Requesting advice to appraise many unknown large toroids - Page 1
Forum section: electronics / beginners

author: Cliff Matthews
author_rank: Unknown
timestamp: May 19, 2015, 02:19:16 am
content: Lately I got some work shutting down and scrapping some big tri-phase inverters. Many of the parts (like Toshiba 6inch IGBT's) were taken by another tech - but I agreed to take the toroid rings that were placed along large cabling within the cabinets (some sets were wrapped in the biggest heat-shrink I've ever seen!). These were never taped-up or wound by enameled copper, so they're are in star condition - however:Without knowing permeability, how would I test them for specs?I have a 200mhz quad storage scope and a 150khz AF gen now, (thanks to a member here) what can I look for?

author: djQUAN
author_rank: Frequent Contributor
timestamp: May 19, 2015, 02:36:26 am
content: I have built this circuit to test my DIY inductors and it works a treat.http://ludens.cl/Electron/lmeter/lmeter.htmlYou'd have to wind a test coil on the inductor (you'd have to do it anyway with an inductance meter) but with some additional calculation, you can also obtain data on how many amps before the core saturates. It will also be different if they are ferrite or powdered iron so you can tell what type core it is too.

author: mzzj
author_rank: Super Contributor
timestamp: May 19, 2015, 06:34:44 am
content: Quote from: Cliff Matthews on May 19, 2015, 02:19:16 amJudging by the look and original use I would say that most of them are high permeability EMI filter types. Permeability probably in the 3000-15000u range.Useless for making inductors since saturation current will be very low, somewhat usable as a power transformer core. Best suited for.... EMI filteringSome of those yellow ones might be powdered iron type low permeability cores suitable for inductor cores, if you wind couple of turns of wire through them you can tell the AL value and likely material. Or just try to cut them with file or hacksaw. Ferrite types are too hard to be cut with file, powdered iron or mpp cores are softer and file/hacksaw is able to nick them.http://www.tek.com/document/application-note/capacitance-and-inductance-measurements-using-oscilloscope-and-function-ge

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 19, 2015, 10:20:41 am
content: SeeexxxyGood bet the black ferrite ones from cables are common mode chokes. High mu. Good for pulse transformers, CMCs (duh..) and FBs for LF use. OK for power conversion (with reduced ratings at medium frequencies). No good at RF.Look up bi-color types here,http://www.micrometals.comI see yellow and green, which are probably yellow-white #26 and green-blue #52. The latter has half the losses of the former, but they're both quite lossy. Suitable for filtering chokes with low current ripple. Commonly seen in computer PSUs where the low cost dominates over poor performance.Gray, green and other enameled cores can be powder or ferrite. You'll have to test (or at least begin guessing from context -- CMC vs. differential filter?).Testing:Take the dimensions. Actual core dimensions will be slightly less than the outside dimensions with enamel, but not too far off. You want l_e and A_e.l_e is the average path length. Average OD and ID, multiply by pi.A_e is the cross sectional area. Take (OD - ID) / 2 and multiply by height.I find mm (mm^2, etc.) are most convenient, but cm are commonly seen as well (Micrometals uses both..).Inductivity:Put on N turns (note: a single pass through the center counts as one) and measure the inductance. Divide by N^2 = A_L.You can measure this by resonating the inductance with a capacitor, and measuring the resonant frequency. This should be performed at a low voltage, under 1V, and at a modest frequency, like 10-100kHz. Repeat the measurement at different voltages: if the frequency changes, you're testing at too high a voltage. Ferrites in particular are nonlinear, and the inductivity increases then decreases with rising voltage!You will find L = 1 / ((2*pi*F)^2 * C).You can also take note of the loss factor of the core, by setting up a voltage divider circuit (between signal generator, dropping resistor, and LC tank), and when it's adjusted to resonance, the tank is pure resistive, and that resistance represents the core loss. Generally speaking, losses tend to dominate above some cutoff frequency, which also helps to identify it (~100kHz for high-mu ferrites, ~1MHz for "power" ferrites, >10MHz for NiZn RF types, etc.).You can further calculate mu (permeability) from:A_L = mu*mu_0 * A_e / l_eIf A_L == nH/t^2, A_e == mm^2 and l_e == mm, then mu_0 = 1.257 nH/mm.Typical values should be 2k-10k for ferrites (>5k for CMC type ferrites)You can also test saturation with a pulsed circuit like this,http://seventransistorlabs.com/tmoranwms/Elec_Inductor.htmlbut this shouldn't be necessary after identifying the cores, or testing them.Tim

author: Cliff Matthews
author_rank: Unknown
timestamp: May 19, 2015, 12:55:14 pm
content: Thank you all! Since I am likely to get more attrition-type tear-downs (mainly Cummins Onan) I plan to build the tester illustrated by user djQUAN. Tim, that math does get scary but I will try to absorb your awesome essay with the Tek pdf and come up with some kind of method.djQUAN, for that circuit, can I swap a BUZ11 FET (.04ohm 50v spec'ed for 120amp pulses) with 2 screw-lug 250volt 9,100uf caps instead of the 12 x 3300uf LV radial ones illustrated?(I don't have that many 3,300 radials and I'm guessing to swap at little less uf for lower EMF and big connectors..)

author: djQUAN
author_rank: Frequent Contributor
timestamp: May 19, 2015, 01:06:36 pm
content: It shouldn't be much of an issue what you use to build it as the design is pretty forgiving as I found out.The mosfet should be fine. I made mine with a compact 20pcs of 2200uF caps. I also added an on board CC/CV regulator based on TL431 and LM1085 so I can use a 12V battery if a bench PS is not available.You can have a look in here:http://quan-diy.com/b/

author: Cliff Matthews
author_rank: Unknown
timestamp: May 19, 2015, 01:21:15 pm
content: Quote from: djQUAN on May 19, 2015, 01:06:36 pmMy God man, you do neat work! (and nice clear photo's too)The open case design in IMG_3965 makes all those caps look like header pipes on an engineI'm afraid my tester will have to live in a Texan-sized lunch box with those big caps I have..Maybe I can hang them out the back, like mufflers on a rice-rocket (all sound and no fury)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 20, 2015, 01:21:31 am
content: You don't need nearly so much capacitance -- I used a couple 2200's in mine, IIRC. Easy way to tell: if the supply voltage is drooping during the "on" pulse, consider using more turns (= less current!) or more capacitance.Oh, and when doing pulsed tests, you're only exercising half the B-H curve -- any remenance (remaining field -- a side effect of hysteresis) will reduce your operating range, making it saturate easier than you might otherwise expect. This is usually small for powdered iron (which is effectively self-gapped) and gapped ferrite cores (usually E-E shapes and whatnot, with a visible gap or shims), modest for ungapped ferrite (maybe 10-30%), and large only for special types ("square" ferrite 70-90%; permalloy and Metglas, 90-98%).(If you happen upon such a specimen, demonstrating the conditional saturation can be quite interesting. There's usually a plastic-encased toroid on the secondary side of ATX power supplies -- this is a saturable reactor that is used to control the +3.3V rail, independently of the other outputs. You could wind maybe 100 turns of fine wire to get a high enough impedance to see the "snappy" saturation effect with a common function generator.)Tim

author: djQUAN
author_rank: Frequent Contributor
timestamp: May 20, 2015, 01:49:34 pm
content: Quote from: Cliff Matthews on May 19, 2015, 01:21:15 pm

author: albert22
author_rank: Regular Contributor
timestamp: May 21, 2015, 01:41:38 pm
content: Interesting thread, I am still trying to understand magnetics and wanted to experiment by using some of the cores that I collected over many years.djQUANThanks for the link. Nice work on your tester !Tim,QuotePlease can you help me to understand a little more on this ?AFAIK the core will go to one side of the hysteresis curve when volt/secs are applied and it will return to some point when it "discharges" thru the diode. I am not sure where it is. What will be the state of the core at the end of the cycle ? Is it going to have a remaining field due to the remeance ?I would think that there should not be any remaining field once the core discharges so it wont saturate easier. If the core does not fully discharge then it will saturate easily and escalate in subsequent cycles until saturation. Please correct me if I am wrong,When the core is driven by a full bridge it will exercise the full BH curve but saturation will occur at the same point (Bsat)Thanks

author: Cliff Matthews
author_rank: Unknown
timestamp: May 21, 2015, 03:49:55 pm
content: Quote from: albert22 on May 21, 2015, 01:41:38 pmMaybe it's not a field, but a propensity to resist the first opposite cycle? (depending on resting domain states)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2015, 01:09:25 am
content: Correct, remenance is even the technical term.In reality, it will be slightly past that, because of circuit capacitance and diode reverse recovery causing some "hangup" where the coil's terminal voltage stays in the "discharging" direction for a period of time beyond the point when coil current = 0.The amount of "force" applied to bring flux density to zero is called the coercivity.The product of these two comes to units of power density (the B-H product), which is either a measure of hysteresis loss for a soft material, or a figure of merit for hard materials (NdFeB has the highest product, amounting to so much hysteresis loss that a single cycle magnetization physically heats up the material a noticeable amount!).Like I said, if you happen upon a saturable core, like the mag amp core found in a usual ATX supply, it's interesting to play with. The remenance is high (usually >90% of saturation), and coercivity low, so it doesn't take much current (given some number of turns, of course..) to flip it on and off. If you drive it with a square wave through a diode, you'll find it only absorbs a small blip before letting the rest on through undisturbed; but if you apply an opposite voltage through a resistor (enough current to overcome the coercion), the flux discharges proportionally, blocking proportionally more and more of the square wave!Tim

author: The Electrician
author_rank: Frequent Contributor
timestamp: May 22, 2015, 10:00:00 am
content: Quote from: T3sl4co1l on May 22, 2015, 01:09:25 amA small point. It's spelled "remanence":http://en.wikipedia.org/wiki/Remanence

author: albert22
author_rank: Regular Contributor
timestamp: May 22, 2015, 02:13:18 pm
content: Thanks.Here is another method:http://info.ee.surrey.ac.uk/Workshop/advice/coils/BHCkt/index.htmlAlthough it may be more suitable for small cores or un gapped ones, where saturation is reached more easily.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 22, 2015, 08:20:03 pm
content: Quote from: albert22 on May 22, 2015, 02:13:18 pmThanks, many good pages around the subject there. The circuit looks a little incomplete but why is the input to the core a sine wave? Aren't we trying to get comparisons of saturation speed and to see what happens at the top of the curve? (ferrite vs. identifying air gap compounds).On the other-hand, I'd suspect a j-FET op-amp input may not survive flyback from square-wave pulses

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 22, 2015, 10:16:01 pm
content: Doesn't much matter: as long as it's periodic, it will scan a useful display.Square voltage gives triangle current, so there's no flyback.Tim

author: albert22
author_rank: Regular Contributor
timestamp: May 23, 2015, 03:10:31 pm
content: CliffI dont think that the circuit is incomplete. It does not get much more complicated than that. There are a lot of similar arrangements on the web to plot the BH curve. That link is the best description of the method that I found so far. I rather use one that only needs a single winding.The core saturates at Bsat, meaning that you reach a maximum B (flux density). If you apply a rectangular pulse of voltage to the coil, the core saturates when a certain product of volts x seconds is reached. This represents the integral of voltage respect to time on the faradays law.There is not a saturation speed of a core but a point in the BH curve where as H (magnetic field) increases, B does not get any higher than Bsat.H on the other side is proportional to the current flowing and this is stated in the ampere law.The method that I linked exercises the full BH curve by applying positive and negative voltages (or currents) and it is a different approach to measure the parameters of the core but both relay on the same laws and the BH or hysteresis curve.I find both very instructive. The pulsed circuit allows to test at very high currents. The second method may require a high power source to reach such a high currents but you can get also info on coercivity, remanence and loses.

author: Cliff Matthews
author_rank: Unknown
timestamp: May 23, 2015, 06:20:39 pm
content: Thanks everyone, I'll be building a tester soon. Today I'm laying sod in my yard... (pre-cut grass strips, for use when things are so far gone that the weeds are doing the victory dance).For any interested, here's a small library to visit:http://www.voltech.com/Support/Search.aspx