On Tue, 21 Aug 2007 04:35:22 GMT, Ross Herbert
wrote:

On Tue, 21 Aug 2007 11:13:31 +1000, Peter Dettmann
wrote:


Here we have a communications problem.
You referred to the normal slugs which do in fact enclose the
magnetic
path (not the magnetic loop or circuit) While there is some leakage
flux outside the iron circuit. the major operating flux is through
the
iron core for the coil, which is path to which I referred.

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.

With the exception of Ron(UK) all the posts are still centered on DC
relays whether slugged or not. Ron said "some did have a D shaped
shading pole set into the armature end of the pole".

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 :
How does an AC relay work? I understand that the magnetic field of a
DC relay coil attracts the contact arm and I assumed an AC relay just
had a diode to convert coil current to DC - however when I tried to
find an AC relay fault there was no diode. I thought 50 or 60 Hz
alternating magnetic field cannot produce such a corresponding
movement in a mechanical contact arm so I would have thought the net
magnetic effect would be zero (no overall attraction or repulsion).
Given the relay obviously operated before, I just can't see how. I
did think the 'AC' rating just meant the contacts but I don't see why
this wouldn't just be a current rating.

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




There is a slot cut into the pole face (to which the armature is
attracted). The slot divides the pole face into two sections with a
ratio of about 1/3. In this slot is usually just a solid D shaped
copper piece which forms a shorted turn on the smaller pole face
section. What happens is that the flux attracting the armature is
the
sum of the fluxes from the two sections of the pole face. With an AC
energised coil, the smaller pole face flux actually lags the flux in
the un-shorted larger pole face section, so that even when either
pole
face section has zero flux (and zero pull) there is still flux in the
other section. Therefore the armature always has some pull from the
pole face while the coil is energised. For larger relays, the
magnetic circuit is made up of laminations, but the same style of
having two distinct sections of pole face, with one having a shorting
coil around one section.

I hope this is clear, and I would rather have shown a diagram, but
with the painters in I have rather limited access to my library to
get
at the old basic theory books. I should add that I have had many
years working directly with relay, protection and control equipment

Peter Dettmann

I am not saying you don't know your relay stuff. It just seems that
you are referring to the types of relay found in general AC and DC
usage. Telephony relays are far more specialised and have the ability
to be critically adjusted to suit a particular application.