Hi people, I'm trying to set up a couple of solenoids (end to end) with a gap between them where I can put a test cell to do some work on MR fluids. I require between 0.5 - 1 Tesla of flux density within this gap (gap about 50mm across), so it will be N-S across the gap. I have up to 5 amps of current to play with per coil. I'd like the coils to be about 50mm diameter and 150mm long. I'm trying to work out the number of turns per coil from these parameters, but bearing in mind a solid iron rod inside the coil am finding it difficult to ascertain the correct equations to use. My main sticking point is what permeability I need to use for the iron rod. I'm trying to work this into the equation : B=u0NI where u0 is permeability of free space. Also, however, this is for an infinitely long solenoid and probably does not apply in my case as length of solenoids are not over 10 times the diameter?? And these values are for the centre of the solenoid anyway. I know permeability(of the iron) is variable dependent on the field strength but this is where I am getting stuck. Do the physical dimensions of the rod come into play at all?. Because of this variability I guess there isn't just a one-off value I can use and plug it into the equation in place of the u0 value. All ways I've seen to measure permeability is to do a BH Curve of the material but i'm failing to see how I can do this without putting it in a field in the first place which I do not have?? I'm sure there is some really basic thing I am missing here. BTW, I've seen u figures of 2500, 3000 etc bandied around for an iron core. However, plugging these sort of values into the equation yield ridiculous values for N. I really am a dunce! Please help
Hi Deepuz, A few educated guesses from me. There's quite a lot in your questions that I'm not sure about... 1 Tesla is a huge field - about 20,000 times the Earth's natural field. I think some MRI machines might produce fields that strong, but I doubt you'll get it with your apparatus. Hmmm... thinking about this. See below. It won't be too far out, as long as the gap between the solenoids is not too large compared to the diameter of the solenoids. The field is almost constant everywhere inside the solenoid, not just at the centre, so the formula applies everywhere inside. (Not quite true - very close to the windings the field will be a little stronger.) I don't think the dimensions of the rod will matter. Hmmm... I think the problem might be that using those values for u gives the field inside the iron itself, which will not be the same as the field in the air gap outside the solenoids. The field in the air will be smaller, but still larger than it would be without the iron cores. My problem is that I can't recall enough of my magnetic materials theory to be able to tell you how much lower the external field will be. I hope somebody else will be able to help you.
James, I appreciate your reply. Thanks for trying. Regarding the flux density. You are probably right, 1T is quite ambitious, in fact, having looked through my notes it is more like between 0.1T and 0.5T, whic I'm pretty certain is possible as I have used some similar apparatus before on a dynamics rig used for eddy current damping (and I measured the field in the gap with a Teslameter, for between 1 - 5 amps. The problem is that rig has too small an air gap for me to currently use (and I'm not allowed to take it apart anyway). So really I'm trying to build from scratch, with a slightly wider gap than before and I'm trying to find the number of turns I'll need for each solenoid. I thought it would be simple but I'm a mechanical guy and electronics is just not my cup of tea! If anybody else can help it would be much appreciated.
Hi Deepuz, I don't have time to respond in full right now, but have you considered just using permanent magnets? One can easily obtain neodymium iron boron magnets with a field on the order of a tesla at their poles. It seems like a couple of these guys would suffice for your purposes, or are you looking for something more systematic? I've played with MR fluid myself using such magnets, so I know you can see interesting things. Also, if you're looking for a really uniform field, you might consider using a helmholtz coil. The magnitude of your field would be smaller, but you could get a very uniform field.
Hi Deepuz, I think you need to be more concerned with the saturation level of iron than with the permability (should be easier to find than calculate the required amper turns) You could make a test coil, and with the meter you have access to determin it experimantally also. - Near saturation the increase in field strngth will become linear, as if the iron were not there. I think Physics Monkey is giving you good advise, if you can afford it. - As I recal, the rare Earth magnets were quite expensive. One thing you also might like to consider is getting your hands on an broken micro-wave oven. The magnitron (I am assuming they still make the micro-waves with one) has a strong magnet in it, and a gap large enough to consider. I do not know exactly what you want, but if you had saturated iron of diamater large compared to the gap, the field should be reasonable uniform. Why does it need to be so strong?
Actually, neodymium magnets of around one Tesla in field strength are pretty cheap, less than ten dollars. The question is whether you can get the kind of field you want at strength. A tesla is 10,000 Gauss. It's not that overwhelming a field strength to get. Here is the Wiki link. It says that most ferromagnetic materials saturate at about 1 to 2 teslas, which works out to 20 amp-turns per inch, so it looks like getting your 1 tesla field isn't going to be at all difficult, but take this with a grain of salt. The entire question gets a bit more complex than this.
Guys, Thanks for the replies, I have to get to class now but will reply more in depth later. I just wanted to point out that I need to use electro-magnets because part of the problem that I'm trying to investigate is the time response of the fluid's change in rheological behaviour between on and off state of field. (generally quoted to be in the order of milliseconds.) Most apps of these fluids use electromagnets to 'smartly' adjust the viscosity, and hence, yield stress of these materials (usually for damping purposes) and the electromagnets give much greater control than permanent magnets. I will look into iron saturation levels as opposed to permeability and also the idea of using Helmholtz coils. What I suppose I'm really after falls more into the realm of solenoid design. Not a lot of what I can find in the literature falls into exactly my scenario, but there must be a relatively simple way to find out number of turns over the two coils (albeit with an airgap between them), knowing that input current will be 1 - 5 amp and the resultant flux density should lie somewhere between 0.1 and 0.5Tesla. I'm just struggling with the equations at the moment as no theoretical scenario I've seen in textbooks / on the web fits my setup exactly??? Thanks again for your suggestions.
