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Metalworking (rec.crafts.metalworking) Discuss various aspects of working with metal, such as machining, welding, metal joining, screwing, casting, hardening/tempering, blacksmithing/forging, spinning and hammer work, sheet metal work. |
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#41
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Magnabend
On Jan 16, 11:57*pm, "Wild_Bill" wrote:
I just saw the force listed in the eBay auction shown in this thread as 6 tons. The retarded seller doesn't even clearly state what the size of the machine is.. one might assume that it's 48" from the model number, but I didn't look it up.. and the machines are from a metricized country, so the model number may not be relative to the size in inches. -- WB ......... It's in sillimeters, a Model 2500=2.5 meters. Big enough for anything I'm likely to dream up. Smaller the model number, the shorter the bed. And "Tonnes" is likely metric tons, 2200 lbs, thereabouts. So probably closer to 5000 lbs if it's factory listed at 4 tonnes. Wonder if you could rig a magneticly operated punch off that? Would be kind of neat to do pop rivet holes on the same unit. Stan |
#42
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Magnabend
Rick wrote:
More detailed manual and schematics: http://www.aaybee.com.au/MagnaBend%20UserManual.pdf That is an interesting de-mag ckt. :-) ...lew... |
#43
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Magnabend
On Jan 15, 9:25*am, "Wild_Bill" wrote:
The steel in transformers isn't particularly hard.. it bends freely and cuts quite easily with tin snips. A quality hacksaw/bandsaw blade shouldn't have any difficulty cutting thru the welds. A fiberglas reinforced cutoff wheel will make easy work of cutting thru the weld of microwave xfmers, or most any welds for common steels. The entire weld doesn't need to be cut away to separate the E and I sections, just cutting from the outer surface to the junction of the E-I sections will do. Cutting thru multiple layers of steel (where the welds aren't located) can present problems if the layers start to flex or shift, which could cause pinching or binding of a saw blade. -- WB ......... "Denis G." wrote in message ... Nice work and good ideas! *Did you need anything special to cut the MOTs in half? *I haven't tried it, but I thought that they were high silicon steel metal and quite hard. I remember when I took apart transformers as a kid, I think that they were bolted together and the lamination slid apart quite easily. We used to smash them with hammers on the sidewalk and they would shatter. They probably don't make them like that anymore, so maybe the steel is nothing special nowadays and they don't make them like they once did. |
#44
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Magnabend
Jon Danniken wrote:
.... On a side note, I played with the little transformer I posted pics of earlier, using it as a lifting magnet. ... I turned the power on, and it lifted all 30 pounds, ... That was my big fun for the day. Well, I just had some big fun of my own. Based on your report, I had pretty much dismissed the idea of MOTs being converted to any kind of serious magnet. But, what the hell, I was curious about the MOTs that I had converted, so I did some testing. Mine is a MOT that I had cut in half & used the secondary. The cut surface was bandsaw flat, but not machined. The core is 2+ x 4+ & I put a 3/4" plate across the poles. I applied 120v, full wave rectified. I started out about where you had been: 20-30 lbs. No problem, so I started adding more. Well, I overflowed the bucket that I was putting weight in (my MOT collection), switched to a milk crate and filled that with all the MOTs, plus all the lead I had. Beefing up the lifting system as I went. I got to 207+ pounds and had to stop 'cause the milk crate was full and near its breaking point, and the lift system was also at its limit. So, I'm going to bring in my engine hoist, scrounge up some more weight and see just what this baby will do. Not because I think that it might actually be strong enough for a magnetic brake (it won't be), but "I gots to know" (Dirty Harry). Bob |
#45
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Magnabend
Bob Engelhardt wrote:
Jon Danniken wrote: ... On a side note, I played with the little transformer I posted pics of earlier, using it as a lifting magnet. ... I turned the power on, and it lifted all 30 pounds, ... That was my big fun for the day. Well, I just had some big fun of my own. Based on your report, I had pretty much dismissed the idea of MOTs being converted to any kind of serious magnet. But, what the hell, I was curious about the MOTs that I had converted, so I did some testing. Mine is a MOT that I had cut in half & used the secondary. The cut surface was bandsaw flat, but not machined. The core is 2+ x 4+ & I put a 3/4" plate across the poles. I applied 120v, full wave rectified. I started out about where you had been: 20-30 lbs. No problem, so I started adding more. Well, I overflowed the bucket that I was putting weight in (my MOT collection), switched to a milk crate and filled that with all the MOTs, plus all the lead I had. Beefing up the lifting system as I went. I got to 207+ pounds and had to stop 'cause the milk crate was full and near its breaking point, and the lift system was also at its limit. So, I'm going to bring in my engine hoist, scrounge up some more weight and see just what this baby will do. Not because I think that it might actually be strong enough for a magnetic brake (it won't be), but "I gots to know" (Dirty Harry). Bob COOL! Thanks Bob. I anxiously await the next chapter. --Winston |
#46
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Magnabend
"Bob Engelhardt" wrote in message ... Well, I just had some big fun of my own. Based on your report, I had pretty much dismissed the idea of MOTs being converted to any kind of serious magnet. But, what the hell, I was curious about the MOTs that I had converted, so I did some testing. Mine is a MOT that I had cut in half & used the secondary. The cut surface was bandsaw flat, but not machined. The core is 2+ x 4+ & I put a 3/4" plate across the poles. I applied 120v, full wave rectified. I started out about where you had been: 20-30 lbs. No problem, so I started adding more. Well, I overflowed the bucket that I was putting weight in (my MOT collection), switched to a milk crate and filled that with all the MOTs, plus all the lead I had. Beefing up the lifting system as I went. I got to 207+ pounds and had to stop 'cause the milk crate was full and near its breaking point, and the lift system was also at its limit. So, I'm going to bring in my engine hoist, scrounge up some more weight and see just what this baby will do. Not because I think that it might actually be strong enough for a magnetic brake (it won't be), but "I gots to know" (Dirty Harry). Bob What AC current are you seeing when you do this? Is the winding getting hot? How ling did you leave it powered on? Since you are putting DC thru the MOT, it isn't acting as an inductor and the secondary resistance is all that is limiting the current. If there isn't any significant heating you could try putting 240VAC into the rectifiers for increased mag flux and holding power. Check for heating again. Art |
#47
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Magnabend
Bob Engelhardt wrote:
[snip] Mine is a MOT that I had cut in half & used the secondary. The cut surface was bandsaw flat, but not machined. The core is 2+ x 4+ & I put a 3/4" plate across the poles. I applied 120v, full wave rectified. Holy mackerel, 2x4 cross section on a core is a big sucker, sounds like it must have been out of a commercial oven? I started out about where you had been: 20-30 lbs. No problem, so I started adding more. Well, I overflowed the bucket that I was putting weight in (my MOT collection), switched to a milk crate and filled that with all the MOTs, plus all the lead I had. Beefing up the lifting system as I went. I got to 207+ pounds and had to stop 'cause the milk crate was full and near its breaking point, and the lift system was also at its limit. I'm impressed, and anxious to see what you can get up to. I'm also encouraged that you were able to do this with the secondary winding. Jon |
#48
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Testing MOT as electromagnet - this just in
This continues to amaze me. I had tested with 207 lbs, then set up a
better test rig and added 85 lbs more (292 total) - no problem, 52 more, to 345 - no problem, then 393 (48 more) - still holding!! But at 425, my engine hoist bent! It was homemade, by the that-looks-about-right method, so perhaps to be expected. Back to the drawing board, as they say. Then inspiration: use 2 equal pole pieces & just load one. I.e., I would be measuring 1/2 the pull. So I got another 3/4" bar and used each of the 2 bars to cover 1/2 of the pole area (the long way) and fastened the load to one of them. I backed off the load, some, to 369 and it _just_ lifted it before letting go. I had _finally_ found the limit. That means that this little MOT electromagnet is capable of holding 740 lbs! WHOA! Double WHOA! The coil is 3-3/4", i.e., in a magnetic brake it would take 3-3/4" of clamp space. And the magnetic pull rate would be 740 divided by that, or about 200 lbs/in. Compared to the 250 lbs/in of the MagnaBend. Now, if it isn't saturating at 120v & could be run higher, with more flux, and more pull, we could be in the DIY magnetic brake business. Some tidbits: it's not easy finding a 400 lb load - I had 2 milk crates FULL of MOTs, lead ingots, exercise weights, chunky steel stock, and a lathe tailstock. It's also not easy lifting that much - I was using a come-along & doing one click at a time. At the 345 level, I tried something. I backed off the coil voltage (I was using a variac) until it released the load. It was at the 30/100 mark. 36 volts if I hadn't wired in the boost section of the variac. I don't know if this means that the core runs saturated at 120v, or what. But I intend to find out. It would be much better if it didn't have to run at 120v. An aw-**** moment: when the magnet did let go, a piece of lift chain smashed into the coil & cut through a couple of turns. It still works, but probably not for long. But I have 2 other MOT electromagnets, and 8-10 untouched MOTs. Stay tuned, Bob |
#49
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Magnabend
Artemus wrote:
What AC current are you seeing when you do this? I was thinking it was an amp, but I just looked at FunWithMOTs & the 1 amp is a secondary in air (no core) with ac on it. I'll have to measure it. Is the winding getting hot? Warm, but not too hot to touch. How ling did you leave it powered on? A minute or two each trial. Duty cycle would definitely be less than 100%. (The MagnaBend has a duty cycle of 30%.) Since you are putting DC thru the MOT, it isn't acting as an inductor and the secondary resistance is all that is limiting the current. Well, there is definitely a lot of ripple, so that portion of the current would be inductor affected. If there isn't any significant heating you could try putting 240VAC into the rectifiers for increased mag flux and holding power. Check for heating Yes. First, I intend to determine the saturation voltage and pull at that point. Then heating, by change-of-resistance method. Bob |
#50
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Testing MOT as electromagnet - this just in
Bob Engelhardt wrote:
(Very intriguing MOT as electromagnet stuff) Stay tuned, If you are not at saturation yet, you could stack a couple secondaries and run them in series or parallel for double flux. --Winston |
#51
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Magnabend
On 2011-01-17, Bob Engelhardt wrote:
Rick wrote: More detailed manual and schematics: http://www.aaybee.com.au/MagnaBend%20UserManual.pdf Good stuff! It makes me want one even more! Lots of data one could use for building one, too. The most significant spec is the 3 tons ("tonnes") of clamping force on the 24" brake. That's 250 lbs/running inch. I'm skeptical that a reworked MOT is going to come anywhere near that. I suspect that the magnet is a continuous stack of E-cores the full active width of the brake -- all in a single coil. If you had a bunch of MOT cores, you would have areas where the grip was a lot weaker than you need. Enjoy, DoN. -- Remove oil spill source from e-mail Email: | Voice (all times): (703) 938-4564 (too) near Washington D.C. | http://www.d-and-d.com/dnichols/DoN.html --- Black Holes are where God is dividing by zero --- |
#52
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Magnabend
DoN. Nichols wrote:
I suspect that the magnet is a continuous stack of E-cores the full active width of the brake -- all in a single coil. If you had a bunch of MOT cores, you would have areas where the grip was a lot weaker than you need. It's true that the field would be uneven. My test magnet, for instance, has coil width of 3.75" & a core width of 2.25". 40% of the length would be field-less, more or less. However, if the average field/grip is the same, the 4 x 5/8" clamping bar should be beefy enough to provide grip in the field-less areas. In other words, if the total clamping force is the same, the clamping bar should spread it out evenly (enough). Bob |
#53
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Magnabend
On Mon, 17 Jan 2011 13:22:29 -0500, Bob Engelhardt
wrote: Jon Danniken wrote: ... On a side note, I played with the little transformer I posted pics of earlier, using it as a lifting magnet. ... I turned the power on, and it lifted all 30 pounds, ... That was my big fun for the day. Well, I just had some big fun of my own. Based on your report, I had pretty much dismissed the idea of MOTs being converted to any kind of serious magnet. But, what the hell, I was curious about the MOTs that I had converted, so I did some testing. Mine is a MOT that I had cut in half & used the secondary. The cut surface was bandsaw flat, but not machined. The core is 2+ x 4+ & I put a 3/4" plate across the poles. I applied 120v, full wave rectified. I started out about where you had been: 20-30 lbs. No problem, so I started adding more. Well, I overflowed the bucket that I was putting weight in (my MOT collection), switched to a milk crate and filled that with all the MOTs, plus all the lead I had. Beefing up the lifting system as I went. I got to 207+ pounds and had to stop 'cause the milk crate was full and near its breaking point, and the lift system was also at its limit. So, I'm going to bring in my engine hoist, scrounge up some more weight and see just what this baby will do. Not because I think that it might actually be strong enough for a magnetic brake (it won't be), but "I gots to know" (Dirty Harry). Bob It would also be interesting to see how much force you get when a non-ferromagnetic gap of 1/16" or so (e.g., .062 aluminum) is introduced. |
#54
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Magnabend
If the faces are shaped they will focus the forces.
I used to use a research magnet that had cone shaped with small flats - an when we turned it on, all of our pocket things, watches, rings, etc were in another room. A ring would get red hot near the poles. A company I worked with had one that was the diameter of 48" and had a focus cone down to 8" diameter. It was to energize wafers :-) Martin On 1/17/2011 9:41 PM, Bob Engelhardt wrote: DoN. Nichols wrote: I suspect that the magnet is a continuous stack of E-cores the full active width of the brake -- all in a single coil. If you had a bunch of MOT cores, you would have areas where the grip was a lot weaker than you need. It's true that the field would be uneven. My test magnet, for instance, has coil width of 3.75" & a core width of 2.25". 40% of the length would be field-less, more or less. However, if the average field/grip is the same, the 4 x 5/8" clamping bar should be beefy enough to provide grip in the field-less areas. In other words, if the total clamping force is the same, the clamping bar should spread it out evenly (enough). Bob |
#55
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Testing MOT as electromagnet - this just in
Testing for saturation: I added a small secondary, so I could get an
induced voltage to measure. Then I measured this induced voltage vs. the applied voltage in 10v steps up to 120v, applied. My thinking is that there will be a linear relationship until the core saturates and then the induced voltage will level off. That didn't happen so far. Before I rig to test up to 240v, I want to confirm that my thinking is correct about output voltage at saturation. Anybody know? Thanks, Bob BTW - in the microwave oven, this coil generates 2000v, so 240v won't be a problem (current might be). |
#56
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Testing MOT as electromagnet - this just in
Bob Engelhardt wrote:
Testing for saturation: I added a small secondary, so I could get an induced voltage to measure. Then I measured this induced voltage vs. the applied voltage in 10v steps up to 120v, applied. My thinking is that there will be a linear relationship until the core saturates and then the induced voltage will level off. That didn't happen so far. Before I rig to test up to 240v, I want to confirm that my thinking is correct about output voltage at saturation. Anybody know? Thanks, Bob BTW - in the microwave oven, this coil generates 2000v, so 240v won't be a problem (current might be). The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Can you place a small value sense resistor in series with the 'common' line to your coil? The way I misunderstand magnetic core saturation is that we expect a 'knee' where current begins to increase in a nonlinear fashion in relation to applied voltage. You could set your 'scope up as an X-Y display to show this nonlinearity very clearly. --Winston |
#57
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Testing MOT as electromagnet - this just in
On Tue, 18 Jan 2011 21:47:49 -0500, Bob Engelhardt
wrote: Testing for saturation: I added a small secondary, so I could get an induced voltage to measure. Then I measured this induced voltage vs. the applied voltage in 10v steps up to 120v, applied. My thinking is that there will be a linear relationship until the core saturates and then the induced voltage will level off. That didn't happen so far. Before I rig to test up to 240v, I want to confirm that my thinking is correct about output voltage at saturation. Anybody know? Thanks, Bob BTW - in the microwave oven, this coil generates 2000v, so 240v won't be a problem (current might be). I thought you had a bridge rectifier in there. Are you exciting it with AC or with rectified AC which is DC with ripple? |
#58
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Testing MOT as electromagnet - this just in
On Jan 18, 9:47*pm, Bob Engelhardt wrote:
Testing for saturation: I added a small secondary, so I could get an induced voltage to measure. *Then I measured this induced voltage vs. the applied voltage in 10v steps up to 120v, applied. *My thinking is that there will be a linear relationship until the core saturates and then the induced voltage will level off. That didn't happen so far. *Before I rig to test up to 240v, I want to confirm that my thinking is correct about output voltage at saturation. * Anybody know? Thanks, Bob BTW - in the microwave oven, this coil generates 2000v, so 240v won't be a problem (current might be). If I recall correctly when one designs a transformer, you calculate what the ET product is. That is the voltage times time. E{avg}= 4 f N A B{peak} is the basic formula So rearranging and eliminating constants. ET = N A B ( peak ) Where T is the inverse of f. And N, the number of turns, and A the cross sectional area are constant for a transformer. So ET ~ to B ( peak ) So the flux ( B ) depends on the voltage and frequency. And using the same line frequency and the voltage at 240 volts RMS ( much less than the usual 2000 volts on the secondary, you should not have to worry about saturation. ( this assumes you are using AC voltage.). Dan |
#59
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Testing MOT as electromagnet - this just in
Winston wrote:
The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Yes and no, I suppose G. 'Though I did lop it off, I replaced it with a shorting bar, so it shouldn't be drastically different. Can you place a small value sense resistor in series with the 'common' line to your coil? The way I misunderstand magnetic core saturation is that we expect a 'knee' where current begins to increase in a nonlinear fashion in relation to applied voltage. You could set your 'scope up as an X-Y display to show this nonlinearity very clearly. Funny you should mention that. I was going to do that and had dragged my scope out, only to find it busted! Dang - those things are expensive to fix! Bob |
#60
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Testing MOT as electromagnet - this just in
Don Foreman wrote:
I thought you had a bridge rectifier in there. Are you exciting it with AC or with rectified AC which is DC with ripple? Oops - you're right, I was going to switch to AC for the test & kinda' skipped that part. Still, there's enough ripple to give me output & the DC component helps with getting to saturation. Bob |
#61
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Testing MOT as electromagnet - this just in
On Tue, 18 Jan 2011 21:47:49 -0500, Bob Engelhardt
wrote: Testing for saturation: I added a small secondary, so I could get an induced voltage to measure. Then I measured this induced voltage vs. the applied voltage in 10v steps up to 120v, applied. My thinking is that there will be a linear relationship until the core saturates and then the induced voltage will level off. That didn't happen so far. Before I rig to test up to 240v, I want to confirm that my thinking is correct about output voltage at saturation. Anybody know? Thanks, Bob BTW - in the microwave oven, this coil generates 2000v, so 240v won't be a problem (current might be). Basically correct but with most ferrous materials onset of saturation is gradual and not sharply defined. Jim |
#62
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Testing MOT as electromagnet - this just in
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#63
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Testing MOT as electromagnet - this just in
Bob Engelhardt wrote:
Winston wrote: The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Yes and no, I suppose G. 'Though I did lop it off, I replaced it with a shorting bar, so it shouldn't be drastically different. Now's the time to break out your $0.99 inductance bridge. I haven't tested this but it looks reasonable: http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_meter.jpg.html http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_method.jpg.html Can you place a small value sense resistor in series with the 'common' line to your coil? The way I misunderstand magnetic core saturation is that we expect a 'knee' where current begins to increase in a nonlinear fashion in relation to applied voltage. You could set your 'scope up as an X-Y display to show this nonlinearity very clearly. Funny you should mention that. I was going to do that and had dragged my scope out, only to find it busted! Dang - those things are expensive to fix! Not if it is old enough. Got schematics? --Winston -- Borrow an 'X' to repair an 'X', where 'X' = anything. |
#64
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Testing MOT as electromagnet - this just in
On Wed, 19 Jan 2011 09:39:14 -0500, Bob Engelhardt
wrote: wrote: If I recall correctly when one designs a transformer, you calculate what the ET product is. ... snip So the flux ( B ) depends on the voltage and frequency. And using the same line frequency and the voltage at 240 volts RMS ( much less than the usual 2000 volts on the secondary, you should not have to worry about saturation. ( this assumes you are using AC voltage.). Thanks, that helps! I've been thinking about those 2000 volts. If that's the normal voltage on this coil, it's probably going to take close to that to saturate the core. I mean, wouldn't they design for the core to be close to saturation, to minimize the core size needed? My interest in saturation is 2 fold: I don't want to run beyond saturation 'cause of the extra heat, and at saturation is where the maximum pull will be. Now, about ACC. To use as an electromagnet, I have full-wave rectification, unfiltered. Although the coil's inductance will do some smoothing (I wish my scope was working). My intuition is that the DC component of the current will be determined by the coil resistance and that will produce flux proportional to the number of coil turns. So, the question is whether the DC current will saturate it before 240v. Or be too high for the coil's wire guage. But the bottom line is that I'm going to be using 240v max (full wave) and as long as it isn't saturated then, I'll be getting the maximum pull available. Thanks, Bob You're right, DC current will be determined by coil resistance and applied voltage, and flux will be proportional to current and # of turns. Focus on keeping the current to a level that won't burn out the winding. Don't worry about saturation in a DC electromagnet. Having it saturate is not a problem. Saturation is a big problem in a transformer, but not in an electromagnet. It's a bit surprising that, for given core cross section and winding window area, the max flux possible with a copper winding is independent of wire size. Voltage and current change, of course, but Bmax doesn't for given current density in the copper. The attraction force you get will be B^2*A / 2*muzero where B is flux density, A is area, muzero is 4*pi*10E-7 newton/amp^2. Flux density will depend on the total magnetic circuit including whatever is being attracted. |
#65
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Testing MOT as electromagnet - this just in
Bob Engelhardt wrote: Winston wrote: The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Yes and no, I suppose G. 'Though I did lop it off, I replaced it with a shorting bar, so it shouldn't be drastically different. Can you place a small value sense resistor in series with the 'common' line to your coil? The way I misunderstand magnetic core saturation is that we expect a 'knee' where current begins to increase in a nonlinear fashion in relation to applied voltage. You could set your 'scope up as an X-Y display to show this nonlinearity very clearly. Funny you should mention that. I was going to do that and had dragged my scope out, only to find it busted! Dang - those things are expensive to fix! What kind of scope? Brand? Model? -- You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated. |
#66
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Testing MOT as electromagnet - this just in
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#67
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Testing MOT as electromagnet - this just in
On Jan 19, 11:39*am, Don Foreman
wrote: You're right, DC current will be determined by coil resistance and applied voltage, and flux will be proportional to current and # of turns. * Minor correction. The flux will be proportional to the current and # of turns up to where the core is saturated. Or at least that is how I remember it. Dan |
#68
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Testing MOT as electromagnet - this just in
Winston wrote:
Bob Engelhardt wrote: ... dragged my scope out, only to find it busted! Dang - those things are expensive to fix! ... Got schematics? No. Not the skill, either. The idea of a 10,000v power supply is another deterrent G. Bob |
#69
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Testing MOT as electromagnet - this just in
Michael A. Terrell wrote:
What kind of scope? Brand? Model? Analog - Tektronix 465B Bob |
#70
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Testing MOT as electromagnet - this just in
On Wed, 19 Jan 2011 15:06:08 -0500, "Michael A. Terrell"
wrote: Bob Engelhardt wrote: wrote: If I recall correctly when one designs a transformer, you calculate what the ET product is. ... snip So the flux ( B ) depends on the voltage and frequency. And using the same line frequency and the voltage at 240 volts RMS ( much less than the usual 2000 volts on the secondary, you should not have to worry about saturation. ( this assumes you are using AC voltage.). Thanks, that helps! I've been thinking about those 2000 volts. If that's the normal voltage on this coil, it's probably going to take close to that to saturate the core. I mean, wouldn't they design for the core to be close to saturation, to minimize the core size needed? Only at line frequency. With DC exitation the same flux density will be reached when the DC reaches the same level as the normal line freqency magnetising current (i.e the no load current). With DC, the voltage needed to reach this current will typically be about 5% of the rated line frequency voltage provided the magnetic circuit is closed (no air gap) My interest in saturation is 2 fold: I don't want to run beyond saturation 'cause of the extra heat, and at saturation is where the maximum pull will be. Now, about ACC. To use as an electromagnet, I have full-wave rectification, unfiltered. Although the coil's inductance will do some smoothing (I wish my scope was working). My intuition is that the DC component of the current will be determined by the coil resistance and that will produce flux proportional to the number of coil turns. So, the question is whether the DC current will saturate it before 240v. Or be too high for the coil's wire guage. Because the rated line frequency full load current is very much larger than the no load magnetising current, DC excitation will saturate the core long before rated full load current is reached. This applies when the magnetic circuit is as fully closed as in the original transformer. If it is only partially closed with perhaps inferior iron or small airgap, proportionally more current will be needed. But the bottom line is that I'm going to be using 240v max (full wave) and as long as it isn't saturated then, I'll be getting the maximum pull available. 240V should be ample. Jim |
#71
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Testing MOT as electromagnet - this just in
Don Foreman wrote:
... Focus on keeping the current to a level that won't burn out the winding. Don't worry about saturation in a DC electromagnet. Having it saturate is not a problem. ... I have kinda' backed into that conclusion - given that it's not saturated when it's outputting 2000v, and seeing the heating that I'm getting so far (120v). It's a bit surprising that, for given core cross section and winding window area, the max flux possible with a copper winding is independent of wire size. Voltage and current change, of course, but Bmax doesn't for given current density in the copper. It makes sense from the fact that flux is a function of amp-turns. 1 amp through 100 turns or 100 amps through 1 turn. In my MOT electromagnet it's a lot easier to put higher voltage & less current through 2000 or so turns of secondary than small voltage & large current through 120 turns of primary. The attraction force you get will be B^2*A / 2*muzero where B is flux density, A is area, muzero is 4*pi*10E-7 newton/amp^2. Flux density will depend on the total magnetic circuit including whatever is being attracted. I suppose before I had started my experiments I could have used that to calculate force. I would still have verified it, but it would have given me a better idea of where to start. Thanks, Bob |
#72
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Testing MOT as electromagnet - this just in
Michael A. Terrell wrote:
Are you removing the secondary windings? Actually, I'm removing the primary and exciting the secondary. Bob |
#73
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Testing MOT as electromagnet - this just in
Today's results:
I abandoned my search for saturation. I will not be applying the kind of voltage to the coil that will get it anywhere near saturation. Also, with the kind of force that I've measured already, I don't need to saturate. What the limiting factor will be is heat. Exciting the MOT's secondary causes a lot more current that when it's used in the microwave. I gathered some data on heating: with 160v AC applied to the bridge, it drew 1.8A initially and dropped to 1.2A after running for 4 minutes and heating up. The average coil temperature had risen 210F, to 280F! (There was some bubbling sound from the coil potting material!) This is 24ga wire+-. I let it cool for 13 minutes (to give a 30% duty cycle) and ran through a couple of iterations of 3 minutes on, 7 minutes off. The peak temperature kept climbing, reaching a max of 240F rise to 310F. At this point I decided that this test is not realistic. The 30% duty cycle would likely give the same average temperature as an operating brake, but the long on period was giving exaggerated max temperatures. In use, the full on time would only be seconds and 1/2 power on a few seconds more. I am going to build a rig that will do 30% duty cycle with short on times (10sec maybe). And maybe do 10% duty cycle also. Let me add that the temperatures that I was seeing were not alarming me. I had looked into transformer temperature ratings and found some interesting stuff. First, their ratings are not so much limits above which they shouldn't operated, rather they are values which can be expected to not be exceeded when the transformer is operating at capacity (80, 115, & 150 degree C rise). It's more a rating of transformer efficiency (hot is less efficient). And, independently, the real limiting parameter is the winding insulation temperature limit. Typically 220C (428F). I was not anywhere near this insulation limit. See http://www.copper.org/applications/e...fficiency.html Damn, I'm having more fun than a boy in a sandbox, Bob |
#74
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Testing MOT as electromagnet - this just in
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#75
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Testing MOT as electromagnet - this just in
Bob Engelhardt wrote: Michael A. Terrell wrote: What kind of scope? Brand? Model? Analog - Tektronix 465B Damn! I had a lot of spare parts, but all I have left is a power transformer. -- You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated. |
#77
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Testing MOT as electromagnet - this just in
On Wed, 19 Jan 2011 19:41:33 -0500, Bob Engelhardt
wrote: Today's results: I abandoned my search for saturation. I will not be applying the kind of voltage to the coil that will get it anywhere near saturation. Also, with the kind of force that I've measured already, I don't need to saturate. What the limiting factor will be is heat. Exciting the MOT's secondary causes a lot more current that when it's used in the microwave. I gathered some data on heating: with 160v AC applied to the bridge, it drew 1.8A initially and dropped to 1.2A after running for 4 minutes and heating up. The average coil temperature had risen 210F, to 280F! (There was some bubbling sound from the coil potting material!) This is 24ga wire+-. I let it cool for 13 minutes (to give a 30% duty cycle) and ran through a couple of iterations of 3 minutes on, 7 minutes off. The peak temperature kept climbing, reaching a max of 240F rise to 310F. At this point I decided that this test is not realistic. The 30% duty cycle would likely give the same average temperature as an operating brake, but the long on period was giving exaggerated max temperatures. In use, the full on time would only be seconds and 1/2 power on a few seconds more. I am going to build a rig that will do 30% duty cycle with short on times (10sec maybe). And maybe do 10% duty cycle also. Let me add that the temperatures that I was seeing were not alarming me. I had looked into transformer temperature ratings and found some interesting stuff. First, their ratings are not so much limits above which they shouldn't operated, rather they are values which can be expected to not be exceeded when the transformer is operating at capacity (80, 115, & 150 degree C rise). It's more a rating of transformer efficiency (hot is less efficient). And, independently, the real limiting parameter is the winding insulation temperature limit. Typically 220C (428F). I was not anywhere near this insulation limit. See http://www.copper.org/applications/e...fficiency.html Damn, I'm having more fun than a boy in a sandbox, Yeah, and a whole lot fewer cats hang around the shop than a litter box. We're having fun with you (albeit vicariously) too, little Bobby. Carry on. -- Live in the sunshine, swim the sea, drink the wild air... -- Ralph Waldo Emerson |
#78
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Testing MOT as electromagnet - this just in
On Wed, 19 Jan 2011 13:54:15 -0800 (PST), "
wrote: On Jan 19, 11:39*am, Don Foreman wrote: You're right, DC current will be determined by coil resistance and applied voltage, and flux will be proportional to current and # of turns. * Minor correction. The flux will be proportional to the current and # of turns up to where the core is saturated. Or at least that is how I remember it. Dan Right. Thanks for that correction. It isn't really proportional in the nonlinear region below saturation either, but it works as a first approximation. |
#79
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Testing MOT as electromagnet - this just in
Winston wrote:
Bob Engelhardt wrote: Winston wrote: The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Yes and no, I suppose G. 'Though I did lop it off, I replaced it with a shorting bar, so it shouldn't be drastically different. Now's the time to break out your $0.99 inductance bridge. I haven't tested this but it looks reasonable: http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_meter.jpg.html http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_method.jpg.html You can also measure inductances of this size using your PC and tone generating software. Wire up the inductor with a capacitor, apply the tone, and look for the resonant frequency. I did this a few years ago with a MOT, testing the difference in inductance by using different sized paper shims between the "E" and the "I" laminations. There's a webpage describing this somewhere out there, but of course I can't find it right now. I think I made the mistake of saving it instead of adding it to my bookmarks. Jon |
#80
Posted to rec.crafts.metalworking
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Testing MOT as electromagnet - this just in
Jon Danniken wrote: Winston wrote: Bob Engelhardt wrote: Winston wrote: The inductance of that coil fell significantly when you lopped off the 'I' core, yes? Yes and no, I suppose G. 'Though I did lop it off, I replaced it with a shorting bar, so it shouldn't be drastically different. Now's the time to break out your $0.99 inductance bridge. I haven't tested this but it looks reasonable: http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_meter.jpg.html http://www.aronnelson.com/gallery/main.php/v/BinOfBrett/Inductance_method.jpg.html You can also measure inductances of this size using your PC and tone generating software. Wire up the inductor with a capacitor, apply the tone, and look for the resonant frequency. I did this a few years ago with a MOT, testing the difference in inductance by using different sized paper shims between the "E" and the "I" laminations. There's a webpage describing this somewhere out there, but of course I can't find it right now. I think I made the mistake of saving it instead of adding it to my bookmarks. It's simple: Connect a known value capacitor across the coil. Put a 1K ohm resistor in series with the generator to the hot side and connect the ground to the other end of the coil. Then use a scope or AC voltmeter to look for resonance across the L/C pair. -- You can't fix stupid. You can't even put a band-aid on it, because it's Teflon coated. |
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