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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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AC relay theory
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
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AC relay theory
On Fri, 17 Aug 2007 08:01:28 -0700, Steve wrote:
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. 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
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AC relay theory
In article .com,
Steve wrote: 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. 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. -- *Gaffer tape - The Force, light and dark sides - holds the universe together* Dave Plowman London SW To e-mail, change noise into sound. |
#4
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AC relay theory
On Fri, 17 Aug 2007 12:13:25 -0400, Straw Man wrote:
On Fri, 17 Aug 2007 08:01:28 -0700, Steve wrote: 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. 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. 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
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AC relay theory
"Dave Plowman (News)" wrote in message
... In article .com, Steve wrote: 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. 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. -- *Gaffer tape - The Force, light and dark sides - holds the universe together* Dave Plowman London SW To e-mail, change noise into sound. 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 address) "In theory, there isn't any difference between theory and practice. In practice, there is." - Yogi Berra |
#6
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AC relay theory
Straw Man writes:
On Fri, 17 Aug 2007 08:01:28 -0700, Steve wrote: 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. 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. 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! --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#7
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AC relay theory
On 17 Aug 2007 19:58:41 -0400, Sam Goldwasser
wrote: 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! 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
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AC relay theory
"Steve" wrote in message oups.com... 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. They have a shorting bar like a shaded pole motor. |
#9
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AC relay theory
On Sat, 18 Aug 2007 03:16:00 GMT, Ross Herbert
wrote: On 17 Aug 2007 19:58:41 -0400, Sam Goldwasser wrote: 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! 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. 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
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AC relay theory
On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann
wrote: 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. 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. 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 |
#11
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AC relay theory
On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann
wrote: On Sat, 18 Aug 2007 03:16:00 GMT, Ross Herbert wrote: On 17 Aug 2007 19:58:41 -0400, Sam Goldwasser wrote: 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! 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. 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 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/repositor...k_complete.pdf WARNING: Over 3MB. It will take some time to download. |
#12
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AC relay theory
Ross Herbert wrote:
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. 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
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AC relay theory
On Sun, 19 Aug 2007 16:27:02 +1000, Peter Dettmann
wrote: On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann wrote: 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. No there is a difference here Ross, in the AC relay, the "slug" does not cover the whole of the magnetic iron path, I didn't say it did. 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. 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 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. 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 could be used, but fast operating speed was critical. 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
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AC relay theory
On Mon, 20 Aug 2007 12:35:20 +0100, "Ron(UK)"
wrote: Ross Herbert wrote: 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. The common GPO 3000 had the copper slug embedded into the armature. 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. http://www.englishclocksystems.co.uk/slugrelay.html |
#15
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AC relay theory
Ross Herbert wrote:
The common GPO 3000 had the copper slug embedded into the armature. 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. You are probably correct copper/brass non ferrous anyway impulsing relays used an adjustable residual stud (brass screw and locking nut) so that the residual gap could be set according to specific requirements. I imagined that was to adjust the throw of the armature to provide and also take care of the residual hang. Here is a pic of a 3000 type relay with a 1" heel end slug and an adjustable residual stud. 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! http://www.englishclocksystems.co.uk/slugrelay.html 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
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AC relay theory
In article ,
Ross Herbert wrote: 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. Think 'magnetic material' is the wrong term. It is capable of being momentarily magnetised - but isn't actually magnetic. -- *What boots up must come down * Dave Plowman London SW To e-mail, change noise into sound. |
#17
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AC relay theory
On Mon, 20 Aug 2007 12:04:01 GMT, Ross Herbert
wrote: On Sun, 19 Aug 2007 16:27:02 +1000, Peter Dettmann wrote: On Sun, 19 Aug 2007 14:39:50 +1000, Peter Dettmann wrote: 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. No there is a difference here Ross, in the AC relay, the "slug" does not cover the whole of the magnetic iron path, I didn't say it did. 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
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AC relay theory
On Mon, 20 Aug 2007 13:39:23 +0100, "Dave Plowman (News)"
wrote: In article , Ross Herbert wrote: 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. Think 'magnetic material' is the wrong term. It is capable of being momentarily magnetised - but isn't actually magnetic. 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
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AC relay theory
On Mon, 20 Aug 2007 13:26:24 +0100, "Ron(UK)"
wrote: Ross Herbert wrote: The common GPO 3000 had the copper slug embedded into the armature. 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. You are probably correct copper/brass non ferrous anyway 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. impulsing relays used an adjustable residual stud (brass screw and locking nut) so that the residual gap could be set according to specific requirements. I imagined that was to adjust the throw of the armature to provide and also take care of the residual hang. 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. Here is a pic of a 3000 type relay with a 1" heel end slug and an adjustable residual stud. 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! 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
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AC relay theory
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. 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. 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. |
#21
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AC relay theory
In article ,
Ross Herbert wrote: On Mon, 20 Aug 2007 13:39:23 +0100, "Dave Plowman (News)" wrote: In article , Ross Herbert wrote: 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. Think 'magnetic material' is the wrong term. It is capable of being momentarily magnetised - but isn't actually magnetic. 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. -- *The early bird gets the worm, but the second mouse gets the cheese * Dave Plowman London SW To e-mail, change noise into sound. |
#22
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AC relay theory
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. |
#23
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AC relay theory
On Tue, 21 Aug 2007 22:41:21 +1000, Peter Dettmann
wrote: SNIP 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=gES...sKmenpcNVhKamU 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..... |
#24
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AC relay theory
On Tue, 21 Aug 2007 11:16:54 +0100, "Dave Plowman (News)"
wrote: 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. 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/sDef...543303,00.html |
#25
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AC relay theory
On Tue, 21 Aug 2007 11:16:54 +0100 "Dave Plowman (News)"
wrote in Message id: : *The early bird gets the worm, but the second mouse gets the cheese * I GOT to remember that one... |
#26
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AC relay theory
On Wed, 22 Aug 2007 06:49:47 GMT, Ross Herbert
wrote: 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=gES...sKmenpcNVhKamU 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 |
#27
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AC relay theory
On Wed, 22 Aug 2007 23:51:10 +1000, Peter Dettmann
wrote: On Wed, 22 Aug 2007 06:49:47 GMT, Ross Herbert wrote: 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=gES...sKmenpcNVhKamU 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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