Engineering magnetics -- practical introduction to BH curve

today on Applied Science we're gonna take a practical look at engineered magnetics I've actually wanted to make this video for a long time because it's common to teach electronics with batteries and wires and make it very understandable but for some reason when it comes to magnetics it's typically taught heavy on the math and the theory so after watching this video you'll have a really good conceptual grasp of magnetics and if you want to design your own transformers or motors this is a great place to start it so I mentioned batteries and light bulbs and this is actually a really great place to start over here we've just got a d-cell some wire and an incandescent light bulb and this is even taught in grade school level I mean it's pretty clear what's happening here we've got an electric current that's flowing through the wire making the light bulb glow and flowing back into the battery and then you know a few years later we introduce the concepts of there there's a voltage here and there's a resistance in the light bulb and then a current flows through here that's determined by the voltage and the resistance and it's actually almost exactly the same thing for magnetics let me show you so I have a little magnet between these two pieces of steel and then we've got a steel track here making a circuit and a compass just to show what the field is in this gap here and then I've got another magnet here just to pull the compass sideways so that we can see when there's no field and so if I put the thing in here you can see the compass needle moves of course and if I turn it around it goes the other way nothing too shocking just yet but what to notice is that if I hold the magnet out here and turn it the compass moves a little bit because the magnetic field can even go through the air but if I put it into this steel piece the magnetic field has a much easier time making its way out to the compass and so to notice the same exact quantities that we get in electric circuits apply to magnetic circuits we essentially have something like a voltage source we have something like a resistance and there's actually a flow through here a quick note about terminology there's actually quite a lot in the field of magnetics and it makes it hard to understand because there's just so many new terms and so I've tried really hard in this video do not introduce terms that aren't necessary but unfortunately when you go off and search for more information on the internet they're gonna crop up so we're gonna have to cover some like for example in electric circuit we call it resistance when there's an opposition to a flow of current and in a magnetic circuit we call it reluctance and so the trouble is that if you go searching for stuff like magnetic resistance you won't really find it on the internet you have to search for reluctance so I'll do my best to describe each term as it comes up and keep in mind we're trying to keep it minimal and I'll always do my best to describe them first it helps a lot to visualize what's going on in here so I took the magnet with the two steel pieces out of that circuit and I'm just going to sprinkle some iron filings on here and you know you've definitely seen this demo before but the thing that's important to note is that there's sort of field lines flowing out of the ends in making a circuit even in air right like everything is an electric circuit everything is a magnetic circuit and the trick is that electricity really really doesn't want to flow through air so we can usually neglect it in thinking about electric circuits it's actually very fortunate because all of our circuit boards require air and insulators to work properly but with magnetics even though the field doesn't really want to go through air we'll talk about what the exact numbers are later it can still make it and so there's no there's a magnetic field flowing around here and we could do the analysis and figure out just how much is flowing through there based on the reluctance of air so let's put this into the circuit so you can see how that changes the field lines okay now we'll try the same thing with this in circuit notice that there is quite a bit less activity here and the reason is that these steel pieces are pulling away the magnetic field it's easier for the magnetic field to flow through these low reluctance pieces of Steel rather than flow through the air so you know it's there's two camps on whether you want to anthropomorphize physical phenomenon but you could say the magnetic field would rather flow through the steel than it would flow through the air or you can just think of it in terms of reluctance the steel has a lower reluctance and so it conducts more of this magnetism than the air around it and if we took these gaps out and made it perfect you would have even less field leaking out into the air so we've shown that a magnet acts like this source of magnetic force it's kind of similar to voltage in an electric circuit but there's an important difference between a magnet and a battery a battery is an electrical energy source I mean it's converting chemical energy into electrical energy and we're actually pulling electrical energy out and burning it up in this tungsten filament lamp but a magnet is not an energy source a magnet is much more like a compressed spring so at the factory they started with a piece of material that was not magnetic and put a lot of energy into it to make it into a magnet so think of this as kind of like a compressed spring so it's providing force for us because it's a compressed spring and we can squeeze it harder or you can uncompress the spring and get the energy output only once so it's much more like a small storage vessel that's filled up at the factory and you really just can't get you know forever energy out of it of course ok let's try something else so instead of the permanent magnet with the two pieces of steel I have just a single piece of steel here with about 50 turns of wire wrapped around it and when I close the switch here about 3 amps flows through there and you can see the compass needle turns when I put current through there so it's as if this piece of steel is becoming a magnet when we put power through and when we stop putting power through it is no longer a magnet you could almost say it's an electromagnet right okay so we'll try it with a different material let's try a piece of brass okay I'm turning it on and off and you can see it's the same three amps and we'll zoom in on the compass so you can see this a little more closely but there's barely any movement okay now I'm going to switch to another third type of material this will be a piece of stainless steel it may be hard to see that the compass is actually moving a little bit more than the brass but less than the steel so let me switch to a meter so we can actually put some numbers on this instead of just looking at a compass moving so I've got this magnetic meter that I got from Amazon and it reads off in units of military and this is kind of the sensor on the end of this probe here a Tesla is a unit of magnetic flux density so it sounds complicated but really if you think about these field lines connecting one side of the magnet to the other the flux are the lines themselves and the density is just how close the packed they are together so the Tesla is used as a unit of describing kind of how you know intimidating your magnet is you know what met up an MRI machine is like a 1.5 Tesla or a 3 Tesla machine a large neodymium magnet could be something like half a Tesla at the surface and so here's a couple of them stuck together with a little space in between and if we put the probe right on the surface we're getting about you know 460 millet has load so about half a Tesla at the surface now the interesting thing is if I turn the probe 90 degrees we get almost zero in fact I can turn it to a point where in fact it is zero and I'm just turning the probe on the surface here and this makes sense because this probe only measures magnetic flux in one direction this way you can kind of see there's like a little dot on the on the surface there and that's that's the sensor itself and on the other two faces there's no dot so it only measures in this way and since the magnet is polarized so that the North Pole is this way if we put the probe so that it's cutting through all those so that it's lined up with the magnetic field then we get the full reading but if it's 90 degrees off we get nothing so anyway so about half a Tesla at the surface but if we move away from the surface the reading goes down really fast really fast so for this far away and the reason is that air has high reluctance to this magnetic flux so even though the magnet is trying to push a lot of magnetic flux through the air the air is not very good at conducting it and that's why this drops off so quickly there's just not a lot flowing so when you measure a magnet you have to specify where this flux measurement is happening okay so let's measure the little electromagnets that I've made here I'll go back to the sensitive scale adds a decimal point and we'll put the probe here and it's already measuring a teeny bit but now I'll turn the current on and with the current on it's doing about you know 10.5 Millett aslan okay let's try the stainless steel little bit of residual there with the current on about 1.4 mil otezla and with it off about 0.5 and with the brass 0.4 with almost no residual so 0-2 0.4 so we can tell that there's something about the material itself that determines how good of a magnet it becomes when we put it into a coil of wire and run some current through the coil so if we were gonna graph that out it might look something like this I should also mention that if we put more current through the coil we get more Tesla's out of the end of our electromagnet so and instead of clicking the button I'm going to turn it up slowly on the supply and so if you watch the current going up over here you can see as we put more current through we get more Tesla's out the magnet so we're getting 30 mil of Tesla at about 8 amps and we're getting about 20 mil of Tesla at 5 amps and so on okay let's kind of graph out what's going on here so the the input to our system here is the current through the wire and the output of our system are the number of Tesla's that we're getting out of our electromagnet so when we put the probe here and we turn the current off or getting more Tesla's out okay it's pretty good as it turns out the input to this electromagnet is the number of turns times the current that we're putting through it divided by the length of the coil and the reason that this you know is the case is that one turn of Electress of one turn at a certain current is the same thing as two turns at half the current right you think about it if you're if we drew a black box around this coil you could not tell the difference if you were inside the middle of the coil between this two turns at one amp and one turn at two amps it's the same thing so the unit is actually pretty convenient it's just amps times turns divided by meters and the meters is like the length of the coil and the reason that we throw that in there is because this is actually like the intensity of this magnetizing field and so if this was spread out among a whole meter yeah that makes sense that's less intense than if we're all crunched up over into one little centimeter like this one is so we include this length there because this is actually the magnetizing intensity so you've probably heard of this BH curve before it's it's actually not that mysterious but for some reason if folks don't explain it sort of simply so I hope this is making sense I mean it just think of it as the y axis b which is the symbol for this magnetic flux density is dependent on how much current and how many turns were basically putting into it so how good of an electromagnet and how much work are we putting into this electromagnet is basically what this graph is going to show so we saw these three materials we had brass stainless steel and steel and we saw that each one of them has a different characteristic so even with the same number of turns the same current and the same length of the coil on each one of these pretty much the steel performed really well the stainless steel was kind of intermediate and the brass was was basically nothing and I should point out that brass really is pretty close to nothing the only reason that we saw any fields coming out at all is because because there's a little bit of air actually in here I mean the brass is not really playing apart but let's let's not worry about that too much just at the moment okay so if we were gonna plot out the performance of these coils we might have a really good performer like this so this is maybe the steel and then we've got kind of an intermediate performer and that's the stainless steel and then we've got a really low performer and that's maybe the brass and what we want to do is basically come up with a description you know a number that describes how good these materials are at conducting at becoming electromagnets when we put them in a magnetic field and as it turns out that's basically the slope of this of this graph and we call that mu so mu is a description that it's one number that describes how good these materials are and actually is a fun side note there is such a thing as a negative material the set graph actually keeps going and these are diamagnetic so air itself or even empty space is actually on this graph it's let's let's consider it basically brass or air is pretty much the same thing air an empty space or true vacuum are basically the same for this for this argument and it has a very very low permeability but it's there it's actually a universal constant it's just a number and so to make things easy when we're talking about steel or stainless steel what we're doing is describing how much better they are at conducting magnetic fields than air or empty space is so a lot of times you'll see mu with a subset R meaning relative so mu relative of steel may be like a thousand and mu relative of air is one I mean that's that's what we're measuring relative to however when we do the numbers later on we'll actually plug in what the real value for air or empty space is so just keep in mind that when you hear permeability of a thousand or two thousand or a hundred or whatever that's just the relative permeability compared to air or vacuum the actual number is something like times 10 to the minus 7 it's just not convenient to throw around even though we're starting to get further away from this electronic analogy when understanding magnetics it helps to keep a few things in mind one is that in electronic circuits we're almost always using copper as the conductor so if you don't really care about the material properties so much in electronics because it's always copper and so copper has a certain resistivity to it it's a material property and then if you want to flow you know an amp through a wire you basically look up on a table to see how big the wire should be because we're not really caring about you're not gonna switch to a different material basically whereas with magnetics choosing the material is actually a pretty important part of it and they each have different permeabilities like we just said each one of these has a different slope on the graph another thing to keep in mind is that there's multiple unit systems unfortunately for magnetics but there's a couple of things that will help you out one is that the English system is almost never used so you can't even blame this on us Yankees because the you know the American units or whatever are almost never used so you can ignore these then unfortunately it seems to be about half and half split between the CGS system which is centimeter Graham's second and true SI units which is metre kilogram second and if you look through here you'll probably recognize some of these different units for this video I'm only going to use SI units but if you're searching around on the internet and you come across let's say Gauss just keep in mind that there's a conversion factor and a Gauss is the same as a Tesla there's just a conversion you have to do an over sted is the same as amp turn per meter so in our previous chart here H could be amp turn per meter and the vertical axis is B and Tesla this could be gouged over here and it's the same concept the same everything it's just you know just different units unfortunately the conversion factors get to be kind of weird because the geometry comes into effect so 1 Tesla is 10,000 Gauss that one's pretty easy but the conversion between some of these other things are not so straightforward I would recommend if you're cruising around the internet reading about this kind of stuff just focus on one unit system and I would recommend SI units but it's up to you and then when you get really comfortable with what all these different things are then you know just do the conversion in your head or just when you see something like Oersted you just know that's amp turns per meter with a conversion factor but until you get a really good conceptual grasp of all these different things I would recommend sticking to one system okay so I keep saying that the magnetic circuit is just like the electric one and we can do the same type of resistance ohms voltage kind of analysis on it so let's let's actually do it we've got the simple one up here to start with let's say we're given that the voltage is 1.5 the EMF or electro-motive force and we're also given that the resistance of the bulb is 5 ohms pretty easy the current is equal to the voltage over the resistance 0.3 amps no problem now the same kind of thing will happen for the magnetic circuit one of our Givens is that the MMF or the magneto motive force is 250 amp turns and in this case our coil has 50 turns and we're going to put 5 amps through it so 250 next our circuit has kind of two resistive elements one of them is this steel bar that makes a circuit through here and the other element is the gap and the total amount of reluctance because it's that's basically magnetic resistance will equal the sum of those two things it's actually very easy just add them together so we can calculate the reluctance of the core or the steel part and then the reluctance of the air-gap add them together and that's our total reluctance so our Toth our core plus our gap and if we look over at the unit's here reluctance is actually denoted by this stylized R and I really don't like the the weird symbols and stuff because again it just makes the material harder to understand but there aren't too many this is really it and this is as advanced as the math is going to get in this video so the reluctance for any kind of material is the length of it divided by the permeability times the area and this is actually exactly the same as in electronics it's just again we always use copper and so we're not really starting with the resistivity of copper but if you have a really thick copper wire that can carry a whole lot more current it has a lower resistance or if you have a short copper wire as opposed to a long one that also has lower resistance even though the resistivity the intrinsic property of doesn't change the same in both cases or any case same thing here if you know the permeability of your material the length and the area are what determined the reluctance are like the total resistance to magnetic flux so let's do the corer first by the way a core is just a material that's good at conducting magnetic fields so if you're building a transform or a motor or something the core is basically the metal that's carrying this magnetic field so in this case the core has a length of 324 millimeters and when you're doing the math always use the right unit system I would strongly recommend MKS so everything in here is metre kilogram second so instead of 324 it's point 3 to 4 which is the length from from here to here and then the permeability of the steel I'm estimating at 500 remember we talked about relative permeability the actual permeability of empty space happens to be this number so 4pi times 10 to the minus 7 is just a universal constant it's just the way it is it's one of those very few universal constants that defines how the whole universe works and it's just there and then the area is pretty straightforward it's an 8 millimeter diameter and if you calculate it out it's 5 times 10 to the minus 5 so if you calculate all this out it comes out to be 1 times 10 to the 7 and the units are a little weird if we look over here the units for reluctance our amp turns per favore or I'm gonna say Weber because that's a little bit more comfortable don't worry too much about the unit's here just remember that the reluctance is 1 times 10 to the 7 for this core ok now we'll do the same thing for the gap but we're already running into a problem what's the area of the gap like if we were to cut the gap with a plane through it perpendicularly I mean it's it's infinite area there's it's air I mean there's no there is no yeah there's no area to it so what we use is a approximation and the way to do it is to add the length of the gap the distance to each of the dimensions in space to calculate it out so since this is a circle I'm going to use point zero zero four four millimeters is the radius plus 0.03 which is the length squared times pi to get the area if it were a square cross-section you would add the length to both the width and the depth and multiply them together and that's actually a very good approximation and the reason we do that is because the field lines sort of bow out through this gap right like you've you've seen in that with the iron filings that the magnetic flux kind of tends to bow out from these air gaps and so we use this area approximation so if we do the math for that one we get six point six times ten to the six for the reluctance of the gap and then just like with the electric circuit the total flux and the unit of this is the Weber and you can remember this one because it sounds like ampere so amperes vapors and it's equal to the magneto motive force divided by the total reluctance and we know all these numbers and if we plug them in we get 1.5 times 10 to the minus 5 this is great but it's it's actually very tough to measure this in an electric circuit it's easy to measure the current we can just put an ammeter in circuit and measure it or we could even use a clamp meter but in magnetic circuit it's very tough to measure the total flux so instead what we want to do is measure the flux density that's something that I can measure directly with the meter over here and to figure out the flux density all we do is divide by the area again so if we know the total flux is 1.5 times 10 to the minus 5 and we want to know what the density is we just divide by the area that's pretty straightforward so be the flux density we'll use the area from the gap again it's the same the same area here is here and we got out for milites low not too bad right let's try it out so if we come over here we'll take the compass out and put the probe in and we can see there's a little bit of residual it's coming up at about point four and if I turn the current on lo and behold we get about four mil otezla that's pretty cool now it's true that if I move the probe around like put it over here it drops precipitously I mean it's only about 0.7 in the middle if we go to the other side it's about three so what this is telling us is that our approximation for the area is close but not great but actually it's pretty good considering we just went from Universal constants measuring the geometry to a value that's within you know even within 20 or 30 percent or even 50 percent is pretty good pretty cool let's do another one okay so in this one we've got a steel ring and I put a hundred turns of copper on there so this time we have the magneto motive force of 500 because it's a hundred turns at five amps so 500 the total reluctance is still the reluctance of the core plus the reluctance of the gap and it's the same math as in the previous one I'm still estimating the permeability of the steel here at 504 for pi e negative seven is the permeability of free space we end up with a reluctance for the core and a reluctance for the gap and one thing that's interesting here this is 2.1 times 10 to the 7 this is three point one times 10 to the 6 so that tiny little air gap in this system actually has 10 times the magnetic reluctance of this whole steel core it's a really important thing to keep in mind air is 500 times less permeable than steel than a lot of steels and for some decent materials used in transformers it's even several thousand times so if you put an air gap into your magnetic system most of the reluctance and in a lot of cases all of the you know reluctance that you have to worry about is in the air gap so anyway we do the the flux this is the total amount of magnetism flowing through the circuit 2.1 times 10 negative 5 vapors and since we can't measure that we want to measure B which is the flux density divided by the area and we use that same trick to estimate the area of the gap we get 188 Militello so let's try it out now one thing you'll notice is that we're already measuring 40 Millett as 'la and I'm not even flowing any power through here so we'll turn the power on and we're getting about a hundred so you know it's it's it's a little more than half of what we estimated but like I say the point of this is not to you know the video is not to give you a lot of practice doing the math or showing how rigorous this can be if it's within a factor of two I'm very happy since we're starting with universal constants and geometry and so actually getting even this close I think it's pretty good however there is something to notice here even without any current flowing it mean the meter is pretty close to zero it's reading you know point six now and if I go into the air gap without any current flowing it's actually reading forty Militello so that means that our graph here can't be quite right because H is amped turns per meter and if it's zero then that would mean we shouldn't be getting any B at all so this graph is actually slightly inaccurate there's another couple of factors to take into account here this residual magnetism we keep seeing is something you've probably experienced yourself if you just take a brand new plain old nail out of the package it has a very slight influence on the compass but not much whereas if we magnetized the nail by putting it into a strong magnetic field now it's quite strong or much stronger than it was and so clearly the steel nail has the ability to retain magnetism and so if we were going to put this back on the graph instead of just having a linear relationship here the actual relationship has this hysteresis to it and so what this means is that let's say we start off in the middle here at zero magnetism so zero applied field and also zero magnetism coming out of our electromagnet when we apply power it starts moving up like we said it would and it eventually saturates which we'll talk about in a minute but then if we start reducing the power we can come back to this point here so that H is zero we're not applying any magnetic field we're not trying to magnetize it anymore but we still have being we still have magnetic flux coming out of the device then if we start going negative so we basically switch the polarity of our coil so if H is amp turns per meter now we're giving it negative amps or basically flip the polarity it actually requires us to apply this negative polarity field to it in order to get back to zero B and then we can keep going it's saturates again and then comes back to this point or over here where it's zero power and now it's a magnet in the other direction and so you've probably seen this BH curve and by now in the video we've gotten to this point where we're talking about it maasai click hysteresis to it and all magnetic properties have this there is no such thing as a perfect magnetic material as far as I know that only is a straight line up here forever well error is what I mean paramagnetic diamagnetic materials have this linear relationship that goes on forever but any ferromagnetic core is going to have this more of a shape and so I mentioned saturation what this means is that we can keep trying to make a better and better electromagnet by adding more turns or pushing more current through our coil but what ends up happening is we only get a certain amount of B out so for you know measuring this electromagnet with our meter here like this and we keep putting more and more current in eventually the graph will slide down into a flatness here and no matter how much current we put through we don't actually make it a better electromagnet and this is a material property as it happens for a lot of normally encountered magnetic materials like electrical steel and all this the saturation point is about 1.5 tesla so getting above 1.5 tesla is difficult very difficult because we just don't have materials that can carry that much magnetic flux whenever you encounter a magnet that's higher than 1.5 tesla like a MRI machine is commonly 3 Tesla you have to use other technology like superconducting coils or or just quote a lot of current with coils close together remember if we put if the coils are close to each other like this and we put our probe in the middle here even if this is air remember air still has some amount of permeability it's just that we start losing the the help from the steel so if we're trying to make a really good magnet using steel works up to 1.5 tesla and then beyond that we have to use just air or superconducting coils basically okay in this setup we have an isolated variac with its output connected to these clips and it goes through a clamp meter this measures the current going through here and then there's a hundred turns on this steel core with an air gap and the air gap has the probe for the magnet meter the Tesla meters stuck in there and I've modified the Tesla meter so that its output I added this jack to the side so its output actually goes into the oscilloscope and the output of the current meter also goes into the scope so basically we're just going to plot magnetic field in the gap versus current and let's see how that looks okay so I'm going to start increasing the current and at this low level you can see we've got current versus time here the magnetic field versus time here and then this is the XY plot so we've got milites 'la and the y axis and amps on the x axis this is pretty much the BH graph so we're gonna turn it up and you can see something starts to happen I'm gonna stop the scope so that I can turn the power off and we'll retain it here you can see that there's definitely this saturation effect right at about 120 mil of Tesla so the thing behaves linearly up to a point and then breaks over and becomes pretty flat and when a current is 0 we actually have quite a bit of retained magnetic field in fact it's about 80 Millett as low in this set up and you can see the current is pretty higher this is topping out at about 17 amps if we scroll through here you could figure out exactly how many amps it takes to saturate this material another very important thing to keep in mind is that the this whole system is dominated by the air gap remember how we said that the reluctance of the air gap is much higher than the reluctance of the material itself if we wanted to use this set up to measure the material itself without the influence of the air gap we have to come up with another scheme of doing it if we use the air gap to basically insert our Tesla meter in there then we affect the whole system and what we're actually measuring is the saturation of the whole system including the air gap so steel saturates much higher than 120 milli Tesla the only reason that it's showing up this way on our graph is because of that air gap so let's go back to the set up and see if we can figure out another way to do this if we add another little coil to this metal ring we've kind of made ourselves a simple transformer so we've got the primary over here putting the magnetic field into this steel ring and then we've got another coil over here that we're using as a sensor and we just said that this whole thing saturates it you know 150 militares or something like that what that would mean is that when we get up to the saturation point we shouldn't get much additional output out of this secondary coil this is why saturation is typically bad in your transformer design so we're putting more and more current in here but we're not getting any more magnetic field out because the the system has been saturated so what we're going to do is add an oscilloscope probe to the second coil and then look at the voltage while we do the same experiment and see what we can get with that okay so we're still looking at B with the Tesla probe here and then current going into the primary coil of our new little transformer and then this sense voltage is the output of our transformer so you can see I can scale up and down and everything is still pretty much the same so I'm going to scale up and then stop just sweet so I don't have to run current through there since we're doing you know 17 amps that thing heats up pretty quick so I don't like to leave it on for too long we can see the same phenomenon same saturation and everything and this is the voltage that we're getting out the transformer are secondary basically and it's horribly distorted the voltage we're putting in is pretty close to a sine wave and what we're getting out is this horrible thing here now the trick is how do we use this to figure out what the magnetic flux is in the coil so up until now we've only been talking about currents in magnetic fields this is the first time that voltage is cropped up and as it turns out the voltage is proportional to the change in magnetic field with respect to time and that's just sort of another universal law to sort of deal with but it's convenient that we can just measure the voltage and then figure out what the magnetic field is doing in there and since I said that the voltage is proportional to the change in magnetic field what we want to do is mathematically integrate this now we aren't going to do the math ourself we're actually going to use the oscilloscope to do it so I'm going to turn on the math trace and we can edit it and with a formula that I'm going to use is the integral of channel eight which is the voltage that we're picking up from that sense coil plus the voltage on channel one and channel one is just this very simple voltage divider just a potentiometer and the reason that I need that is without this offset control the integrator is going to drift and I'll show you what I mean in a minute so we'll say that's fine okay and currently this XY plot is plotting channel three on the x-axis which is current going into the transformer and channel 2 on the y-axis which is the direct measurement of the magnetic field with the Tesla probe however if we change the y-axis to be a mathematical calculated field look at that it's almost exactly the same will flip back and forth so that's the actual measured field and chattin the math channel which is the integral of the sense voltage is this almost the same thing it's actually working it's always nice when things work it's kind of surprising sometimes now the scale is not set up properly this has units of micro volts micro volt seconds but the cool thing is that since we have the magnetic field probe in there and we know that the flux is the same at all points in the circuit because it's a circuit so the flux is necessarily the same kind of everywhere and the area is kind of almost the same in the gap as the core we can use this to calibrate our system and then we can actually use this thing to measure the real magnetic permeability of any material we want if we can make it into a ring or make it into a transformer here's what I mean about the integrator drifting so I've just got this potentiometer here that's feeding a tiny little voltage into channel one we can put it up there it's it's just a flat looks looks varying it's very small that you can see it goes up and down when I turn the potentiometer and it's the the integration that we set up is basically adding channel one to this and so if it integrates a constant it basically just splays out but we don't want this and so we kind of tune the pot in to give us a really decent looking eh curve I thought there might have been a way to do this in the scope itself but as it turns out this is actually easier because I have really fine control over it and it's it's easier to just dial it in like that and then we can go up and current again you can see it's it's starting to separate a little bit here so you can use this to kind of dial it in just right and then another interesting quirk is that to get the to get the plot centred on 0 I'm using the level trigger and triggering off the sense voltage that comes back and so we want this thing to trigger it basically just the right level so that it's plotting the zero at just the right way okay let's take a look at another material this is a ferrite transformer it's two separate coils found on there and previously we were looking at this steel ring that I made myself but this is the real ferrite transformer that's purpose-built now we can't put the probe into the gap anymore because there is no gap so we just connected the input power to here and we're going to use our sense and integrate technique on that side to see what this looks like okay here we go I'm gonna start increasing the power and then I'm gonna stop the oscilloscope so I can turn the power down and let's see what we've got here first you'll notice that the BH curve looks quite a bit different than it did for the steel some things to notice the remain the retained magnetism when the power goes to zero is actually quite a bit less than a steel so the last vocabulary word for the video is the coercivity of a material which describes how much magnetism it retains when the field has been turned off so this BH curve actually gives us quite a lot of information about the material 1 the permeability which is the slope of the line likely you know what we saw in the earlier part of the video the saturation which is where this thing levels off and you just can't get any more magnetism out of it and the coercivity which is how much magnetism remains when you cross 0 another interesting thing that you can get from this graph is the area inside this curve if you add up all the area inside here this describes how lossy the transformer is based on the core losses you have to basically spend energy to make this core into a magnet and then you flip back the other way each time each cycle so if you're feeding this thing AC and it's taking a lot of energy to create a magnet then you have to switch everything around and undo everything you did see for a transformer you typically want something with a low coercivity a high permeability and are really high saturation the problem is that you can't always get everything that you want but you can get what you need and so for example in low frequency transformers the best material choice is typically steel it has a high saturation it the coercivity is reasonably high but that's kind of ok because it only flips polarity 60 times a second which is relatively low so even though we waste a little bit of energy with this coercivity problem the fact that the saturation is really high and the permeability is also quite high that ends up being the best material choice for a transformer that operates at 50 or 60 Hertz now if for higher frequency transformers like this little toroid this could be designed to work it may be 10 to 100 kilohertz or something like that and in this case that high coercivity would be a really big problem at at 10 or 100 kilohertz we'd be wasting so much power that you wouldn't really be a good material choice and so instead these little toroid x' are often made of ferrite and ferrite has a lower coercive 'ti as we saw on the oscilloscope so even though the saturation is also lower when you do all the math and figure out how much material would I need how much does it cost how hard is it to process ferrite ends up being a better choice for high-frequency transformers and steel ends up being the best choice for low frequency transformers also don't confuse the shape with the material choice like for example this is a toroid but it's actually a steel core so is this right this is a thing that I cut up myself but they make toroidal transformers out of Steel it just happens to be that most of these smaller high frequency transformers are toroidal and made out of ferrite there are also ferrite transformers that are not toroidal for example this one so what's the difference between a toroid and Ecore or something that's not a toroid if these are both ferrite transformers why would I pick one or the other again it comes down to more of a manufacturing cost balance type thing winding a toroid is actually relatively expensive if you if you're holding a roll of wire try to think about the difficulty and sort of winding it around the Tauri it's actually not that easy you need a specialized winding machine the benefit is that a toroid has no air gaps and it's essentially self shielding like all the fluxes contained within that core whereas this Ecore has some sharp edges and you have to join it together and so there might be a gap and again it's it's really kind of more of a manufacturing thing don't think that toroid x' and other types of transformers are all that different it's just a different way to accomplish the same thing there's also quite a few different kinds of ferrite teenies are marked 43 and then these have a little sticker on there and this one is painted oftentimes the toroid that are painted like this one are actually not ferrite it's powdered iron so they take iron powder and mix it with epoxy and form it into a toroid and you know these things are all relatively similar usually the differences involved how well it works at a specific frequency range so for example maybe this is really great at a megahertz and this other one is really great at a hundred kilohertz but really it all comes down to the BH curve diagram so they have differing amounts of coercivity and differing saturation points and different permeability but it has this additional problem of the BH curve being dependent on the frequency at which you test it and so not only do all these variables exist they also change depending what frequency you're putting through this thing which is why magnetics get to be fairly complicated and then of course if you're using a square wave that's a combination of frequencies and so you can get complicated right in a hurry let's finish up by taking a look at a couple unusual magnetic things that you might find in everyday items this is a flyback transformer from an old CRT television set and you'll notice that there's a little gap in here it's a ferrite core but there's an air gap in here and at first you might think well it's you know it's just manufacturing tolerance they obviously snapped this thing together and didn't quite make it but actually this is intentional you can see they even put some glue over here to set the gap thickness very precisely and the way that they designed this thing they actually want there to be some extra reluctance in the core at first you might think that's pretty dumb why didn't they just use a smaller core that would achieve the same thing right as it turns out not quite because the air gap the reluctance in this air gap helps this thing store power on each cycle that goes through and the way that a flyback transformer works is that you want to like charge up the magnetic field and then you stop charging it up and the magnetic field collapses and goes into the other coil that's in here I think the purists would claim this is actually a coupled inductor not really a transformer because of the way it's used but the trick is that the air gap actually allows it to store energy between cycles and that's how it works you might find other transformers that have careful controlled air gaps as well not the most common thing in the world now here's one so this is actually an iron core in fact this is an inductor this is not a transformer because it's only got two leads but you can see they put some wood or some cardboard or something in here to make an air gap even in this iron core again because they didn't want too much magnetic field in here this adds reluctance and prevents the field from getting too high another way to achieve this which is pretty weird this is a microwave oven transformer and this is the primary and this is the secondary and if you look in there you can see there's actually metal that's shorting out the magnetic field so if we were going to draw the magnetic field circuit diagram for this thing you would actually have a short in there and if you knock those pieces out this is what they look like it's a shunt a magnetic shunt and it serves the same purpose of putting a gap in these transformers you can sort of control how much magnetic field goes through the transformer and the point of these shunts is to limit how much current you can get out of the secondary the way that a microwave oven works is it tries to pull a large amount of current out of this coil and it relies upon the transformer doing the current limiting and that current limiting is achieved by bleeding off some of the magnetic field and making the transformer less effective as it gets up into higher currents even though it seems like the ideal material would have no power loss sometimes that's actually exactly what you want in an application like this these little ferrite beads are clipped on to this USB cable with it here this one clips on but these are molded on and the idea is that the ferrite will prevent high frequency noise from going through here and that and actually if this is lossy that's even better because we want to get rid of this high frequency noise we don't want to couple it to anywhere so sometimes you actually want a material that is purposefully lossy because we want to get rid of that high frequency signal which is interference in this case in the case of magnetic storage media like this we actually want a material with a really high coercivity right because we want to set the magnetic information in here and then have it not change until we're ready to set it again so these typically have higher coercivity and the you know the permeability and that sort of thing doesn't really matter that much in this case so the point is that depending what you're doing with magnetics you can search for a BH diagram that sort of meets your need and then make sure that the frequency that you're running at you get that BH characteristic at that desired frequency IFS of course a whole lot more to cover but I think this actually does cover all the aspects of magnetic circuit design and I hope some of this has made sense and made it easier for you to get started in the field once you start reading on the internet you'll find that people try to dump the math on you and it's not necessary I think it's really much better to sort of keep things conceptual until you really really need the math for a specific reason and then you can dive in and do the heavy duty analysis okay see you next time bye

Channel: Applied Science

Views: 614,249

Keywords: magnetics, magnets, motors, transformers, magnetic circuit, telsa, oersted, amp turns, bh curve, B field, H field, coil, applied science, krasnow, oscilloscope, measure bh curve, torroid, ferrite, iron powder, inductor, magnetic field

Id: 4UFKl9fULkA

Length: 49min 57sec (2997 seconds)

Published: Sun Aug 12 2018

Reddit Comments

[u/mosinut]

I could have used an explanation like this so much back when I was in school!

[u/datums]

This is like the nerdiest shit to make it to 70 point here ever.

It's excellent.

[u/nhremna]

They used to make computer memories out of these.