# AC relay theory

Discussion in 'Electronic Repair' started by Steve, Aug 17, 2007.

1. ### SteveGuest

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.

2. ### Straw ManGuest

Enough latent magnetism in the core to somewhat negate the alternating
field? Same goes for an AC solenoid as in a doorbell plunger I suppose.

3. ### Dave Plowman (News)Guest

AC coil relays are common. A magnet of any type will attract a magnetic
substance regardless of polarity - it's just if the second substance is -
or gets - magnetized that they can repel. So you use a low permeability
substance for the armature. Something like soft iron.

4. ### Derek GeldardGuest

No.

An AC relay is just an electromagnet operating a set of contacts.

An electromagnet attracts ferromagnetic material during both halves of
the AC cycle. You could try it at home with a home wound electromagnet
and a battery, the electromagnet will attract iron no matter which way
round the battery is connected, or for that matter which way round the
coil is wound, (same difference !) .

The only difference is that the wound coil of a solenoid (esp. with an
iron core) forms an inductor which has an impedance which acts to
reduce the AC current through the coil for any particular AC voltage
and frequency, it does this without causing electrical energy to be
wasted (For the pedants I'm not saying it's perfect).

This figures significantly in the design of AC relays and
elecromagnets.

HTH

DG

5. ### DaveMGuest

I haven't seen anybody describe the real difference between AC and DC relays.
A relay has a coil and the pole piece, or armature. Voltage applied across the
coil causes current to flow in the coil, creating a magnetic field, which causes
the armature to be pulled into the center of the coil, thus energizing the
relay's contacts. Both types of relays operate on the same principle of
electromagnetism.
The difference between the DC and AC relay is that the AC relay has a shading
pole, or a heavy shorted turn imbedded into one end of the armature. Its
purpose is to maintain a high flux level in the armature when the current in the
main coil goes through zero. This acts to eliminate buzzing or chattering that
is evident when you drive a DC relay with an AC voltage.
There is no diode in an AC relay. A diode is commonly used across the coil of a
DC relay to eliminate the high reverse EMF caused by the collapse of the coil's
magnetic field. If a diode were used on an AC relay, it would create a short
circuit every half cycle, something you want to avoid.

An AC relay can be used in a DC circuit, but not vice versa. If you drive a DC
relay from an AC source, the lack of a shading pole in the relay will cause
buzzing, possibly allowing the contacts to bounce during zero crossings of the
power source.
If you drive an AC relay from a DC power source, the shading pole on the AC
relay will cause the relay to be slow to release when power is removed.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

"In theory, there isn't any difference between theory and practice. In
practice, there is." - Yogi Berra

6. ### Sam GoldwasserGuest

You will usually find a copper "shading ring" wrapped around the end of the
pole piece. The current induced in the shading ring delays the decay of the
magnetic field long enough to smooth it out between cycles. There is no
diode or smoothing cap!

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7. ### Ross HerbertGuest

In the telephony industry these are known as "slugged" relays. Such
relays have a solid copper slug of a specific length - eg. 1/2" or 1"
depending on delay period required - the same diameter as the coil
itself. The slug could either be at the armature end or the heel end
of the coil depending upon whether a predominantly slow operate or
slow release was required. For ac operation it hardly matters which
end is slugged and if pushed for a part you could use a relay with a
spare winding on it and simply short circuit this winding to produce a
"slugging" effect.

8. ### James SweetGuest

They have a shorting bar like a shaded pole motor.

9. ### Peter DettmannGuest

No there is a difference here Ross, in the AC relay, the "slug" does
not cover the whole of the magnetic iron path, it is typically only
applied to about a quarter of the iron circuit. The process is to
delay the decay of flux in that slugged path so that there is a useful
magnetic pull during the time that the un-slugged path has zero flux,
(and therefore zero magnetic pull). Using the DC relay slug is not
really useful for the AC case as it covers the whole magnetic path.

Peter Dettmann

10. ### Peter DettmannGuest

For completeness I should have added that we did extensively use
relays on AC fed from a full wave bridge rectifier, and without
capacitor for smoothing. This gave a tendency to chattering as there
is still a pulsating current to the relay, however this chattering was
overcome by the use of an armature end slug (as you describe) which
A longer slug could be used, but fast operating speed was critical.

Peter Dettmann

11. ### Ross HerbertGuest

Hi Peter, there is NO difference.

The principle is exactly the same even though the use of telephony
relays is predominantly DC usage. However, they also were used in AC
applications such as the detection of ringing voltage in ring trip
circuits before the advent of semiconductor rectifiers. A relay which
would chatter in response to 16-2/3 c/s ringing would not be very
effective as a ring trip relay so the slug performed the same function
as in modern AC relays at 50 or 60 Hz.

In the telephone type relay I was referring to the slug does NOT cover
the full length of the winding bobbin either. As I said the slugs are
in different lengths eg. 1/2", 1", 1-1/2". The winding bobbin length
is always a fixed length for the relay type and the slug was located
either at the armature end or the heel end. A typical ring trip relay
would have a 1" armature end slug.

Here is a typical 3000 type telephony relay as used in UK and
Australia in SxS. http://www.britishtelephones.com/autorel.htm

In fact this particular picture appears to show the winding bobbin
with a copper front cheek which is the smallest armature end slug of
all. In normal "donkey" relays the front cheek is always bakelite.

For further study of the BPO 3000 type relay data see
http://www.samhallas.co.uk/repository/documents/yellow_book_complete.pdf

12. ### Ron(UK)Guest

The common GPO 3000 had the copper slug embedded into the armature. They
would work on AC but were much happier on DC

My father manufactured amusement machines and I entered the business
even before leaving school.

There are still many GPO type relays and boxes of contact blades kicking
around in my mothers house. (Also Quite a lot of those wonderful
uniselectors). The ability to strip them down and reassemble them in
many permutations of contact arrangement, made them a wonderfully
versatile device, and pretty reliable also.

Ron(UK)

13. ### Ross HerbertGuest

I didn't say it did.

I beg to differ. As others have described, A relay, whether DC or AC
is simply a coil of wire on a core of suitable magnetic material
(usually soft iron) with a closed magnetic loop which passes through
the pivoted armature. It is only the inclusion of a delaying mechanism
- in the AC case, a shorted copper turn or slug - which results in a
delay to ensure the armature stays held while the AC curent passes
through each half cycle.
For DC operation an armature end slug produces a predominantly "slow
operate" function because the slug produces an opposing magnetic field
to that produced by the winding when energised. Only after the field
which is set up by the winding has stabilised, and the corresponding
field produced by the slug collapses to zero, does the armature pull
in. While an armature end slug also delays the release of the
armature, it predominantly affects the operate time. If you required
the relay to operate as fast as possible the slug must be on the heel
end and not the armature end. In most AC operation situations, unless
either fast operate or fast release is required, it hardly matters
whether the actual delay occurs on closing or opening the magnetic
circuit.
A longer slug simply increases the length of the delay period. As I
said earlier, for the fastest operation the slug must be on the heel
end (furthest from the armature) of the winding. In cases where fast
operation and slow release (or vice versa) is required, a relay will
often employ a secondary winding which can be shorted or opened as the
case requires, to produce the slugging effect, rather than using a
fixed copper slug.

14. ### Ross HerbertGuest

No Ron, the 3000 type relay armature was fitted with a small brass
residual stud to ensure the armature was not unduly held by residual
magnetism in the core on releasing, NOT a copper slug. Impulsing
relays used an adjustable residual stud (brass screw and locking nut)
so that the residual gap could be set according to specific
requirements.

Here is a pic of a 3000 type relay with a 1" heel end slug and an
http://www.englishclocksystems.co.uk/slugrelay.html

15. ### Ron(UK)Guest

You are probably correct copper/brass non ferrous anyway

I imagined that was to adjust the throw of the armature to provide and
also take care of the residual hang.

I don't recall seeing one just like that, all the GPO types we used had
the coil full length of the frame, some did have a D shaped shading pole
set into the armature end of the pole.

We also used a later type of GPO relay, exactly the same in design but
slimmer frame and coil.

Life was so much simpler back then: GPO relays, Bulgin microswitches,
Crouzet motors, Honeywell timers... Selenium rectifie... erm well praps
not!

Amazingly, I have one of those slider rheostats in my workshop right
now. I converted it into a dummy load for testing power amplifiers.

Ron(UK)

16. ### Dave Plowman (News)Guest

Think 'magnetic material' is the wrong term. It is capable of being
momentarily magnetised - but isn't actually magnetic.

17. ### Peter DettmannGuest

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. 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".

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

18. ### Ross HerbertGuest

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.

19. ### Ross HerbertGuest

Copper is too soft and quickly becomes thinned out requiring the
armature to be replaced too soon. Brass is much harder and takes a lot
longer to wear out.
I don't understand the term "residual hang".

The travel of the armature was adjusted by bending it in a armature
bending tool. This was set to allow the specified travel and
functioning of all springsets fitted tothe yoke. The residual stud
without unduly affecting the magnetic force attracting the armature to
the pole piece on operation while at the same time allowing periodic
resetting to compensate for wear.
GPO relays are exactly what I have been talking about.

That relay in the pic was a 3000 type with heel end slug. The 3000
type relay was the standard relay used in all BPO Pre-2000 (later
issue), 2000 type SxS and SE50 exchange equipment from the 30's til
late 60's. The smaller brother to it was the 600 type relay but these
found only limited use in exchange equipment. None of the relays used
in exchange equipment that I worked on from 1956 - 1993 were fitted
with a D shaped shading pole. All 3000 type relays which required a
slugging effect were fitted with a cylindrical slug as shown in the
pic.

20. ### Ross HerbertGuest

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