Rectifier for extreme current - Page 1
Forum section: electronics / projects

author: najrao
author_rank: Regular Contributor
timestamp: January 05, 2018, 07:51:04 am
content: At the outset, I must request that any reader of this strange help - needed notice not dismiss it as from a demented soul.I once constructed an a.c. generator which needed excitation of 2000A @8V(dc). I could show great improvement in the cost per kVA and weight. The rectifier needs to rotate along with the rest of the rotor. I used 6kA hockey puck diodes with precious metal contacts, supplied by Semikron, in a multiphase half-wave formation, to minimize drop. There is also a need to overexcite the generator for under a minute a time; this calls for twice the current, and four times the power. In the environment, the diodes can see inverse voltages to some 10 times the nominal dc (8V in case).If one can make a rectifier for even higher current, to 16kA @1V in this case, the generator. design improvement gets to be even more dramatic. Diodes are out as they drop over 500mV, even Schottky's. Sub milliohm mosfets, numbering hundreds would be a poor solution. One needs microohm mosfets, with usable kA rating. I have not consulted manufacturers to see how far they can go. I understand that power mosfets are intrinsically large numbers of small cells internally paralleled, it may be possible to stretch this to very large numbers.To cover larger and smaller generators over a range, I need rectifier with output ranging from under 1kW to over 25, all at ultra low voltage.I invite suggestions.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 05, 2018, 08:12:50 am
content: That's an really interesting challenge. Joining the thread to see if someone comes up with something else than MOSFETs. That was my first thought, but looks a bit impracticable at 16kA.IGBT modules rated to some 100Amps to kA are quite common, never seen MOSFET modules of low voltage and high current rating. I wonder if a module manufacturer like Semikron or Danfoss would make custom MOSTFET modules for your purpose. IMO just using a lot of standard package MOSFETs isn't the way to go here.

author: Jeroen3
author_rank: Super Contributor
timestamp: January 05, 2018, 08:56:03 am
content: We've made some static exciters up to a few hundred amps. Buttwo thousandis a bit high.Care to share any more details? Sounds interesting.

author: Ian.M
author_rank: Super Contributor
timestamp: January 05, 2018, 09:02:11 am
content: From the description above, it sounds like a large brushless alternator of the typical design: i.e. with its exciter (an AC generator) on the same shaft, powering the alternator field coils via a shaft mounted bridge rectifier.It sounds like the design has been optimised for greatest flux density for minimum winding area, by using a copper bar winding with a very small number of turns, possibly only one, so it only has to be insulated from the rotor core, thus minimising the 'waste' space in each slot taken up by the winding insulation, and maximising the copper and iron CSAs.The tradeoff is the need for very high current very low Vf rectifiers that are in low mass packages, suitable for mounting to a spinning shaft without excessive mass in their support assembly.With 8V or less per turn, and modern thin film insulation materials, I'm not convinced that the tradeoff is favourable vs using ten times the number of exciter and field coil turns, and rectifiers with a tenth of the current rating, operating at ten times the voltage.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2018, 12:23:14 pm
content: I would have thought that MOSFETs and an ideal rectifier controller or Schottky diodes would be the way to go. IGBTs would have far too higher voltage drop.How big is this alternator? I've not heard of anything which requires 16kW of power, just for the excitation, but I don't work for a power company.

author: najrao
author_rank: Regular Contributor
timestamp: January 05, 2018, 03:26:47 pm
content: Thanks to each of you.Capt Bullshot, IGBT's have a voltage drop of over 1.5V, which rules them out. Sub milliohm mosfets go down to just below 1milliohm in the hot condition, so each can take up something under 100A even if given a great deal of cooling. The rectifier would need several hundred of these, and result in an awkward assembly even if the costs are accepted. There is also the question of connecting up so many tiny device packages to the high current bus. If only a silicon foundry would repackage a good few dozen chips into a workable module, this can take off. There is also the question of driving the gates at 10+V, to be correctly timed to coincide with the conduction period, but this may not be a problem. Note that the mosfet needs to have as low as 20V Vdss rating, but must have solid connections and extra low thermal resistance. Note also that the parasitic diode will always block (the mosfet can only be used in the conventional current direction). The frequencies are low enough to ignore all switching losses, below 1kHz.Jeroen2, the first prototype was designed for 1875kVA 0.8pf continuous. Static excites are a very different ball game, it would be quite impossible to pass such currents through slip rings.Ian.M, my description of the problem did not elaborate on the machine per se, because I thought readers in this forum may be not familiar with such construction. Besides, I wanted out-of-the-box thinkers not to be put off. The benefits of very high current at low voltage are many, and are not due to the reduction or elimination of insulating materials. Suffice it to say, my prototype was 33% lighter than what it would replace. The grandiose plan was to develop a series, to cover 50 to 3750kVA, as needed in the market.

author: tszaboo
author_rank: Super Contributor
timestamp: January 05, 2018, 03:47:34 pm
content: Weel, there is SOT227 FETs, which go up to 200A. Those are the biggest FETs I've ever worked with. So you would need... 80 of those in parallel. Not cheap. But, you can bolt them down on a heatsink, and connect very wide cables or busburs on them.All these SMD FETs probably end up cooking themselves, if you place a lot of them in a small place. Even if they are below 1 mOhm, the connections and everything else would just heat up the entire assembly too much.

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 05, 2018, 03:52:24 pm
content: You may just have to use some sort of specialized rotary converter. You'd have a transformer step the voltage up to the converter first though.

author: capt bullshot
author_rank: Super Contributor
timestamp: January 05, 2018, 07:04:06 pm
content: Quote from: najrao on January 05, 2018, 03:26:47 pmYes, IGBTs are not applicable here. My thought was to ask one the IGBT module manufacturers to put MOSFET dice into such a module, since they are experienced with high current and low thermal resistance stuff. 1kW or more power dissipation is common with those module packages, so one could easily imagine a 1kA or more module with less than < 1kW losses. I don't think you get this done using discrete MOSFETs.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 05, 2018, 08:41:13 pm
content: Quote from: capt bullshot on January 05, 2018, 07:04:06 pmYou can get MOSFET modules for small traction drive applications (e.g. golf carts).It's kind of a dumb idea, because a module is physically too large for the switching performance a MOSFET is capable of. That is, you're adding so much stray inductance (at least 10nH), yet using devices that are capable of commutating that 10nH+ fast enough to blow the chip (say, 200A in 50ns is an EMF (overshoot) of 40V, which would require 100V devices on a 48V bus). So, you don't see them as much, and you must settle for slow operation.Better is probably packing the thing chock full of TO247 or even TO-220 parts. A few hundred in parallel, secured with clamps (you wouldn't want to use individual screws and hardware here!), gets you there, with the stray inductance distributed and acting in parallel.Tim

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 05, 2018, 10:15:29 pm
content: FETs still sound about as close to what you need as it'll get...You could run these:https://www.infineon.com/cms/en/product/power/mosfet/20v-300v-n-channel-power-mosfet/40v-75v-n-channel-power-mosfet/irl7472l1/at about 200A without too much trouble (but with a hefty heatsink!)

author: najrao
author_rank: Regular Contributor
timestamp: January 06, 2018, 07:42:12 am
content: Tim, I did mention that this needs to be a multiphase rectifier but at low frequency, less than 1kHz. Also, the a.c. source (an on-the-shaft exciter generator) would have considerable source reactance, leading to large overlap during commutation from one phase to the next. Fast switching is not a need here, so lead inductance in nH range would have no perceptible effect.I will research traction type mosfet modules, thank you. I did some sketching of TO 247 and TO220 in the past, but the source pin connection left a lot to be desired. Seems much better to mount just the dies (chips) on to copper work without the intermediate TO plastic. Not too confident I can set this up, need a module assembler, know one or two.

author: oldway
author_rank: Super Contributor
timestamp: January 06, 2018, 11:56:54 am
content: 2000A, it is not at all an extreme current in power electronics.I worked on 12.000A electrolytic rectifiersThe only problem is that the output voltage is low.But this problem also exists with rectifiers for electrolysis.The main point is reliability because dismantling a high power alternator to repair it will cause very high costs.All these Mosfets solutions in parallel must be rejected because they are unreliable.The only valid solution is that of diodes like this one: D5810Nhttps://www.infineon.com/dgdl/Infineon-D5810N-DS-v01_00-en_de.pdf?fileId=db3a304323b87bc20123ffe7cca55c70Maximum voltage drop of 0.92V at 6KA, about 1.840W of losses for 2.000A 8V.

author: najrao
author_rank: Regular Contributor
timestamp: January 06, 2018, 12:33:27 pm
content: Oldway,I cannot disagree with your view on reliability. Nothing like a diode for this. The Semikron diodes I used on the prototype were similar, 0.82V at 6kA. I restricted the load winding design to 2kA because 1. an overexcitation provision is required, 2. the rectifier power loss would be excessive.For higher currents to 16kA, there is just no diode rectifier to keep down the losses. Thus, for 16kA at 1V as the load, the rectifier would have lose 15kW!This concept will work only if the drop can be no more than some 10 percent of the load voltage.I have just sighted data for a smd chip mosfet which claims <30mV drop at 50A and 125C temperature. Hopefully, the Vdss rating will exceed 15V (yet to study). There is the question of reliably paralleling several hundred of these, AND connecting up. Quite far fetched, I suppose.Thanks any way.

author: oldway
author_rank: Super Contributor
timestamp: January 06, 2018, 12:58:25 pm
content: For 16KA, there are many technical solutions using diodes, I will not give you a course in power electronics.And with 16KA, the solution of Mosfets in parallel becomes a true madness.In electronics, there are no miracles .... You want to apply synchronized rectification principles perfectly valid for a SMPS to applications of 2000A or even 16KA, it is completely nonsense.The result will be an unreliable product that will cost fortune the company you work for.In a project, you must choose the right options by prioritizing the most important factors.Here, reliability premium on eficiency.At the limit, you will have to either modify the project to work with a higher voltage and a lower current, or abandon this project to be unsustainable in practice.

author: dmendesf
author_rank: Frequent Contributor
timestamp: January 06, 2018, 01:10:59 pm
content: The design is insane, but let´s add gas to the fire... what about GaNs? This one is 1.3 mOhm @ 40A:https://www.digikey.com/product-detail/en/epc/EPC2023ENGR/917-EPC2023ENGRCT-ND/5247097Even Digikey says "This part is an engineering sample and is not fully tested. This part should only be used for functional evaluation and proof of concept, not for production or end use." but if you´re at the edge of something you must make some tests yourself....

author: oldway
author_rank: Super Contributor
timestamp: January 06, 2018, 02:20:38 pm
content: Quote from: dmendesf on January 06, 2018, 01:10:59 pmOff course, "This 2MVA AC generator is an engineering sample and is not fully tested. This generator should only be used for functional evaluation and proof of concept, not for production or end use." but if you´re at the edge of something you must make some tests yourself....

author: max_torque
author_rank: Super Contributor
timestamp: January 06, 2018, 02:53:21 pm
content: Quote from: najrao on January 05, 2018, 03:26:47 pmreally? P = I^2 x R suggests otherwise does it not? There can't be any significant insulation requirement difference between say 1v and 2v and yet for the same power, the current is halved and the losses quartered?

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2018, 04:54:41 pm
content: Quote from: max_torque on January 06, 2018, 02:53:21 pmYes, arcing can't occur in air, under normal atmospheric conditions, until the voltage exceeds 327V, see Paschen's law, so insulation isn't an issue, until the voltage is much higher.

author: oldway
author_rank: Super Contributor
timestamp: January 06, 2018, 06:35:07 pm
content: Quote from: Hero999 on January 06, 2018, 04:54:41 pmUnder normal atmospheric conditions, breakdown of air is 3KV/mm. 327V is refering at low pressure (10-3 bar if I remember correctly)After ionising of the air, about 25V is enough to maintain the arc.That's the reason why it is possible to weld MMA with a welding transformer of only 50V secondary voltage.

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2018, 06:49:03 pm
content: Quote from: oldway on January 06, 2018, 06:35:07 pmThat's true. Once struck, an arc can be maintained at much lower voltages. I should have been more clear. I was referring to the minimum voltage an arc can spontaneously form in air, at normal atmospheric pressure, at any distance, is 327V. The 3Vk/mm rule is only valid for large gaps. Over small distances, below the mean free path between air molecules, the breakdown voltage actually increases again.https://www.google.co.uk/search?q=Paschen%27s+law&ie=utf-8&oe=utf-8&client=firefox-b&gfe_rd=cr&dcr=0&ei=YBlRWuivG6eZX9GPq8gC

author: Marco
author_rank: Super Contributor
timestamp: January 06, 2018, 07:08:24 pm
content: Forklift inverter modules seem to be closest to what you want, albeit still more than order of magnitude off. Does show commercial use of high numbers of parallel mosfets.

author: oldway
author_rank: Super Contributor
timestamp: January 06, 2018, 07:19:24 pm
content: Quote from: Hero999 on January 06, 2018, 06:49:03 pmAt atmospheric pressure, 327V is only valid for a gap of.....7µm...that's 46.7KV/mm, far higher than 3KV/mm.http://www.academia.edu/9356760/Arc_breakdown_in_air_over_very_small_gap_distances

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2018, 08:05:59 pm
content: Quote from: oldway on January 06, 2018, 07:19:24 pmExactly, like I said, 3kV/mm is only valid for large gaps. Also note that as the gap is decreased further, the breakdown voltage actually increases! That's why the minimum voltage for spontaneously initiating an arc in air is 327V.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 06, 2018, 09:12:50 pm
content: Don't mind oldway -- he'd use tungar bulbs, if they were available in these ratings!Tim

author: RES
author_rank: Regular Contributor
timestamp: January 06, 2018, 10:59:01 pm
content: C&H Technology specializes in high current, high voltage industrial rectifiers with ratings from 10 to 10,000 Amps and 100 to 4,500 Volts. Applications include: welding, alternators, generators, battery chargers, electro-plating, motor drives, and traction markets.http://www.chtechnology.com/rectifiers.html?

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 06, 2018, 11:11:32 pm
content: Quote from: T3sl4co1l on January 06, 2018, 09:12:50 pmLol, I already suggested a rotary rectifier.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 07, 2018, 12:55:36 am
content: For such large currents, wouldn't it be trivial to individually fuse the MOSFETs so one failing shorted will isolate itself?Maybe the better approach would be to make an induction generator with a "small" permanent magnet alternator to supply the power needed to start it? (I'm assuming that to go permanent magnet for the whole thing would be too expensive with the amount of rare earth magnets required.)

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 07, 2018, 01:23:19 am
content: Quote from: NiHaoMike on January 07, 2018, 12:55:36 amThat seems expensive. And also, when they fail, the current would go up on the others, and if too many blow, it would cause an avalanch failure and explode.Yes, you would usually have a seperate exciter generator in this case. But I'm not sure if the OP has already considered that.

author: CopperCone
author_rank: Super Contributor
timestamp: January 07, 2018, 01:51:20 am
content: You would need a fuse monitor circuit, basically each fuse would have AC resistance taken across it with a giant network of gates, so the thing would SCRAM (i think this is appropriate terminology for such power levels) if enough resistors went openeasy enough to implement,i mean, it should be implemented in banks, just bean counters get in the way of such things, maybe, thread starter sounds like he is hell bent on some sick ass shit so maybe it will be implementedyou would just need a heavy bandpass filter so any motor noise etc does not get through (possibly a lockin circuit if its really bad).An analog circuit flags it open, then it gets sent to a big summing junction where enough volts causes the power to trip a contractor (say 25mV per mosfet with a maximum failure of 4)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 07, 2018, 01:54:31 am
content: Quote from: NiHaoMike on January 07, 2018, 12:55:36 amYou're assuming the opposing side MOSFETs are capable of sinking the faulting current required to fuse it.Which, for this many of low resistance FETs, may well be the case; but you'd better have them well matched and shared. Which begs the question, how did the one poor sucker fail shorted in the first place?Best way to design parallel converters: design a small block, that is current limited, or protected, or something current mode in nature. Connect those in parallel. That way you have all the sensing, protection, snubbing, etc. local to each module, and you have no worries about wiring them together. And yes, if one module goes into thermal protection, that'll probably cause a cascade failure, as well it should; just be mindful to prepare for system-level issues like that.Applicable to this case would be, either having enough excess thermal capacity to allow the field winding's stored energy to freewheel through the MOSFET body diodes -- ah, but they won't share evenly -- so you must use the alternative, which is shorting mode commutation, where the default state is on, and when a failure event occurs, you simply short out the supply (which in turn must be a current sourcing inverter) and let the field winding freewheel on its own DC resistance.Incidentally, consider a current doubler configuration: this is a voltage doubler circuit, after series-parallel transformation. Capacitors become inductors (so, uh, you need two filter chokes -- maybe inconvenient where space must be conserved), and the source's voltage output can be doubled as a result -- less copper needed in the rotating transformer. The choke-input filtering configuration is a natural complement to a shorting-mode rectifier, and the inductive load.Tim

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 03:29:14 am
content: Let's simply remember that the rectifier circuit and its possible electronic circuit will be integrated into the rotor of the alternator and will have to operate under extreme conditions of high temperature, vibration, and centrifugal force.The slightest defect will cause very high costs since it will be necessary to dismantle the generator ac to repair the electronic circuit.Without forgetting the costs (which can be astronomical) of production losses that can result from the important downtime for repair of the ac generator.I hope you are ready to assume the financial consequences of the unreliability of your electronic "adventures"!I have known companies that have almost gone bankrupt because of a minor change .... a simple change of supplier or quality of a plastic material ....

author: tszaboo
author_rank: Super Contributor
timestamp: January 07, 2018, 11:03:00 am
content: You assume stuff, which wasn't mentioned. Actually, it looks more like a research project. Also, your solution is about 60% efficient, so totally unsuitable.I've built parallel mosfets in thousands of boards, without individual fusing them. They never failed, because the engineering part was good.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 01:40:55 pm
content: Quote from: NANDBlog on January 07, 2018, 11:03:00 amI did not assumed nothing that was not mentioned: the proof is what najrao wrote in his first post:QuoteYes, this is a research project.....Losses of the excitation circuit of an ac generator is only a very low part of the total losses of an ac generator, so low eficiency of excitation circuit has very low influency on the eficiency of the generator.You can't deny fondamental rule of reliability : reliability decreases with the number of components that are used....And diodes are the most reliable semi-conductors.....diodes and SCR's are the only semi-conductors you can truly protect against short circuits with Ultra fast fuses by selecting fuses with I²dt at least 30% lesser than the I²dt of the semi-conductor.60% efficient ? 0.92V drop on the diode for an 8V output voltage, that's 89.7% eficiency....

author: Siwastaja
author_rank: Super Contributor
timestamp: January 07, 2018, 02:02:50 pm
content: Quote from: oldway on January 07, 2018, 01:40:55 pmYes, you can deny it, I'll do it right here:I deny such an oversimplistic rule of thumb. It serves a purpose when teaching total beginners, but real engineering requires real analysis.Today's electronics easily uses thousands of components completely reliably every day.Unit reliability of a single component varies so wildly and can get so freaking close to 1 that other mechanisms dominate over the simple component count, almost always.MOSFETs are fairly easy to parallel; this is well-known. Physical construction and cooling is a real challenge, but it can very well be the right direction to go to, in 2018. Note that decreasing losses makes cooling considerably easier. With today's prices, it often makes more sense to use more silicon to reduce losses, instead of more heatsink aluminium. In many cases, energy savings are only a side effect.We are not in 1970's anymore.These are things that are fairly easy to model, test and measure. MOSFETs are extremely reliable when driven within specs. Also, all kind of control electronics have become cheap, and thanks to integration levels, small. This allows you to design basic building blocks that protect themselves (assuring the MOSFETs are driven within specs even in corner cases or changing environmental conditions), as Tim explained above. It's not black magic anymore - nor expensive.

author: tszaboo
author_rank: Super Contributor
timestamp: January 07, 2018, 02:53:15 pm
content: Quote from: oldway on January 07, 2018, 01:40:55 pmYes, I can deny it, because it is not true. A chinese power supply can blow its single power transistor within it's limits on day 1, and a modern GPU can work with 21.1 billion transistors for decades.And if you are soo good at reading the first post, read the 16KA@1V also. In fact, I was very generous with the 60% value.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 03:24:18 pm
content: You compare apples with pears.....Compare two circuits of different power but using Mosfet in the same conditions. One with only one MOSFET and another with 20 Mosfets in parallel.....The two circuits have the same reliability ?Look how a modern ac generator is made....I did not copy the picture because I do'nt know if it is copyright or not....http://apachehelicopter.tpub.com/TM-1-1520-238-T-6/css/TM-1-1520-238-T-6_103.htmThe subject of the topic is to replace the diode assembly by a Mosfet rectifier.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 03:51:54 pm
content: QuoteAn ac generator which needed an excitation of 2000A@8V is an ac generator of at least 150KVA....not the kind of equipment you can make at home !!! for sure not an ac generator constructed by najrao himself.....For me, it is clear that it is an industrial project.....development of a new product....

author: max_torque
author_rank: Super Contributor
timestamp: January 07, 2018, 06:50:25 pm
content: Quote from: oldway on January 07, 2018, 03:51:54 pmi'd rather hope it was 150 MVA! 150 KVA is a trivial amount of power..... (relatively speaking. a modern PM eMachine can output 150KVA from just around 15kg of active material....)

author: Siwastaja
author_rank: Super Contributor
timestamp: January 07, 2018, 07:47:04 pm
content: Quote from: oldway on January 07, 2018, 03:24:18 pmYes, properly designed, the reliability of the MOSFET section is so darn close to 100% that all the other factors dominate. The reliability difference between the design choice to parallel or not is lost in the "noise" - there are so many aspects to the reliability.What I mean, if you can't design a system with 20 paralleled MOSFETs in such a demanding setup, I bet you can't do it with 1 big MOSFET either. Of course, itmayhappen that somehow the designer succeeds to design a barely functioning MOSFET section that is on the brink of failing all the time, with MTBF of, say, 1000 hours, which replicated 20 times would become apparent more quickly. But this marginal operation is unlikely and easy to avoid by knowing what you are doing and verifying the design margins.MOSFET itself, as a component, is fairly forgiving what comes to paralleling. Still, designing self-protecting blocks like Tim suggested is highly recommended.This is, really, quite some basic stuff in today's power electronics. I bet many people posting in this thread have designed such systems and know what they are talking about.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 08:03:21 pm
content: No need to make any comment about what you wrote....

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 08:39:54 pm
content: Quote from: max_torque on January 07, 2018, 06:50:25 pmAn a. c. generator 180KVA Leroy Somer model LSA46.2 M3 weight570 kg.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 07, 2018, 08:49:23 pm
content: And an elephant weighs 6000 kg.

author: JoeN
author_rank: Frequent Contributor
timestamp: January 07, 2018, 09:20:32 pm
content: Another old technology. Too bad this appears to only work with high voltages and will probably give you a sunburn as a side effect.

author: oldway
author_rank: Super Contributor
timestamp: January 07, 2018, 09:24:09 pm
content: The diodes support in an a.c generator.This is mounted inside the generator and rotates with the rotor.The internal temperature is about 105°C in full load.

author: najrao
author_rank: Regular Contributor
timestamp: January 08, 2018, 04:50:02 am
content: As the OP in this case, it is about time I get back to restore a semblance of order.Thanks to everybody, my purpose of letting out-of-the-box thinking flourish has taken off in good measure. A lot of straying from the topic, yes, but that is a good way to garner new ideas.I was careful to define my need well in the starting post: how to make a better rotating rectifier for extreme currents. I avoided the term alternator then just to keep the focus, not to hold back information.Yes, it was a research project in a then big manufacturing company. An actual 1875kVA prototype was made, largely to learn what it takes to practise the concept. One observation was that there is even more to be gained by increasing the current (and reducing the voltage). Hence the posting.It is not that I am unaware of the requirement of reliability, and hence simplicity. Diodes are best, no doubt. Even these were fused in earlier days, and an external non-contact diode failure annunciator offered. A single diode failing would let operation continue at full rated load without interruption. Such is the demand on reliability.But then, as elegantly argued already by several contributors, you do not give up a prospect of better engineering by a new idea just because it is a little more complicated, or costly at the moment. Simplicity is not the only way. Ask any inventor. Would you have the computer or IC engine otherwise?My take-aways from the discussion so far are:Mosfets are about the only devices that can cater to 1V 16kA.There are enormous complications in using currently available devices in such huge numbers - connecting up if from die level, providing redundancy and clearing faulty chips, designing gate drive circuit in the face of multiphase current overlap, heatsinking the inevitable kW, and making a robust symmetrical assembly to rotate at 2160 rpm, -----. Till these are resolved, any such new generator has to wait.If there is any information about this project required to make more contributions, please ask away.Thanks to all once again; it was a rewarding exercise.

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 09:15:38 am
content: Quote from: najrao on January 08, 2018, 04:50:02 amThank you for confirming the accuracy of almost all my posts ....Indeed, anyone who knows an a.c. generator can only conclude that:- it's an industrial research project- that you have not built yourself this generator.- that reliability is essential and the most important factor.- that the solution diodes is the best and the only possible currently.- that this project is not feasible in the current state of technology.The only thing that surprises me is that you are hoping for a lot of progress on the mosfets, especially with regard to their power handling capacity, but you do not envisage the possibility of progress in the area of diodes .... for example, diodes with very low voltage drop.However, you have forgotten an important information: to specify the temperature at which this rectifier will operate.I quoted 105 ° C, it will be in all cases higher than 85 ° C.

author: IanB
author_rank: Super Contributor
timestamp: January 08, 2018, 09:25:48 am
content: Quote from: najrao on January 08, 2018, 04:50:02 amOne thing I am perhaps not following is why this is the case? Since magnetic flux is proportional to current per turn times number of turns, I do not follow why you cannot increase the turns and reduce the current, leading to a higher voltage and lower current with the same magnetic flux produced. This would lead to an optimum trade-off between current and voltage, turns and current per turn, where the rectification can be handled by available technology.What are the characteristics of the system that drive the optimum to the extreme of very high current and very low voltage?

author: filssavi
author_rank: Frequent Contributor
timestamp: January 08, 2018, 10:02:34 am
content: Quote from: oldway on January 08, 2018, 09:15:38 amThe voltage drop in MOSFETS is determined only by channel resistivity and so by throwing more silicon at it you can achieve almost whatever you want, provided sufficient heat extraction.on the other end the forward drop in diodes (be it schottky or normal pn ones) is limited by the physics of the device and as such the only way to decrease it significantly would be to introduce a whole new diode structure.now given that in both cases extremely disruptive innovations are unlikely (also because of where the power electronics market is going) but progress in the area of diodes will be incremental at best, without the introduction of a whole new class of devices (highly unlikely)

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 10:04:10 am
content: @ IanBIn my opinion, the goal is primarily to reduce the manufacturing costs of the rotor (inductor) by using a technology similar to that of squirrel cages of asynchronous motors.The squirrel cage is made by injection of aluminum alloy into notches made in the rotor.Here, the goal is to form an excitation winding of 1 or 2 turns made by aluminum injection into the rotor.Due to the large difference in conductivity between iron and aluminum, it is not necessary to insulate aluminum from iron.EDIT: of course, it is 1 or 2 turns /pole....The prototype was a 4 poles ac generator

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 10:11:23 am
content: Quote from: filssavi on January 08, 2018, 10:02:34 amNew technology has been recently discovered for fast diodes (Silicon Carbide Diodes) ...Why not another one for low voltage, high current diodes ?

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2018, 10:56:49 am
content: Quote from: oldway on January 08, 2018, 10:04:10 amThe downside to aluminium is it's less efficient than copper, as it has a higher resistance. I believe that newer motors and alternators have copper in the rotor, to comply with EU energy saving directives.I would have thought, that with a large alternator, made in small volumes, reducing manufacturing costs, is of less importance than higher efficiency.

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 11:10:12 am
content: Quote from: Hero999 on January 08, 2018, 10:56:49 amOn the contrary, aluminum is increasingly used for power transmission lines and even for transformers.The main reason is that aluminum resources (bauxite) are much larger than those of copper, and aluminium is cheaper than copper.The resistivity of aluminum is about twice that of copper, it is enough to double the section to give the same resistance.EDIT: There is also a detail you have not paid attention to ....najrao wrote: "making a robust symmetrical assembly to rotate at 2160 rpm"2160 rpm is the overspeed of an alternator 1800 rpm 4 poles 60Hz ..... so certainly not intended for the market EU 50Hz.In my opinion, if I had to guess where this prototype is, I would say Brazil .....And if that's right, the big manufacturer that no longer exists would be the firm Negrini ..... but of course, there, I can not say anything for sure, these are just riddles on my part ....

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 08, 2018, 12:02:30 pm
content: Quote from: oldway on January 08, 2018, 10:11:23 amPlenty of low barrier schottky out there -- still, none can guarantee anything close to what MOSFETs can do.Examples:https://assets.nexperia.com/documents/data-sheet/PMEG2020EPK.pdfhttp://www.vishay.com/docs/94151/vs-19tq015p.pdfStill need a few hundred in parallel, and the leakage is much worse.I'm not aware of any fundamental technology that can improve upon this. You can make semiconductor junctions in lower bandgap materials, like germanium, but the leakage is about as bad, and more importantly, the maximum operating temperature is the pits. High bandgap is the wrong direction, even for low schottky barriers AFAIK (no one makes them so I don't know, but wide bandgap is useful for high voltages, hence the SiC parts).Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: January 08, 2018, 12:17:33 pm
content: Making something custom and fancy is all great. But the customer won't be able to get spare parts quickly or technicians capable of diagnosing and repair when it's broken.I see this all too often from some machine that used some weird method from a vendor that is gone, but made something completely shit to repair with custom wound transformers, special made varistors and custom potted diode rings.Customer wants repair in hours, but does take days. Customer down for a week. But at least it was cheap to buy!

author: najrao
author_rank: Regular Contributor
timestamp: January 08, 2018, 12:53:15 pm
content: Old way, I respect your opinions a lot, but you would certainly let me take my own decision in the matter of pursuing this further to a logical end. This entails higher currents.You are right about everything, except: I did build this prototype, with the 8V 2kA winding. It had many issues to address, and the testing was incomplete. I wrote 2160rpm simply because any manufacturer would do the design to cater to 50 and 60 Hz without much change; indeed we designed to cater to both on the same stock machine, taking advantage of the higher standard voltage (460 or 480V) at 60 Hz. So your conclusion that my business vests in 60Hz territory has little basis. And I did not use aluminium. The rectifier heatsink/s remain quite cool, say below 80C in normal working, but can get much hotter at up to 120C just after a severe overload; a rotating rectifier needs to only so big even dissipating kW, because of very high self-generated air velocity.Tim, the best Schottky's still drop 400 to 600mV, so you cannot go down to 1V on the load. They are only just a little better than high conductance regular junctions, @800 to 900. The latter are available in single packs to at least 6kA, making them much more attractive.Ian B, you would need to get into much detail of design to understand why the load winding would benefit by an extra high current at low voltage.

author: filssavi
author_rank: Frequent Contributor
timestamp: January 08, 2018, 01:04:17 pm
content: Quote from: oldway on January 08, 2018, 10:11:23 amSilicon Carbide is not a new technology by any stretch of the immagination, it is a different material, so the internal device structure and manifacturing process might be different, leading to better dynamic properties, but it still is a schottky barrier diode, and as such the fundamental principle of operation is the same.

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 01:22:28 pm
content: Quote from: najrao on January 08, 2018, 12:53:15 pmOn a forum, if you want to avoid the wrong answers or off- topic, whoever opens the topic should give sufficient information.Here, you do not give anything,not even a photo of the generator or the rotor that would allow us to realize the available space and the possibility of adapting rectifier modules.As for the riddles about where the prototype is, it's just a joke as indicated byOn this forum, we are all volunteers, You don't have to take your own decision in the matter of pursuing this further to a logical end, we are doing our best, so stay in your place because there is a problem in this topic: it is an industrial research project and you use volunteers for free in a project aiming at a financial profit ....That's not fair at all.Finally, when you say you built yourself a 1.8MVA alternator, let me tell you that it is not credible ....You need industrials means to do this, if only because of the weight.Where are the proofs (photos) of your statements?

author: tszaboo
author_rank: Super Contributor
timestamp: January 08, 2018, 02:10:16 pm
content: Quote from: oldway on January 08, 2018, 09:15:38 amSomeone has trouble reading...I shall call these write only forum members.

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 02:27:43 pm
content: Yes, indeed, and you must not be that lazy reading only one lineMosfet is simply an impossible solution actually.How much MOSfet in parallel do you need for rectifying 16KA with an ambiente temperature up to 120°C ?Quote from: najrao on January 08, 2018, 04:50:02 am

author: Zero999
author_rank: Super Contributor
timestamp: January 08, 2018, 03:23:35 pm
content: Quote from: oldway on January 08, 2018, 11:10:12 amOverhead lines are the only place where it works out more economical to use aluminium, rather than copper, because it's the most conductive material, given its weight.In the case of transformers and motors, it works out cheaper to use copper, even though it's more expensive than aluminium, because the greater efficiency gained, saves enough energy to cover the extra cost of the copper, in a fairly short space of time.As far as the EU rules, they don't stipulate that copper shall be used in motors, but in order to comply with the efficiency requirements, copper needs to be used rather than aluminium.https://ec.europa.eu/energy/sites/ener/files/documents/sec_2009_1013.pdfhttps://ec.europa.eu/energy/sites/ener/files/documents/20141211_GuidelinesElectricMotors%20cover.pdf

author: oldway
author_rank: Super Contributor
timestamp: January 08, 2018, 05:57:14 pm
content: Aluminium used in asynchrone motors.

author: Someone
author_rank: Super Contributor
timestamp: January 08, 2018, 09:38:49 pm
content: Quote from: Hero999 on January 08, 2018, 03:23:35 pmThere are good reasons to continue using copper is space constrained applications such as motors and transformers, but aluminium has and is used widely for bus bars in power plants, switchyards, and substations.

author: najrao
author_rank: Regular Contributor
timestamp: January 09, 2018, 02:08:19 am
content: Oldway, when I said "I built this generator ---", I meant "I was the principal engineer" behind its design. It was built in a large factory. Since I no longer work with that organization, I consider it improper to publish data or photograph. This company is unlikely to resuscitate my project.I do not have a business interest in seeking the views of forum members as you allege. With one foot in the grave and the other dragging towards it, I have neither the need nor the ability to set this up into a business for profit. For years now, I have wondered of what could have been if this project had proceeded, so you will forgive my enquiry in this forum.

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2018, 07:08:30 am
content: Quote from: najrao on January 09, 2018, 02:08:19 amYou should have understood that telling a story that does not hold water on a forum like EEVblog where there are many engineers of great skill could not work ....So, you are you looking to find out if your project is viable or not.I think the simple problem of rectifying 16KA is only a very small part of the answer.To analyze the viability of a project, it is necessary to be able to compare the advantages and disadvantages.The company where the prototype had been made had much more information than us to judge it .... And she abandoned this project .... I think everything is said and they answered your question, even if the answer displeases you.In the life of an engineer, it happens that some projects do not succeed, everyone has the right to make errors.At least, they made o prototype, that's the proof they give your project a chance to turn a comercial product.Do not make it a personal problem, I'm sure in your career you have had many successes that more than make up for that disappointment.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2018, 09:16:48 am
content: Quote from: oldway on January 09, 2018, 07:08:30 amAre you denying that anyone working on MVA scale systems would show up on EEVblog? Not sure how to parse that...Tim

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2018, 09:32:19 am
content: Quote from: T3sl4co1l on January 09, 2018, 09:16:48 amI am denying the possibility to built a 1.8MVA ac generator without industrial means. najrao himself recognized that I am right:He did not built this generator himself.QuoteEDIT: And all these explanations he is given now? Why not tell the truth about the situation from the beginning ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 09, 2018, 11:33:45 am
content: Quote from: oldway on January 09, 2018, 09:32:19 amYup, that's how English works. If you're involved in something, you get to say you did it. This works whether you're the financier (paying a contractor to build a house --> "we built this house"), manager, lead or just a worker.That's why movies have credits rolls, and they'relong. Everyone involvedmadethat movie.If there were some sort of prohibition on saying you were involved in a thing... resumes would be a lot more boring, and a lot less useful.Tim

author: najrao
author_rank: Regular Contributor
timestamp: January 09, 2018, 12:19:05 pm
content: Oldway and Tim, I am NOT disappointed that my company did not carry on with this work. In fact, they did too, but the staff I left behind could not add much value thereafter. I left when the testing was just started, and I got to know the remaining issues only to an extent. In the next couple of years, they faced severe financial problems, and R&D would be neglected altogether.I worked there for 30 years, starting at the bottom and had built up a.c. generator and dc machines business to be its biggest and most profitable from what was -- well, scratch. I had received my rewards all the way and had nothing to sneeze at. But we all get old, and my company had a strict age policy. At the end, I had moved on, and oversaw all technology development in the company.I am forced to say all this, because Oldway is unnecessarily sceptical of my basic competence. So much so, he even bet I did not make the prototype at all! I want to assure you I would know to evaluate viability of this concept. We did not spend literally tens of thousands on the tooling for nothing. That more work is needed is not proof of failure.If you still need the last word, proceed please.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2018, 12:25:09 pm
content: We'd be more interested in what factors were driving your design towards low turn count field coils carrying massive currents rather than getting the same number of Ampere Turns with a higher turn count (and field voltage) at a lower current.

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2018, 01:08:40 pm
content: Ok, Tim, thank you for these explanations, my mother language is French, I also speak fluently Portuguese, but for English and Dutch, my knowledge is insufficient to understand all the finesse and do without the help of google translate.I am also an engineer (retired) and I can not conceive that another engineer asks a technical question without providing the essential elements to the answer: the technical specifications.It's so basic that I feel like Najrao is trying to manipulate us to get the answer he wants.For exemple, the maximum ambient temperature at which the rectifier will have to operate is absolutely essential, the maximum dimensions also ... No information on the MTBF is provided .... Does an engineer not know that correct data is needed to solve an equation ?Why do so many mysteries about an abandoned project that has become worthless?

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2018, 01:17:45 pm
content: Quote from: oldway on January 08, 2018, 05:57:14 pmYes, aluminium has been used in the rotor of induction motors for years. The EU wants it to be replaced with copper, to save electricity and thus CO2emissions.Quote from: Ian.M on January 09, 2018, 12:25:09 pmMe too. I think most people here, would go for as higher voltage as possible, limited by the insulation materials used.

author: najrao
author_rank: Regular Contributor
timestamp: January 09, 2018, 01:39:18 pm
content: I am nobody to manipulate esteemed members of this forum, I wouldn't try to. All I wanted was to see how others look at the problem of extreme current rectification. There was a lot of digression, necessary I thought for promoting free thinking.I did say in a previous post that the rectifier needs to operate with heat sink temperature of about 80C normally, and at up to 120C at the end of a severe overload. The dimensions are dictated by the heatsink and connections, and at 2kA were not a constraint. At higher current, space MAY be a problem, but we must cross the bridge when we get there. Alas, I have committed the grave sin of not specifying an expected MTBF, except that we talked of reliability ad nauseam. I still don't know any better.I was NOT seeking a detailed design from Oldway or anybody else. All members of this forum are honourable men, and I hold them in high esteem. Please forgive me for my excesses if any.

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2018, 01:44:29 pm
content: Quote from: Hero999 on January 09, 2018, 01:17:45 pmHere the reason why the higher resistivity of aluminium is needed for the squirrel cage.http://www.electricaleasy.com/2014/02/double-squirrel-cage-motor.htmlCosts of fabrication are also highly reduced by injection of aluminium.The same resistance (and the same losses) can be achieved with aluminium by multiplying the section by two

author: Zero999
author_rank: Super Contributor
timestamp: January 09, 2018, 01:53:41 pm
content: Quote from: oldway on January 09, 2018, 01:44:29 pmThat's not the case for a motor driven by an inverter.QuoteYes, that is true, although they will be offset by a reduction in power consumption.EDIT:That might change. Newer manufacturing methods have been developed, which could see copper rotors more widely used in induction motors.http://www.machinedesign.com/news/industry-could-take-shine-copper-rotor-motorshttp://www.kitraind.com/txt.php?pgid=1131QuoteWhich will result in a smaller core area or a larger motor, for the same power rating. You can't change the fact that using a higher resistivity material, will reduce the efficiency of a motor, with everything else remaining equal.

author: oldway
author_rank: Super Contributor
timestamp: January 09, 2018, 02:35:53 pm
content: A small difference of the core area does not affect the induction nor the motor characteristics significantly because of the existing gap between rotor and stator ...I do not continue to discuss this because it's off topic ... .Please, open another topic if you want to develop this subject.

author: magicsmoke
author_rank: Regular Contributor
timestamp: January 10, 2018, 05:51:28 am
content: I am also curious about the design factors pushing to very low voltages and high currents for the field winding. It isn't obvious to me from first principles what advantage that would provide at least from a magnetic perspective. The ampere turns of the field winding are sized on the magnetizing path of the field excitation and the overload necessary. A little savings in insulation might be achievable at low voltages and high currents but a big gain isn't obvious to me. The idea of casting the field winding is an interesting one. Najrao are you able to provide any more hints about the design factors?

author: oldway
author_rank: Super Contributor
timestamp: January 10, 2018, 09:23:49 am
content: Even for the project of the rectifier, it would require much more information .... it is a rectifier multi-phase .... but how many phases? 3, 6?What is the diagram of the auxiliary generator? probably a star circuit but we are not sure of it.With an ambient temperature of max 120 ° C, there is not much room for cooling. (Mosfets junction temperature of max 150 ° C or 175 ° C according to models)The current is not clearly defined .... 2KA or 16KA? It makes a huge difference ....We could first look for what model of Mosfet would be usable under these conditions and how much should be put in parallel.At first glance, a 8V 2KA would not be that difficult to design....Greatest problem would be to reach high reliability at such high temperature (120°C max)

author: oldway
author_rank: Super Contributor
timestamp: January 10, 2018, 10:04:36 am
content: QuoteThis idea of aluminium casting the field windings is mine, but Najrao said it is not right, he used copper.

author: najrao
author_rank: Regular Contributor
timestamp: January 13, 2018, 04:02:32 am
content: Magicsmoke, please understand that I am not authorized to describe how the generator benefits by this extreme current field winding; though I was the author of it in the first place, I believe the IP vests with the company I did this work at. Suffice it to say, we could show a 30% reduction of overall weight, even with the 8kA.Old way, I had used just three phases for 8kA. I said "multiphase" just to keep the option open for more phases, as may be appropriate, or even inevitable, for higher current. Note that, given the overlap, the current is always shared by at least two phases, even for three phases. The exciter was star connected with the neutral used as the negative dc terminal.If you chose to read more carefully, I only said that the heatsinks operate at up to 80/120C, This is a very different kettle of fish from ambient temperature of ditto. The cooling air temperature would remain below 60C, and self generated air velocities are very high indeed.From the beginning, I have said that 2kA was the choice for the prototype because more could not be rectified. The 16kA was a target for further escalation of the concept. Indeed bigger machines would benefit by even more, and I had mentioned 100kA if feasible.I do NOT seek a detailed mechanical design of the rectifier.