AC relay theory

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.

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.

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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

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!!!

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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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no

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.

They have a shorting bar like a shaded pole motor.

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

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 was only about 1/16" long. A longer slug could be used, but fast operating speed was critical.

Peter Dettmann

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useful

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.

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

WARNING: Over 3MB. It will take some time to download.

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)

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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.

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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 adjustable residual stud.

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!

Ron(UK)

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

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    Dave Plowman        dave@davenoise.co.uk           London SW
                  To e-mail, change noise into sound.

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

through

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.

armature.

residual

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.

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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 adjustment on the armature was to allow release time adjustment 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.

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pole

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praps

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.

magnetic

the

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.

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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.