grounding chassis power transformer bodies?

[coppercone2]

So should power transformer bodies be grounded?

I think its confusing for a EI core lamination. I have multiple same model # HP power supplies that have the same transformer (6200 series). They all have ground, line and neutral solder points. But some of them have a chassis ground point that is 2x standoffs + internal star washers that touch the transformer plates and screw into the chassis walls. The other one has a extra like serrated jumper + wire that goes from transformer body to the ground solder point terminal on the primary of the transformer, and also the standoff + serrated washer (so you can say its a 3 point ground, 2x standoff to chassis and 1x serrated solder point to ground wire).

But I also think that these things are laminated. I mean what is it doing to ground a single lamination via a serrated washer, be it to the ground wire or the chassis?

How imporant is it that I take the earlier revisions with the slightly smaller transformer and add the serrated solder point so the transformer lamination is connected to the ground wire solder point, and not just the chassis? The bolts are isolated with a special washer, as not to make a shorted turn, so even that serrated solder point is still only possibly contacting a single lamination.

Like with a welded transformer it makes sense I guess, since its all electrically connected. But with the individual compressed laminations it seems just dodgy to ground a single lamination? Do they just assume the varnish job is bad and their not really isolated plates?

[IanB]

A picture would help. Isn't there some kind of frame/bracket that holds the transformer and bolts it to the chassis? All EI core transformers I have seen are like this. And then the ground wire is grounding the frame, which is an exposed metal part you could touch. It's probably unnecessary, but these days lots of things are done for "safety" just because there is a one in a million chance of something bad happening.

[coppercone2]

its from 1960

its like this
https://www.barbouri.com/2018/05/19/hewlett-packard-6200b-power-supply-repair/

You see the 2 hex standoffs going from the transformer to the chassis? 3/4 units have just those standoffs (they have a serrated internal star washer between the standoff and the transformer).

The 4th one has serrated solder point under one of the standoffs that solders to the ground pin on the transformer. So instead of just a serrated washer to couple the transformer to the chassis, they put a serrated washer with a solder point that solders to a jumper wire that solders to the earth ground wire that is soldered to the transformer (like a actual solder point on the transformer windings).

All the same model.

I drew the green stuff to show where the extra grounding was done on the unit with the slightly different transformer

Like should I add the serrated solder washer to the other units? What does grounding that lamination even do?  Is it just a safety incase it melts and the transformer body goes hot? The ground situation on that entire chassis is a little dodgy, I did my best to clean it up and put grease on the different screws. Maybe the concern was that the transformer could short out, the body panel could isolate and it could leave the left side body panel of the power supply hot?

[David Hess]

In sensitive applications the transformer core should be grounded because it is very likely that the electrostatic shield between the primary and secondary is wired to the transformer core.  Careful examination will likely find this connection with a wire coming out of the core and crimped under a screw.

[coppercone2]

ok I will take a look, that one I just popped the cover off of. But the other 3 which I varnished the transformer I am sure there is nothing under the screw because I took them apart for varnishing since the studs were in the way

[Terry Bites]

All metal parts should be at the same [ground] potential as the chasis unless there is a specific reason not to.

[coppercone2]

I gotta go through all these varnish jobs with a high current source to make sure the grounds are still good.

[coppercone2]

btw who else hates dealing with old transformers? God damn I gotta bite the bullet and learn to rewind so I am not so paranoid about this stuff. Heart attack when you put it on the table at the wrong orientation with all those thin wires. 

[jonpaul]

Bonjour cher Coppercone,

Good question.  Thus affects safety and shieldiing.


As a magnetics designer and manufacturer, a few comments please.....

Laminated mains transformers are assembled from lams that are insulated, brackets and screws that are not designed for reliable conductor path to chassis. The finishing is usually varnish impregnated which further limits conduction to chassis.

A separate internal shield may be included in the winding, eg   a black flying lead.

So,  you can bond the transformers frame to chassis or not, but you cannot depend on the frame as a shield.

Hope this helps you

Jon

[coppercone2]

so it seems the only way to make it reliably touch safe is to put a shield on it thats grounded

thats a interesting project, make a dummy transformer out of wood so you can form a shield over it.

[mag_therm]

Looking at some small transformers here made to UL, listed 252L,
they have all lams welded together with 2 very neat  beads of fusion weld right across the stack at the top to join all "E"s to"I"s
Along the 2 bottom corners 2 welds attach all "E"s to a square steel plate with 4 mounting holes.
So the lams are shorted in 4 places.

Search "252L transformer", will show photos of transformers by the various vendors.
I would like to know when that type of weld started to be used, how it is done, & how it affects I_mag.

Edit. While Hammond makes the 252L transformers to the above method, the similarly sized Hammond transformers for tube amplifiers are still made with through bolts, no welding. There is a high res photo here:
https://www.hammfg.com/electronics/transformers/classic/200

[coppercone2]

well it makes eddies I read about peoples experiments with MOT transformers that are welded supposedly the efficiency goes up (less heat) if you grind away the welds and use another method of fixture it. Part of what you can do other then getting rid of the mag shunts, but for a 2kV device.. not recommended. However if you turn it into a current supply, I guess the safety requirements change. Its not that much because the material is lossy and by the time you get to the weld it is kinda low