Impedance Measurement of physical big inductor using calibration - Page 1
Forum section: electronics / rf-microwave

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 06, 2024, 12:06:20 am
content: HiI am working on a problem with a product that exceeds radiated emission limits, and in order to solve the problem I am measuring the characteristics of all the components and building a model.The unit is a 3MW converter, and one of the components I am characterizing is a big 5uH/2000A+ inductor. The terminals are about 100 cm apartSo normally I use a bode 100 VNA to do the measurements, using a BNC cable and at the end of the BNC cable I have clips that can be connected to the component. I would do open, short and load calibration, so I have a reasonable good measurement up to at least 10MHz.But for this component, the terminals are so wide apart, that calibration is difficult. The positioning of the leads during calibration needs to be the same as when measuring on the component, since otherwise the calibration is off.I was thinking to add wide copper tape to one of the terminals and bringing that close to the other terminal, so I have a low inductance path. I would off course do the same for calibration, but I am unsure if there is a better method.One downside to the copper tape is that it add capacitance effects, which again corrupts the calibration.Does a better way of measuring large component exist?ThanksKlaus

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2024, 04:07:31 am
content: Well, two approaches.One: if you want a more, I guess canonical, measure: set up a ground plane in front of the terminals (I'd say "set the inductor on a table/ground plane", but I'm guessing the logistics are a little different in this case..!) and measure it that way.It should stand to reason, given the size of the thing, there isn't much to be done beyond low 10s of MHz, and even then the error due to CM, radiation, and nearby stuff / proximity, will be significant (some percent).Two: more pertinent to the system, how is the part installed? Where is it in relation to enclosure, connecting cables/bus bars, etc.? If it's, say, in series between mains/rectifier and inverter, how about shorting one of those, and measuring at the other end? Those sets of connections are likely to be closer together and thus more representative of the inductance.But a secret third option also exists.Measure it as a two-port, with respect to the enclosure as ground plane. You need a VNA to do this properly, but some approximation can be done with a function generator and scope, or insight gained with it. This will be valid to much higher frequencies, limited by the wires connecting to the terminals/cables/buses, andBut even more to the point: radiation is telling. For equipment of this size, I mean sure it could be coming out along the cables, but that can also be ruled out with some X and Y caps. I have a sneaking suspicion that it's coming out between panels/doors/sections instead. What's the wiring look like? Any EMI gaskets? How about switching loops and wiring?Tim

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 06, 2024, 09:11:55 am
content: >Well, two approaches.>>One: if you want a more, I guess canonical, measure: set up a ground plane in front of the terminals (I'd say "set the inductor on a table/ground plane", but I'm guessing the logistics are a little different in this case..!) and >measure it that way.Yes, it's a little difficult, the inductor is actually a 3 phase reactor in front of a converter stack. It weighs about 500kg, so I cannot move it.>>It should stand to reason, given the size of the thing, there isn't much to be done beyond low 10s of MHz, and even then the error due to CM, radiation, and nearby stuff / proximity, will be significant (some percent).The biggest problem we have is a resonance peak at 250kHz, which is good in at least that at that frequency it's possible to measure and make a model of the thing. It has a first self resonance (parallel), and a second series resonance. It's the second that is the problem, since at series resonance the inductor ceases to act as an inductor and just passes noise to the grid from the converter. I am working on a test, where added capacitance on the reactor moves the resonance frequency down, below the 150kHz start frequency. But that does not work on series resonance, since that is inherent in the device parasitics.>>Two: more pertinent to the system, how is the part installed? Where is it in relation to enclosure, connecting cables/bus bars, etc.? If it's, say, in series between mains/rectifier and inverter, how about shorting one of those, >and measuring at the other end? Those sets of connections are likely to be closer together and thus more representative of the inductance.It's more or less directly on the grid, before an active 3 phase rectifier. It's connected with thick cables, and not busbars. There's a distance from the reactor to the converter, so I am looking at optimizing the cable layout for minimum loop area, but it is difficult with the size of the components.I have done impedance measurements directly on the reactor (across the terminals), and also to earth. I have also done thru measurements, from converter side to the grid side. The impedance measurements are difficult, since due to the size, the unpowered converter actually picks up ambient fields, so I cannot connect the Bode 100 directly, I need to use isolation transformer, and use isolation probe and current probe to get the impedance. With long wires, and no stable routing of the test leads, the short calibration is difficult. That's why I was thinking about using wide copper tape, to at least reduce the impact of lead inductance.>>But a secret third option also exists.>>Measure it as a two-port, with respect to the enclosure as ground plane. You need a VNA to do this properly, but some approximation can be done with a function generator and scope, or insight gained with it. This will be >valid to much higher frequencies, limited by the wires connecting to the terminals/cables/buses, andYes, this is somewhat the same as the thru measurement I did. I could combine this with wide copper tape to do the calibration, open, short, 50R.>>But even more to the point: radiation is telling. For equipment of this size, I mean sure it could be coming out along the cables, but that can also be ruled out with some X and Y caps. I have a sneaking suspicion that it's >coming out between panels/doors/sections instead. What's the wiring look like? Any EMI gaskets? How about switching loops and wiring?We have added Y caps, and it helps.As of now we have measurements in far field that show the problem, and I have done measurements on a powered converter. Saw very large currents in PE bonding wires on the structure. And the reason is that the enclosure, as you say, are not optimal. Very bad shielding of the converter. So we are working on a setup where we add brackets every 20cm that connects the structure to the panels. I might even do a test where the paint is removed and add copper tape on all panels, just to see the impact of correct shielding.On top of that, the grid cables are not shielded, so a large CM loop exists there also. For CM noise we could add a metal cable tray, but that is probably the last resort.So there's lot of options to improve, right now I just want to have the simulation model show the resonance at 250kHz, and what can be done to move it. Also looking at the switching speeds of the IGBTs, since if that is lowered, the resonance will not be triggered since the frequency content at the resonance is lowered (might create too much switching losses)>

author: ahbushnell
author_rank: Frequent Contributor
timestamp: August 06, 2024, 02:47:22 pm
content: I measure impedance in power electronic components using a signal generator to produce a sine wave voltage and a resistor in series with what I am measuring. I use between 10 and 50 ohms resistor. I use a scope to measure the voltage on the inductor and the voltage to a the other side of the resistor. I measure the RMS voltages and the magnitude of the impedance is Vind/((Vin-Vind)*R). If you want phase information measure the delta T between the waveforms. I then measure at many frequencies. I have done this by hand and then more recently I wrote a python program to automate the measurement controlling both instruments.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2024, 03:25:18 pm
content: FYI, you can quote more clearly/cleanly by wrapping with a [ quote ] ... [/ quote ] block... press the quote button on a post to see it done automatically on the whole post, or see how I've done it here when you quote-reply this.Note that 5uH isn't much: about as many meters of cable would do. You could do worse than lay some cable around in a couple turns looped, then stick capacitors to ground on either side of that. This would all be placed inside the enclosure, perhaps in a shielded compartment, or an add-on block (basically, custom filter module), and can be shrunk with the help of core materials, but obviously that then brings saturation into question.Whether it's CM or DM matters. Traditionally, low frequencies are stronger DM, but given the scale, even quite low frequencies could have quite strong (low impedance, high amplitude) CM content. There's at least the possibility that a large fraction is CM, and so, easier to filter (VAC and Metglas make some modestly large nanocrystalline cores that can fit around this many amps worth of cable) than otherwise.Also if you're on an isolation transformer (or nearby pad transformer with common earthing, or have an exemption, or generally no priority to avoid, ground-leakage currents), quite large capacitors can be used in shunt (10s uF?). You'll get impedance dips (series resonance) for 1/2 wave lengths of cabling, or various resonance modes in loops and coils, but the valley impedance of those dips can only be so low, and a good capacitor can be lower, still obtaining useful filtering value. The hardest part may be be keeping capacitor lead lengths short: they'll need to be rated for the voltage (480V+?), probably fused, they'll be film-in-oil types most commonly, of which the metal tabs inside the package also count towards the total, so all that wiring adds up quickly. Low-inductance types are available, and if you can arrange those to basically hang off a terminal block inline with the cables, joined with short links of bus bar, that'll do best.There are procedures for making a LISN at these kinds of currents; it's kind of bespoke because you really can't afford to add many uH's for mains frequency alone, but even just decoupling from a few meters of cable looped and laid on the ground (soil), with a ground stake used as ground return/reference for the resulting RF-port signals, would be a start. The test site should have some way to set this up. (Definitely use RF signal limiters before plugging these cables into the analyzer, lol, don't want breaker/contactor/inrush transients popping it from the get-go. Generous CM choking of the cables is probably wise too, as the analyzer and stuff won't be co-located with the signal pick-off point; expect some ground loop.) Filtering mains noise (ahead of the LISN) will be a similar challenge; perhaps the site has some filtering already, but if not, arranging a similarly-bespoke filter as part of the LISN might be required.Note that EMC ground will be earth (preferably metal-grid enhanced, or solid plate ground plane as such), and the enclosure should be well bonded to that (say, wide strip connections bolted near where the power cables exit) to get the most representative mains-port emissions.Regarding the inductor model:The first series resonance can usually be described as a 1/2 wave transmission line stub.That is, consider the wave coming in differentially between the inductor's terminals. Say the inductor is a two-layer winding, conventional build, a few cm across, bobbin wound, right? There's the start at one end of the bobbin, goes across, up to next layer, comes back, ends in the same place different layer. The two layers are basically 1:1 in size (+/- a wire diameter), same turns, but their helices wind in opposite directions (handedness). Consider the relative position along the wires as you go up their length, into the winding, starting from either end: the points are never very far from each other, but they do spiral in opposite directions. If not for the spiraling, it would be a twin-lead arrangement, where both layers follow the same position in the winding versus distance from the end. Twin lead will give Zo about 100Ω, but the real figure is probably somewhat higher due to the variable distance between wires, this sort-of-loop that keeps opening and closing between them as the helices counter-rotate. (The varying distance will also contribute dispersion, but this will only become critical at comparable wavelengths, i.e. near one circumference.)Or put another way: imagine, after making a winding like this, you tried unwinding the wires, start and finish simultaneously: you have to pull them off with each rotating in opposite directions, and the point you're pulling each wire away from, also rotates; the two points only coincide every half-turn, where they cross/overlap. Whereas, for a single bifilar or twin-lead transmission line, they would rotate in sync, trivially coming off together.Anyway, this is all to put in your mind, an idea: that, despite the weebly-wobbly back-and-forth of the counter-wound helices, there is still an aspect of transmission-line-ness we can ascribe to this geometry. And therefore we can treat the two pins as one end of a transmission line, and the far end (where it turns around and jumps to the next layer) as ashort circuit. Thus we have a 1/4 wave shorted stub at one frequency, giving Z peak, then a 1/2 wave shorted stub at the next mode (not harmonic!), giving Z valley.The 1/4 wave mode of course couples strongly into the core, so gets shifted way down when the core is inserted; the 1/2 mode (and various others) mostly balances out from the core, manifesting as leakage modes instead. The leakage may still be loaded by the core, but more modestly (because the winding space is acting as air gap between core legs).Also, for a little bit here, I've been referring to the ends/terminals as "two" wires, when really they're both ends of a single physical wire. The trick is to think of the terminals as one end of a two-wire pair, that happens to be joined (imperceptibly welded, let's say) at the far end.For illustration, consider this:Top waveform is the drain voltage in a PFC boost, QR mode, so the rise time is short and fall time is gentle. Looks totally normal. Bottom is an H-probe (small loop) placed against the inductor winding. Geometry is RM12 core, I think 3 layers, and, I'm not too interested in quantitative here so the exact wire, turns, etc. doesn't really matter. Suffice it to say: the series-resonant mode is visible as current through the boost diode; it's not visible when probing at the core air gap (which looks like the first waveform instead); and it's visible only in certain orientations, namely the probe laying flat against the middle of the winding (outside cylindrical face). More or less suggesting that the top layer has variable current flow across its width, hence the probe is picking up an overall current loop there. Which is what we would expect for a mode like this, it's ringing against distributed capacitance.(Hmm, 65ns implies 13m wavelength. Not that I was intending to get quantitative here, but just for sake of representation -- there was more like 2m of wire on there. So it seems there's a significant loading effect. I didn't characterize the part with core removed, so I don't know if that's it, but it may be the difference.)In particular here, since the converter has both ends of the inductor AC-shorted together (bypass cap to GND at the input, diode to bypass cap to GND at the output), this ringing is a both-ends-shorted 1/2 wave stub mode, i.e. like a folded dipole or slot antenna, rolled up.When you characterize a component by itself, of course you open one end of that doubly-shorted stub, and measure its impedance; the notch impedance is simply the 1/2 wave delayed reflection of the short at the far end.Now, how this applies to your case, where I gather the terminals are on far sides of a big block of metal (or ferrite or powder), that'll be something else, but there should be contributions from the external loops thus created, plus the internal shorted-stub mode.And this suggests some possible plans. If you can afford the losses, of course: you can perhaps put some resistive, or lossy magnetic, material into the gaps or paths where this leakage flux appears. You can choose a different winding scheme that better distributes the turns, or spaces them out (reducing capacitance, increasing leakage between turns), raising the valley frequency and impedance. You might use a smaller choke in series, with its impedance peak positioned near the other's valley; of course this just splits both peaks into more peaks and valleys, but again depending on losses, or nearby components (i.e. shunt C, preferably R+C), the result might end up better. Perhaps you can end up saving total inductance by using a higher filter order, etc.Tim

author: CaptDon
author_rank: Super Contributor
timestamp: August 07, 2024, 01:31:40 pm
content: Is this a product your company builds? Who's EMI specs does it fail? I would expect a 3MW convertor to have near field EMI problems on the cables exiting the cabinet. What is the 3 phase supply voltage and how wired? Open delta, corner grounded, 4 wire Wye, wild leg? Each style of supply may have a unique way of noise suppression especially noise being pushed into PE.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 07, 2024, 03:04:04 pm
content: I expect FCC Part 18 or analogous (CISPR 11?) standards apply, and near field can be ignored by way of OATS with a fairly generous distance to the antenna; I'm not sure offhand if that also increases the size of the antenna so that "radiated" includes much lower frequencies (perhaps low enough to cover everything of interest), or if it's still done as a two-part with conducted below 30MHz (or whatever cutoff if other) (hence my uncertainty about a LISN).Typical off-the-shelf 3-phase filter modules make their own "EMI neutral" by wiring X caps in a wye, which cancels out AC voltage so that quite generous Y caps can be used without ground leakage problems (assuming okay line-line balance and no dropped phase, maybe not a bad assumption, but something to be aware of depending on what kind of equipment and spec you're working on). OTS modules might not be available/practical here, but the same construction used in them still applies here, even if the inductors become raw cable lengths, and CMCs use the biggest amorphous/nanocrystalline (or even metal stripwound) cores you can muster.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: August 07, 2024, 08:29:01 pm
content: easy>..Add #16 bus to bring terminas to a 1" soacing, use normal RLC/LC bridge.we use Wayne Kerr 3245, GenRad 1689, HP 4275A, HP 4195A + imp test fix ,Photo: 1.2 mHy 200A 5kV CONTINOUS1980s..1990s, made 100s of these and no problems.(special rect HPTZ wire..)Have an absolutely fatastic day!Jon

author: KlausKragelund
author_rank: Regular Contributor
timestamp: August 09, 2024, 12:13:06 am
content: Quote from: T3sl4co1l on August 07, 2024, 03:04:04 pmYes, in this case CISPR11 is used. The standard does allow for usage of a loop antenna at 10m OATS to do the measurement, since LISN that can handle 3000A is hard to design and potentially dangerous.We can add a lot of capacitance for filtering, and we are doing that using Y capacitors. This converter has a flying transformer summing point (PE connection lifted), due to the switching of the converter. But even though we add 1kW of loss due to the capacitor losses, that's in the noise margin for a 3MW converter.