanence in the iron core has (the magnetic remanence was left high from a preceding half cycle).
Hugo
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Re: In-Rush Current Correction

Post by Hugo »

Thanks for pointing that out. It makes sense with observations, actually. So from that it seems be related with the magneto static condition of the core rather than the coil behaviour.

https://en.wikipedia.org/wiki/Magnetostatics

Here is a bit more wiki background info.
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blueglow
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Re: In-Rush Current Correction

Post by blueglow »

Yep, its not the transformer causing the inrush, its the power supply caps charging.
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