Very high current constant-current source, how? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: May 11, 2015, 09:51:45 pm
content: Let's say you want to pump up an MRI magnet.Or something else that requires not only hundreds of amps (500-1000A), but precise amount of hundreds of amps into an inductive load.How can it be done?I'm thinking of some sort of a switching regulator, naturally.Back when i made a coilgun, i used something like this:It could regulate the current in the coil at up to 200A.Problem is, it only worked on the scale of a few milliseconds, and the diodes were already warm after one shot.And i'm thinking of something that would run for seconds or even minutes.So, the question is - is making a higher current supply simply a matter of better heat dissipation, or are there tricks and alternative topologies to be used?If the former, then diodes are the hot and hard to find problem - anything in that range will dissipate about a KW of heat each, ones over 300-400A are difficult to find, and they can't be paralleled. How to get around that?

author: Marco
author_rank: Super Contributor
timestamp: May 11, 2015, 10:21:14 pm
content: Hockey puck rectifierswhich can handle those currents are easy enough to find. You could use synchronous rectification (diode handles current for a bit, then a parallel IGBT takes over) but it's more complex to drive and probably more expensive ... you're going to need some massive cooling regardless.

author: LaserSteve
author_rank: Super Contributor
timestamp: May 11, 2015, 10:35:05 pm
content: Which magnet are you curious about? The field magnet or the gradient magnets?The MRI Main magnet is charged once by a brute force DC current regulator once it has been cooled. A short link that is not superconducting during charging is across the power supply. There is a heater around the link, and it is then a ~100 Ohm load across the coil. When the proper current level is stored in the magnet, the heaters are switched off. The superconducting shorting link is rapidly chilled, forming a closed loop that contains the current. The charging supply is then disconnected.This is known as a "Persistent Current Magnet" .The supply is ramped during installation over 24 hours to 500-1000 Amperes, the current ramping being very slow to avoid thermal driven quenching. Quenching is bad as you loose all the liquid helium. Its also VERY loud and activates a blow out panel on the roof. Not that I would know, I was only working on a CT less then 200 feet away...The gradient coils are the coils driven by amplifiers. They use a series of stacked output transformers in series driven by water cooled IGBT bridges. Typical might be 450-1000 amps at 750 V. The stacked transformers will be driven by different modes of PWM. The center bridge will be driven by a high frequency PWM for fine tuning and high frequency sweeps, the others will be low frequency transformers for brute force at low frequency.Take a look at Powerex CM600HU24H IGBTs which were typical of a older Gradient Amp.Steve

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2015, 10:45:26 pm
content: You need as low a voltage drop as possible, to maintain field. Dumping it back into the supply every half cycle gets you a good falling dI/dt (as good as rising), but doesn't help any with maintaining the energy -- you're cycling a portion of it (the delta I) in and out of your supply as reactive power all the time!I'd heard of a superconducting UPS; it had a nice big coil inside, charged to the tune of 1000A, and was maintained with an SCR normally. (For this to be effective, it would have to be ~1V forward drop (1kW!) bleeding it down with a time constant of hours or days, suggesting an inductance of ~100H.) When power is needed (to charge the inductor, or to deliver energy back), the SCR is commutated off, and the inductor connected to the supply rails (probably around 350 or 700V DC) momentarily. If a 10A load is required on the DC rails, the voltage can be maintained for a full delta-I of 990A, which is 99.99% of the contained energy.I don't actually know if this was a real product, or just someone's dream. 1kW seems like an awful lot of dissipation to idle with. 100H seems remarkable, considering ITER's massive coils are only ~10H (I forget what exactly, but you can calculate based on the rated current and stored energy specs -- E = 0.5 * L * I^2). This thing wouldn't be much smaller than a room full of lead-acid, and would incur quite a lot of quiescent power (cooling plus losses), not to mention cost.Tim

author: Artlav
author_rank: Frequent Contributor
timestamp: May 11, 2015, 11:06:04 pm
content: Quote from: blueskull on May 11, 2015, 10:05:50 pmThe supply will be a bank of supercaps, they can take as much power as they deliver. So, no chance of overcharging anything.Anyway, an easy thing to forget about - thanks for noting it.Quote from: blueskull on May 11, 2015, 10:05:50 pmUm, so i should add a third switch parallel to the coil (Q2), and the existing ones are the Q1?Not quite follow your idea.Quote from: LaserSteve on May 11, 2015, 10:35:05 pmI've been contemplating making an MRI-type machine.Not human sized, something much smaller and likely for an element that is easier to detect than hydrogen - i'm mostly interested in the technology than in anything practically useful.So, i want to be able to control the current in the main (resistive) magnet for a start.Quote from: T3sl4co1l on May 11, 2015, 10:45:26 pmFair point, that circuit was designed for a coilgun, where fall time was as important as rise time.Actually, now i don't think the energy recovery is even necessary - the resistive losses will dominate any saving achieved.Which makes blueskull's idea all that much obvious (except for the dumping energy back part) - just a switch to charge/top-off the magnet, a switch to maintain the current, and a few protection diodes for the dead time.I.e. something like this?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 11, 2015, 11:59:22 pm
content: MPI supplies can supply that easily.http://www.magnaflux.com/Products/MagneticParticleInspection/Equipment/MobilePowerPacks/tabid/261/Default.aspxDunno how precisely you can set it.

author: Someone
author_rank: Super Contributor
timestamp: May 12, 2015, 01:02:44 am
content: Inductively sum the output of a bank of interleaved flyback regulators? You could have a low turns ratio on the output but distributing the current across the many parallel phases.

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2015, 01:08:19 am
content: I have built previously a 1500A DC, 3phase, SCR controlled bridge for testing traction motors for locomotives(parallel hopkins). It had about 100v max voltage output. I know it's not directly applicable but what is your anticipated maximum output voltage required to overcome DC resistance? We also had a motor driven variac controlled field supply (3 phase but diodes) of about 1500A and 10volts max for the fields. It was very responsive, we had of course a step down transformer after the variac which drove the diodes.

author: langwadt
author_rank: Super Contributor
timestamp: May 12, 2015, 01:28:22 am
content: Quote from: Artlav on May 11, 2015, 09:51:45 pmuse FETs instead of diodes, look up synchronous buck converter

author: langwadt
author_rank: Super Contributor
timestamp: May 12, 2015, 01:31:59 am
content: Quote from: Artlav on May 11, 2015, 11:06:04 pmHydrogen is the easiest onewhy not use permanent magnets? easy to get much higher fields

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2015, 04:31:19 am
content: Man, just the sheer amount of Nd needed to make a magnet that big... (or even more than 1T -- yes, higher can be achieved despite Bmax = 1.5T ish for NdFeB).http://en.wikipedia.org/wiki/Halbach_arrayIt's possible, but man, the weight and expense would probably make such a monstrosity actually uneconomical compared to a superconducting version. That's saying something!Not to mention the tempco. It certainly wouldn't change temperature quickly, but the nonuniformity and 1/f tail would go on forever.Tim

author: TiN
author_rank: Super Contributor
timestamp: May 12, 2015, 04:36:26 am
content: If you don't need high voltages (<3V e.g), alternative way would may be a multiphase buck DC-DC working in current limit mode.But those would not get very precise, likely in range 0.5-1%. You can get 400-600A pretty easily with these when properly designed.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 12, 2015, 11:58:26 am
content: Quote from: blueskull on May 12, 2015, 01:36:47 amIs a neutron star a permanent magnet?

author: langwadt
author_rank: Super Contributor
timestamp: May 12, 2015, 03:55:31 pm
content: Quote from: blueskull on May 12, 2015, 01:36:47 amHalbach array, we makes some ~200mm diameter, 25mm bore. With the highest strength magnetic material they are ~1.7Tbut they stat to degrade about ~50'C, So high temp magnet are better and gets to about 1.5T. It is not high resolution because that isn't what it is used for but you can get high resolution permanent magnets, but they are expensive it takes a lot of tweaking to get them homogeneousnon-superconducting magnets just doesn't add up, high current or high voltage doesn't matter, they are huge, inefficient, need water cooling and or low duty cycles to get any where near 1TH at 1T is ~42.5MHz

author: langwadt
author_rank: Super Contributor
timestamp: May 12, 2015, 04:01:09 pm
content: Quote from: T3sl4co1l on May 12, 2015, 04:31:19 amthe tempco is a challenge, at around 1000ppm/CI'd love to hear where you can get a superconducting magnet (and run it) for a less than x kilo of Neodymium magnets

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2015, 12:34:43 am
content: Such a magnet will be dangerous, especially in a fault condition where excessive currents flow and therefore excessive magnetic fields are generated. Had a fault in a HV supply, 1500V, 3 phase scr, and the 5mH air cored filter inductor weighing 200kg moved several feet, crunching against the power transformer. Also have a friend that is an MRI technician and he has heard a fatalities of other technicians due to the inadvertent introduction of some tool effected by the magnetism.

author: langwadt
author_rank: Super Contributor
timestamp: May 13, 2015, 12:57:55 am
content: Quote from: moffy on May 13, 2015, 12:34:43 amThis happend a few year ago when a mover thought he would just move the magnet out of the wayhttp://ing.dk/sites/ing/files/styles/w1120_media_right/public/images/54693.jpg?itok=F2H-VmyPthat pallet jack is ~65kg, it too a few few weeks to plan how to get it off again

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 13, 2015, 01:19:00 am
content: Quote from: langwadt on May 12, 2015, 04:01:09 pmAny guesses how big an MRI sized 1T Halbach would be?I have no idea what the price comparison actually is... and when you buy an MRI, you're buying a whole hell of a lot more than a magnet, so the cost of a PM alone might not compare, but that's not apples to apples either.Tim

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2015, 01:31:51 am
content: Quote from: langwadt on May 13, 2015, 12:57:55 am