Author Topic: BrOdin coil - High power Big sparks!  (Read 6890 times)

Offline davekni

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Re: BrOdin coil - insane racing sparks at 2000A OCD
« Reply #120 on: May 02, 2021, 06:03:41 AM »
It could be just the rectifier.  It could instead be that one or more IGBTs fried, which then fried the rectifier.  Unfortunately, that would be my guess.  Fried IGBTs likely have a collector-to-emitter short that you can measure with a meter.  If no IGBTs show as shorted, then I'd run the bridge at low voltage (say 5-50V) and scope the outputs.
David Knierim

Offline fh89

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Re: BrOdin coil - insane racing sparks at 2000A OCD
« Reply #121 on: May 02, 2021, 08:02:38 AM »
It could be just the rectifier.  It could instead be that one or more IGBTs fried, which then fried the rectifier.  Unfortunately, that would be my guess.  Fried IGBTs likely have a collector-to-emitter short that you can measure with a meter.  If no IGBTs show as shorted, then I'd run the bridge at low voltage (say 5-50V) and scope the outputs.

Just checked, no C->E shorts! Seems like I got lucky and it's just the rectifier!

The rectifier was only rated for 40A rms and I was pulling 180A+ even before upping the primary current, not only that, but after looking closely at it, the way I had it wired was not optimal, with all of the current going through a single diode pair instead of paralleling all 3 pairs like I should have! Kind of unreal that it didn't blow up before this! Just ordered the 100A version as a replacement.
« Last Edit: May 02, 2021, 08:30:40 AM by fh89 »

Offline Uspring

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Re: BrOdin coil - High power Big sparks!
« Reply #122 on: May 02, 2021, 01:05:01 PM »
An interesting coil you got there. The combination of massive racing sparks together with only tiny topload breakout seems to imply, that the top load voltage can't alone be the source of the racing sparks. A much more likely cause is the voltage induced by the primary coil. The idea is, that there are 2 components to the magnetic field near the secondary. One component is the field caused by the current in the secondary. That field is pretty much constant along the secondary and causes a nearly linear voltage drop from the top down to the bottom.
The other field component is generated by the primary coil. That is localised around the bottom of the coil and causes an additional voltage drop there. The amount of voltage induced depends on the primary current and the geometry of the primary coil and its distance to the secondary.

The field of the primary coil drops off quite rapidly in the direction upward. The primary coil can be seen as a collection of rings, each one carrying the same current. The field of a single ring extends up to roughly its diameter, more precisely: The voltage induced in the region from the bottom to one diameter up the secondary will be about 90% of the total voltage induced along the complete secondary.

So there are two ways to avoid the induced voltage drop, which will cause the racing arcs.

1. Move the secondary away from the primary, i.e. up. The penalty for this is lower coupling.
2. Move the primary coil outward, i.e. make it larger and particularly avoid too small inner rings. That will cause the primary field to extend further up and will distribute the voltage stress along the secondary. There is also a coupling penalty, but it is less than solution 1.

You don't seem to have a large diameter primary, which possibly aggravates the arcing problem. But this just a guess and certainly a lot of work to change.

@davekni: The model implied by the above can be tackled almost analytically by calculating center fields of current rings. These are pretty simple. An even simpler model is a split of the secondary in 2 parts, one strongly coupled and the other not at all. Similar to a magnifier setup. I believe JavaTC obtains all these results but does not export them.

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Re: BrOdin coil - High power Big sparks!
« Reply #122 on: May 02, 2021, 01:05:01 PM »

 


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