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#1
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If you have bolts thru the lams have you torqued them to the point of
twisting them off? If you haven't you might want to loosen then and add some more varnish to the lams while they're loose and then torque them. Another possibility: Many of the transformers in microwave ovens have a bead of weld across all the laminations and I've seen this on rectifiers used in Telco applications. Keeps them quiet and I and has some negatives like letting line spikes thru but on a filament that is meaningless. Lam eddy currents area dead issue as I've heard that the old Bell Labs had accepted this practice. I had a 30S-1 with minor buzz but when I added 100 volts to the screen the hum was about to drive me nuts but torquing to lams solved the problem, I twisted off a couple of bolts. -- 73 Hank WD5JFR "raypsi" wrote in message ... Hey all: I tore down a transformer I got from ebay it was a Zenith TV power transformer from days gone by. Re wound it with 17AWG wire on the secondary and primary. It powers up my 4-1000A filament nicely with 7.25VAC from 120VAC mains. I used a penta-filar winding on the secondary. Shimmed the bobbin in the core window with some 1/8 thick virgin teflon. Epoxied the windings to the bobbin. This was a 240VA transformer and now is a 83watt buzzer / 157watt filament transformer. So what is the cheapest I can gits away with sealing these laminations from buzzing, I used up all my epoxy sealing the windings. 73 n8zu |
#2
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Hmmm!
Bolts through the laminations should have insulators (usually fiber board washers) under the heads. Just one end will do, there's no need for them under the nuts. But shoulder washers are best so the bolts don't short any laminations together deeper into the core. If uninsulated the bolt can form a poorly coupled shorted turn and that in itself can cause noise, heating of the core and the hardware as well as spray flux around where you wouldn't have expected any. How bad it is depends on the locations of the holes in the core. A bead of weld across the ends of the laminations won't add to the eddy currents significantly. Steel isn't a great conductor. The weld's cross section is small and it's very poorly coupled to the primary. Why would a weld across the laminations let line spikes though? "Henry Kolesnik" wrote in : If you have bolts thru the lams have you torqued them to the point of twisting them off? If you haven't you might want to loosen then and add some more varnish to the lams while they're loose and then torque them. Another possibility: Many of the transformers in microwave ovens have a bead of weld across all the laminations and I've seen this on rectifiers used in Telco applications. Keeps them quiet and I and has some negatives like letting line spikes thru but on a filament that is meaningless. Lam eddy currents area dead issue as I've heard that the old Bell Labs had accepted this practice. I had a 30S-1 with minor buzz but when I added 100 volts to the screen the hum was about to drive me nuts but torquing to lams solved the problem, I twisted off a couple of bolts. |
#3
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Grumpy
Years ago in 93 or so on a Telco fiber system by Alcatel we took severe hits when the air conditioner came on and it was traced to the station rectifier. The Gould rectifiers had a weld bead and when we replaced them another make which I can't recall that didn't have weld beads the problem went away. Its quite strange since the rectifiers floated big batteries but a vco in system was sensitive to the spike. After doing some research I found an explanation that I can't recall because I was too busy with other problems and mine had gone away. Alcatel spent several hundred thousand trying to find the problem and I stumbled on to it by shear luck. 73 Hank WD5JFR "Grumpy The Mule" wrote in message ... Hmmm! Bolts through the laminations should have insulators (usually fiber board washers) under the heads. Just one end will do, there's no need for them under the nuts. But shoulder washers are best so the bolts don't short any laminations together deeper into the core. If uninsulated the bolt can form a poorly coupled shorted turn and that in itself can cause noise, heating of the core and the hardware as well as spray flux around where you wouldn't have expected any. How bad it is depends on the locations of the holes in the core. A bead of weld across the ends of the laminations won't add to the eddy currents significantly. Steel isn't a great conductor. The weld's cross section is small and it's very poorly coupled to the primary. Why would a weld across the laminations let line spikes though? "Henry Kolesnik" wrote in : If you have bolts thru the lams have you torqued them to the point of twisting them off? If you haven't you might want to loosen then and add some more varnish to the lams while they're loose and then torque them. Another possibility: Many of the transformers in microwave ovens have a bead of weld across all the laminations and I've seen this on rectifiers used in Telco applications. Keeps them quiet and I and has some negatives like letting line spikes thru but on a filament that is meaningless. Lam eddy currents area dead issue as I've heard that the old Bell Labs had accepted this practice. I had a 30S-1 with minor buzz but when I added 100 volts to the screen the hum was about to drive me nuts but torquing to lams solved the problem, I twisted off a couple of bolts. |
#4
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Howdy Hank,
Thanks for the follow-up! Perhaps they used a voltage regulating transformer. I think some of the telcom rectifiers used them. I know some mainframe computers did. It's still a curiosity to me. Though I've seen enough occult behavior from wound magnetic devices that I wouldn't discount it, I'm tempted to wonder if there's another variable that was coincident with the welded laminations. 73 "Henry Kolesnik" wrote in : Grumpy Years ago in 93 or so on a Telco fiber system by Alcatel we took severe hits when the air conditioner came on and it was traced to the station rectifier. The Gould rectifiers had a weld bead and when we replaced them another make which I can't recall that didn't have weld beads the problem went away. Its quite strange since the rectifiers floated big batteries but a vco in system was sensitive to the spike. After doing some research I found an explanation that I can't recall because I was too busy with other problems and mine had gone away. Alcatel spent several hundred thousand trying to find the problem and I stumbled on to it by shear luck. 73 Hank WD5JFR |
#5
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Hey Grump
A transformer is one of the simplest electrical machines and pretty well understood in the classical design but when some of the parameters are changed we don't have much experience or investigation and probably wont. Hot swappable rectifier modules have made transformer based rectifiers uneconomical kinda like carburetors and fuel injection. If you have the time, check with some of the old time designers of transformer rectifiers. Let me now what you find out. I had some notes but those are long gone. Hank "Grumpy The Mule" wrote in message ... Howdy Hank, Thanks for the follow-up! Perhaps they used a voltage regulating transformer. I think some of the telcom rectifiers used them. I know some mainframe computers did. It's still a curiosity to me. Though I've seen enough occult behavior from wound magnetic devices that I wouldn't discount it, I'm tempted to wonder if there's another variable that was coincident with the welded laminations. 73 "Henry Kolesnik" wrote in : Grumpy Years ago in 93 or so on a Telco fiber system by Alcatel we took severe hits when the air conditioner came on and it was traced to the station rectifier. The Gould rectifiers had a weld bead and when we replaced them another make which I can't recall that didn't have weld beads the problem went away. Its quite strange since the rectifiers floated big batteries but a vco in system was sensitive to the spike. After doing some research I found an explanation that I can't recall because I was too busy with other problems and mine had gone away. Alcatel spent several hundred thousand trying to find the problem and I stumbled on to it by shear luck. 73 Hank WD5JFR |
#6
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On Tue, 28 Oct 2008 00:52:28 +0100 (CET), Grumpy The Mule
wrote: Hmmm! Bolts through the laminations should have insulators (usually fiber board washers) under the heads. Just one end will do, there's no need for them under the nuts. But shoulder washers are best so the bolts don't short any laminations together deeper into the core. If uninsulated the bolt can form a poorly coupled shorted turn and that in itself can cause noise, heating of the core and the hardware as well as spray flux around where you wouldn't have expected any. How bad it is depends on the locations of the holes in the core. A bead of weld across the ends of the laminations won't add to the eddy currents significantly. Steel isn't a great conductor. The weld's cross section is small and it's very poorly coupled to the primary. Why would a weld across the laminations let line spikes though? It's not just the laminations that make noise. The windings can vibrate if they're not locked in place with transformer varnish or epoxy. No amount of tightening the laminations will help if the windings are free to vibrate. This is probably more likely in larger higher power transformers (multiple kW) than in a filament or screen supply transformer, but it can happen. Also, depending on how much magnetic leakage your core has you can couple to the steel walls of the cabinetry the transformer is housed in if you don't maintain adequate spacing from the walls, in which case the steel wall and the transformer can vibrate and make noise even though no parts on the transformer are loose. |
#7
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![]() That's a good point. The filthy leakge inductance will be spraying its flux all about. But there are things we can do a to contain it. A belly band around the winding (but not though the core) helps. End bells (covers) bolted down over the windows work too. There may also be fringing flux spewing out of the unavoidable gaps if it's a butt stack lamination core. Interleaving the laminations will help. I've done all of that on a design with the transformer mounted directly beneath the neck of the CRT in a video terminal. A requirment of the project manager because he feared SMPS noise and demanded a linear power supply. Of course everything had to be jammed into the smallest possible enclosure and a mu metal shield was deemed too expensive :P Jim Higgins wrote in : It's not just the laminations that make noise. The windings can vibrate if they're not locked in place with transformer varnish or epoxy. No amount of tightening the laminations will help if the windings are free to vibrate. This is probably more likely in larger higher power transformers (multiple kW) than in a filament or screen supply transformer, but it can happen. Also, depending on how much magnetic leakage your core has you can couple to the steel walls of the cabinetry the transformer is housed in if you don't maintain adequate spacing from the walls, in which case the steel wall and the transformer can vibrate and make noise even though no parts on the transformer are loose. |
#8
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![]() On Sat, 25 Oct 2008, raypsi wrote: Date: Sat, 25 Oct 2008 10:25:23 -0700 (PDT) From: raypsi Newsgroups: rec.radio.amateur.homebrew Subject: I built a 7.5VAC 21Amp transformer now it has lots of buzz Hey all: I tore down a transformer I got from ebay it was a Zenith TV power transformer from days gone by. Re wound it with 17AWG wire on the secondary and primary. It powers up my 4-1000A filament nicely with 7.25VAC from 120VAC mains. I used a penta-filar winding on the secondary. Shimmed the bobbin in the core window with some 1/8 thick virgin teflon. Epoxied the windings to the bobbin. This was a 240VA transformer and now is a 83watt buzzer / 157watt filament transformer. So what is the cheapest I can gits away with sealing these laminations from buzzing, I used up all my epoxy sealing the windings. 73 n8zu This might be a little "far-out" but, besides just buying more epoxy and just doing it right, maybe you could find a can or metal box, put the transformer into the box, and fill the box up with enough sand that you have 1-2 inches all around the transformer, and that might muffle the buzz. You'll need to worry about heat dissipation through that sand which will act like an insulator, but if you are on the air only a couple of hours per day, it should not get dangerously hot (however, many of the old hermetically sealed transformers were burried in a waxy-tar that was quite hard at room temperature). Your other option is to figure out some clamping arrangement so that the laminations are held close and tight. Re-winding transformers is fun, but next time you want some off-beat voltage, maybe scrounge up higher voltages (eg. car battery chargers) and use either a variac or power resistor (with sufficient heat dissipation) to bring the voltage down to where you want it and save yourself the headaches and earaches. Or, don't take the laminations apart but feed the wire (stranded) through the holes that are left. Or, other kludge-cheapskate approaches. Good luck. |
#9
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![]() It's not so far out of a suggestion. One of my Zenith console radios has some remote control solenoid mounted inside a metal box that's lined with rubber. The sand is a pretty good thermal conductor (compared to air), better than epoxy. In fact they fill the cheaper type of epoxy potting material with coarse quartzite or powdered limestone to improve its the thermal conductivity. It's the air trapped between the grains of sand that provide some thermal insulation. Mixing sand with epoxy or oil fills the interstaces which would otherwise be filled with air. Some one mentioned coating the laminations with nail polish as you stack them up to glue them together. Either that or little varnish would most likely have prevented any buzzing laminations. Not taking the laminations off is a good suggestion too if it's possible. For instance microwave oven transformers just chisel off the secondary and wind a new one. Though for smaller transformers that often isn't possible. Stray Dog wrote in .org: This might be a little "far-out" but, besides just buying more epoxy and just doing it right, maybe you could find a can or metal box, put the transformer into the box, and fill the box up with enough sand that you have 1-2 inches all around the transformer, and that might muffle the buzz. You'll need to worry about heat dissipation through that sand which will act like an insulator, but if you are on the air only a couple of hours per day, it should not get dangerously hot (however, many of the old hermetically sealed transformers were burried in a waxy-tar that was quite hard at room temperature). Your other option is to figure out some clamping arrangement so that the laminations are held close and tight. Re-winding transformers is fun, but next time you want some off-beat voltage, maybe scrounge up higher voltages (eg. car battery chargers) and use either a variac or power resistor (with sufficient heat dissipation) to bring the voltage down to where you want it and save yourself the headaches and earaches. Or, don't take the laminations apart but feed the wire (stranded) through the holes that are left. Or, other kludge-cheapskate approaches. Good luck. |
#10
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On Oct 27, 11:29*pm, Stray Dog wrote:
On Sat, 25 Oct 2008, raypsi wrote: Date: Sat, 25 Oct 2008 10:25:23 -0700 (PDT) From: raypsi Newsgroups: rec.radio.amateur.homebrew Subject: I built a 7.5VAC 21Amp transformer now it has lots of buzz Hey all: I tore down a transformer I got from ebay it was a Zenith TV power transformer from days gone by. Re wound it with 17AWG wire on the secondary and primary. It powers up my 4-1000A filament nicely with 7.25VAC from 120VAC mains. I used a penta-filar winding on the secondary. Shimmed the bobbin in the *core window *with some 1/8 thick virgin teflon. Epoxied the windings to the bobbin. This was a 240VA transformer and now is a 83watt buzzer / 157watt filament transformer. So what is the cheapest I can gits away with sealing these laminations from buzzing, I used up all my epoxy sealing the windings. 73 n8zu This might be a little "far-out" but, besides just buying more epoxy and just doing it right, maybe you could find a can or metal box, put the transformer into the box, and fill the box up with enough sand that you have 1-2 inches all around the transformer, and that might muffle the buzz. You'll need to worry about heat dissipation through that sand which will act like an insulator, but if you are on the air only a couple of hours per day, it should not get dangerously hot (however, many of the old hermetically sealed transformers were burried in a waxy-tar that was quite hard at room temperature). Your other option is to figure out some clamping arrangement so that the laminations are held close and tight. Re-winding transformers is fun, but next time you want some off-beat voltage, maybe scrounge up higher voltages (eg. car battery chargers) and use either a variac or power resistor (with sufficient heat dissipation) to bring the voltage down to where you want it and save yourself the headaches and earaches. Or, don't take the laminations apart but feed the wire (stranded) through the holes that are left. Or, other kludge-cheapskate approaches. Good luck.- Hide quoted text - - Show quoted text - I have always dunked them in varnish but only got some success with this by drawing a vaccum then applying pressure. I used an old compressor from a refrigarator to do this. The tank was made from from pipe and fittings. The best results I got was winding the core on a block of wood cut the same size as the core wrapping the wood with cardboard then windng the coil then pushed then put the core together. I was 18 at the time and a very talented man named Olin Griggs helped me with this project and let me use his shop. Olin was a retired engineer that had worked for GE at their Bloomington plant. Since then the onl transformers I have wound that havent bussed to much was by modifying a ready built transformer. USually this meant one that I on which I could keep the primary winding intact. Lately I have gotten interested in SMPSs for doing this sort of thing. It is very easy to see why industry has has made the move to them. Jimmie |
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