Mains transformer question

[Red Squirrel]

This is more a general question/observation so was unsure if I should still post in one of the technical forums or not but was more a curiosity of mine.

I understand that mains transformers use laminated cores, which are essentially pieces of sheet metal that have an insulated coating on them and then they are press fitted together.  This is to prevent eddy currents.

However when you look at some transformers, they either use metal bolts to hold them together or even welds.  Dosen't that break the separation and end up making them all conductive together?  Or is that not actually an issue?

[jmelson]

However when you look at some transformers, they either use metal bolts to hold them together or even welds.  Dosen't that break the separation and end up making them all conductive together?  Or is that not actually an issue?

If you look closely (you may have to take one apart to see it) they often use a fiber washer under the bolt head, and sometimes a fiber tube on the bolt, to insulate it from the laminations.  Even if the bolt was insulated from the laminations but bolted through metal end caps, that could cause a shorted turn.

The welds (haven't seen them in a long time on transformers, but do see them in motor stators) are placed in strategic alignments where the magnetic field does not cut through the weld.  The field is least at the outer edge of the core.  If these welds were put in the wrong place, it could certainly cause the core to run hot.

Jon

[T3sl4co1l]

Depends where they are.  You wouldn't put a bolt through the middle of a leg span, where all the flux is flowing.  (Although IIRC I've seen this before... go figure!)

The bolt itself isn't all that likely to heat up, because the air gap around it (the bolt doesn't fit perfectly in the hole, that would be awful!) somewhat discourages flux from going through the hole.

Bolts in the corners, or in the middle of the 'E', are most common, and are fine because the flux tracks around the inside corner, leaving little to flow through the hole.

Welded cores take additional losses, but not too much as long as the welds do not form a continuous loop.  Notice they're only welded around the outside edge, and not the inside edge as well.  It's the same as wrapping a bunch of single turn windings around the core, and leaving them floating versus connecting one side of them all together -- the far end is still floating so no current flows.

Tim

[Red Squirrel]

Ahhh interesting, I guess I was thinking of the way flux works too much like how electricity works so that if the laminations are connected anywhere then the entire thing is basically one, but guess it's not quite like that, some parts of the core will have more "flow" than others.  So like if the weld and screws are at the outer end it won't matter as much as if it was right in the middle near the winding.  Guess the flux takes the path of least resistance but also the closest path then kind of spreads out from there.  So the further away the weld/bolts are from the winding the less it has an effect. 

[T3sl4co1l]

NOTHING takes the "path of least resistance".  Feel free to purge that awful expression from your memory banks!

The resistance analogy works just fine; you need to draw the equivalent circuit of reluctances.  In the case of a screw hole in the middle of a limb, you have core material all around it (low reluctance), with a higher reluctance path (air gap + screw in series; the screw will probably be moderately low reluctance (assuming steel), but not as good as transformer iron).  So, even though the core cross section has been thinned in the area, it's still shunting most of the flux through the area, just as a shunt resistor across a higher-resistance meter, say.

Flux around a hard 90° bend, works just the same as current flow around a conductor.  It bunches up along the inside corner, but it certainly doesn't prevent current flow through the outside corner.  There's just a lot less there.

Now, in practical transformers, you do have the matter of saturation.  This is a local increase in reluctance, in response to high flux density.

Indeed, as you approach saturation, flux tends towards taking a longer path length!  This prevents flux from bunching in too tightly around inside corners, or necked-down areas (like around screw holes in the middle of a limb), and forces more to the outside of the bend, or through the air.

This would be equivalent to having a resistive material, whose resistance increases at high current density.  Say you had a sheet of that goop they put in Polyfuses -- it's mostly a low resistance, but it heats up around inside corners and pushes current to the outside when it gets too high.  (That would be a little too dramatic of an example though- Polyfuses are designed to exhibit negative incremental resistance.  A more gentle curve would be right.  The inside curve, in a magnetic core, doesn't oversaturate itself, it still carries Bsat all the while.  It's just that the inside track doesn't carry much more than Bsat, and the total cross section that's sitting at Bsat grows until the whole cross section is filled in.)

It would be in this case, where you expect to see screws heating up more.  Basically, the core loss on such a design, will increase anomalously by, say, Bsat/2 (for a 50% neck-down at the screw hole), and the inductivity falls; and this will continue until Bsat proper is reached, when inductivity completely tanks and the whole core becomes saturated.


Related riddle: what happens if you put steel pole pieces (Bsat = 1.5T) around a superconducting magnet (B = 4T, and assuming the coil maintains the same current flow)?  Does the field increase, decrease or remain the same?  If different, how much?

Tim