AC relay theory

Right, I messed that up, in trying to differentiate between the total magnetic iron loop and the crossectional area of the iron. I tried to point out that I did understand what you meant by a slug, and that it did not cover the whole length of the magnetic circuit.

Ok but Ron did not say that these AC relays were actually an extensively BPO use if at all by them. That was not the question, he just mentioned just that they do exist,and he has seen them. What has happened is that the focus has been drawn in to this red herring path of BPO relays. This was because they were cited as a typical relay one could describe. However in the process the actual original question has been lost in a discussion about DC relays when the subject is (was) basically how do AC relays operate. So the question to be answered (which I believe I have largely done) was :

If a discussion on 3000 type relays were involved, I have my well worn gram balances, contact adjusters, and armature bender at hand.

You are right that the 3000 style as an AC type is rare, as there are much better designs used for AC, but still using the same principle of the divided magnetic path as Ron described.

Peter Dettmann

On Tue, 21 Aug 2007 22:41:21 +1000, Peter Dettmann wrote: SNIP

My initial post was to simply add a comment so that other readers would not gain the impression that the only type of AC relay was one with a "shading ring". It is only because of responses to my initial post that I added further comments to expand on it or to correct those responses but I did not plan for the thread to specifically become one about BPO relays. If no one had cared to respond then it wouldn't have gone further.

Actually, I haven't found any description on AC relays which refers to a "divided magnetic path", or anything remotely resembling this term, so I doubt that it means anything at all. What I did find is a description of the various types of shading ring (as refered to by Ron and others) which might be found on AC relays and/or contactors meant for 50/60 Hz operation. See page 36.

While operating on the same principle, the solid copper slug as used on the BPO 3000 type relay is far more effective for operation at lower than 20Hz while also allowing the tailoring of the delay period by varying the length of the slug.

AC relays commonly used today are not necessarily "better designed" for use on AC than the 3000 type (and similar) relays. The AC relay of today only has to operate on 50/60Hz where the slugging effect of a relatively small shading ring is adequate, thus making the design much simpler and less costly to implement than for the 3000 type relay. In many cases the addition of semiconductors simplifies the design of relays used for AC applications so that shading rings aren't required.

I might add that every relay manufacturer in the world making relays for telephony applications, would have produced similarly slugged relays of a perhaps a slightly different mechanical design - it wasn't just the BPO 3000 type which used this principle. In closing, this type of slugged relay would have been far more common than any other type of AC relay used in any other industry then, and even today. Every final selector in every strowger based switching system the world over used one. They had to be reliable and good for millions of operations.

I don't have an armature bender but I do have the pressure gauges and spring adjustment tools.....

As I said, I am guilty of omitting one word in my phraseology. If by omitting this word you were led to thinking that I was saying the core itself was a permannent magnet then you may be less intelligent than you imagined. If it were a magnetic material as you obviously imagined then the armature would be permanently held operated, so what then would be the purpose of the winding?

Well, that's because there is no single word (technically speaking) - and I didn't infer that there was. There is however, a symbol for Magnetic Permeability, Mu - expressed in Henry's per meter, with which I am sure you are familiar. As in most fields of science there is usually a set of symbols to accompany it and we all know that symbols are basically shorthand representation or descriptor for a term or property. So it can be said that the single character U = Mu (sorry, can't type the actual character) is a one letter word meaning Magnetic Permeability.

I GOT to remember that one...

Ross page 36 does not seem to be relevant, however, your comment that you find no reference to a divided path would indicate that you have no idea what I have been talking about. You need to understand that on an AC relay, the pole face is divided, and one section has the shading ring around it, and the other section has no shading ring (so that unshaded section it is just like you find on a DC relay). The armature is therefore is attracted by the sum of the fluxes from each of the two pole faces. The idea is that while the the coil is energised, then even when the un-shaded pole flux is zero (twice per cycle), there is still flux from the shaded (lagging or delayed) pole, and so that there is no time during the AC cycle when there is zero flux pull on the armature.

Peter Dettmann

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I certainly understand the principle of operation as described on pages 36 and 37, and you seem to be saying very much the same thing as depicted in fig.a(3) along with the accompanying description. As I understand it the total flux in the core is divided where it passes through the pole face and the electromagnetic force on the armature will be the sum of the two forces produced by the two flux paths through the shaded and unshaded parts of the pole face. The electromagnetic force resulting from both flux paths never actually drops to zero throughout a full cycle of current so the armature remains held in.

I thank you for your patience and your comments have been helpful.