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AC relay theory

D

Dave Plowman (News)

Jan 1, 1970
0
On Mon, 20 Aug 2007 13:39:23 +0100, "Dave Plowman (News)"
You are being a bit nit-picking aren't you?... You know what was meant
by the shorthand description. In this case "magnetic material" means
"magnetically permeable material" to be precise.

In terms of the OP's question, no. Magnetic suggests it will attract an
armature etc without being externally energised.

I was searching for a single word that means 'magnetically permeable' but
if it exists it escapes me.
 
P

Peter Dettmann

Jan 1, 1970
0
What is the difference between "magnetic path" and "magnetic circuit
or loop"? It is the same thing in my experience.

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.
But Ron, who claims to be familiar with "GPO relays", is wrong. Not
one of the standard 3000 type relays used in GPO exchange equipment
was fitted with a D shaped slug. He may have come across a relay which
was obtained for use in a specialised piece of equipment but this was
definitely not inthe standard library of relays used by the GPO. I
worked on SxS exchange equipment of the same type as used by the GPO
from 56 - the early 60's when it was replaced by LME ARF102 x-bar
(Aust), and not once did I come across a relay with a D shaped slug.
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
 
R

Ross Herbert

Jan 1, 1970
0
On Tue, 21 Aug 2007 22:41:21 +1000, Peter Dettmann
If a discussion on 3000 type relays were involved, I have my well
worn gram balances, contact adjusters, and armature bender at hand.

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.
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.

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.
http://books.google.com/books?id=gE...ts=197E5vBgGC&sig=IcSAXw39fQrQksKmenpcNVhKamU

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.....
 
R

Ross Herbert

Jan 1, 1970
0
In terms of the OP's question, no. Magnetic suggests it will attract
an armature etc without being externally energised.

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?
I was searching for a single word that means 'magnetically permeable'
but if it exists it escapes me.

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.
http://searchsmb.techtarget.com/sDefinition/0,,sid44_gci543303,00.html
 
P

Peter Dettmann

Jan 1, 1970
0
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.
http://books.google.com/books?id=gE...ts=197E5vBgGC&sig=IcSAXw39fQrQksKmenpcNVhKamU

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
 
R

Ross Herbert

Jan 1, 1970
0
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


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.
 
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