That's a lot of back EMF

[Brainonska511]

Thanks for the extensive explanation and link. That has helped my improve my understanding a lot.

I got mixed up. I incorrectly thought that magnetic shielding was ONLY possible, with moving/changing magnetic fields. But of course magnetic patterns can be shielded, by guiding the magnetic flux through suitable materials.

Worse still. I would have expected it to be the Physics department. So I am feeling even more foolish, as it is the Chemistry department.

I was initially confused by the mention of 600 Megahertz. Because I thought it was low frequency, for the magnet. But I assume the extra coil(s), which allow high frequency modulation of the magnetic field, are what the 600 MHz are talking about. Presumably the 600 MHz is the "excitation" signal (Not sure of proper terminology, as I am NOT an MRI guru). Which makes the radio signals, become emitted from the sample(s), as they fall back to where the electrons were originally orbiting from.

So because the magnetic fields are reasonably shielded, the electronics (and cabling), are relatively straight forward. Except as you go inside the main part of the unit, which then has a huge magnetic field.

Before this thread, I thought/assumed that the huge magnet (of an MRI), was done by using HUGE permanent magnets, such as Neodymium Magnets.
I never realized it was done, by creating huge electro-magnets, and super-conductors.

NMR operation and MRI development are used in all manner of departments - chemistry, biophysics/biochemistry, physics (or any number of other subdisciplines). I myself am in a chemistry program and department and we do biophysics research.

The frequency listed is the Larmor frequency of the proton (1H) at that particular magnetic field. A lot of chemists will refer to a magnet size simply by the 1H Larmor frequency. It's the frequency at which protons will precess around the static magnetic field. But you don't have to apply a MHz field with your radio pulses. When you apply a radio field, this new field will create a smaller magnetic field, causing them to precess differently and coherently. You then record the radio signal as it relaxes back to its equilibrium, non-coherent state.

As for inside the magnet - there are still some metal things inside, like the probe, which is around your sample. If you've ever had an MRI done, the probe (the place where the RF signals are sent from and captured with) is the device they strap around your head or place on your chest. For an NMR, the probe is typically a long, cylindrical piece of metal with some wires and capacitors inside, so that the circuits can be tuned to the appropriate frequency. But for a lot of it, they use non-magnetic metals, so it isn't a problem putting it in and out of the magnet.

I don't think they can get very high fields with permanent magnets yet. But one of the bigger concerns, beyond a high field, is to have a very homogeneous field where your sample will sit. This is a lot harder to do with MRIs, because the bore is much larger than an NMR.

 

[SOFTengCOMPelec]

NMR operation and MRI development are used in all manner of departments - chemistry, biophysics/biochemistry, physics (or any number of other subdisciplines). I myself am in a chemistry program and department and we do biophysics research.

The frequency listed is the Larmor frequency of the proton (1H) at that particular magnetic field. A lot of chemists will refer to a magnet size simply by the 1H Larmor frequency. It's the frequency at which protons will precess around the static magnetic field. But you don't have to apply a MHz field with your radio pulses. When you apply a radio field, this new field will create a smaller magnetic field, causing them to precess differently and coherently. You then record the radio signal as it relaxes back to its equilibrium, non-coherent state.

As for inside the magnet - there are still some metal things inside, like the probe, which is around your sample. If you've ever had an MRI done, the probe (the place where the RF signals are sent from and captured with) is the device they strap around your head or place on your chest. For an NMR, the probe is typically a long, cylindrical piece of metal with some wires and capacitors inside, so that the circuits can be tuned to the appropriate frequency. But for a lot of it, they use non-magnetic metals, so it isn't a problem putting it in and out of the magnet.

I don't think they can get very high fields with permanent magnets yet. But one of the bigger concerns, beyond a high field, is to have a very homogeneous field where your sample will sit. This is a lot harder to do with MRIs, because the bore is much larger than an NMR.

That makes a lot of sense.

Needing much lower frequency (NOT MHz) radio transmitting coils, and receiving coils, is much better.

(After looking up your Larmor (frequency of the proton (1H)) stuff, I also ) discovered that the temperature is down to -271 C. So NOT quite, room temperature super-conductors, yet!
So only a couple of degrees or so, from Absolute Zero!

I think permanent magnets, are a little bit unstable (field varies with temperature and age), which also would stop them being easily used, in this application. I've seen ones, powerful enough (in a museum or similar) to dramatically slow down metal objects vs gravity.

Thanks for all the explanations (you and others, and the OP). I have learnt a lot. MRI scanners, are really fascinating. A sort of real life "Star Trek" device/phenomenon.

 

[BUTCH1]

Just out of curiosity, what exactly are you trying to build/test?.

 

[john5677]

That is a lot of energy.
If i would take the 5kWh :
If i am not mistaken, that is 5000 * 3600000 = 18GJ.
W= 1/2 *L*(I*I)

If the coil would be for example 100H that would be a current flowing of
18.973 A., almost 19kA.
I have to be making a calculation error. That cannot be right.
But then again, I do not know the inductance and it is super conducting.

And does the PSU to charge the coil really have to be able to deliver that current ? That would be a massive psu, with lot of diodes in parallel.

EDIT :
How does current density work out for superconducting wire ?
How thick is the wire of such a coil ?
Can it really be that high ?

5kWh is equal to 18MJ. 18GJ is 1000x larger.

 

[Mark R]

I think permanent magnets, are a little bit unstable (field varies with temperature and age), which also would stop them being easily used, in this application. I've seen ones, powerful enough (in a museum or similar) to dramatically slow down metal objects vs gravity.

They can be used. In fact, I have one myself, in addition to my superconducting ones. The main problem with permanent magnet MRIs is the low field strength (typically 0.5T and lower) and homogeneity (variation in field strength across the magnet bore) resulting in poor image quality and very long scan times compared with superconducting magnets. Certain high end functions are not feasible at low field (e.g. NMR spectroscopy).

Pic

Some other issues (but very minor in reality) are:
Unstable magnetic field (very sensitive to temperature drifts - require constant power for magnet heaters and HVAC, to ensure that magnet temperature is stable). Not that big a deal. After a power failure, the scanner will work, but will need to run a recalibration sequence every 10 minutes or so, for the first 8 hours, until the magnet temperature stabilises.

Huge weight (25-50 tons) and size - requires special building construction, reinforced floors and building design which permits installation/decomissionning . This is only really an issue for whole body MRI, and it's just something to think about during building works (but may preclude installation above ground level).

The advantages are much lower cost ($300-400k compared with $2-3 million), much lower energy costs, often much wider bore or "open" design (less claustrophobic and better for in-surgery use).

In practice, novel designs of superconducting magnets with open designs or wider bores, and the overwhelming advantage of the higher field strength between permanent magnets and superconducting magnets is making permanent magnet MRI almost completely obsolete.

 

[William Gaatjes]

5kWh is equal to 18MJ. 18GJ is 1000x larger.

Yes your right. 5 kWh. I did a multiplication of 1000 by accident.

5 * 3600000= 18MJ.
If i take the same 100H inductance, that makes 600A. That sounds more reasonable. If i did the calculations correct.
Even superconductors stop superconducting if too much current is forced to flow through the super conductor. Has to do with the current density.

 

[SOFTengCOMPelec]

They can be used. In fact, I have one myself, in addition to my superconducting ones. The main problem with permanent magnet MRIs is the low field strength (typically 0.5T and lower) and homogeneity (variation in field strength across the magnet bore) resulting in poor image quality and very long scan times compared with superconducting magnets. Certain high end functions are not feasible at low field (e.g. NMR spectroscopy).

Pic

Some other issues (but very minor in reality) are:
Unstable magnetic field (very sensitive to temperature drifts - require constant power for magnet heaters and HVAC, to ensure that magnet temperature is stable). Not that big a deal. After a power failure, the scanner will work, but will need to run a recalibration sequence every 10 minutes or so, for the first 8 hours, until the magnet temperature stabilises.

Huge weight (25-50 tons) and size - requires special building construction, reinforced floors and building design which permits installation/decomissionning . This is only really an issue for whole body MRI, and it's just something to think about during building works (but may preclude installation above ground level).

The advantages are much lower cost ($300-400k compared with $2-3 million), much lower energy costs, often much wider bore or "open" design (less claustrophobic and better for in-surgery use).

In practice, novel designs of superconducting magnets with open designs or wider bores, and the overwhelming advantage of the higher field strength between permanent magnets and superconducting magnets is making permanent magnet MRI almost completely obsolete.

They are trying to find ways of making much more powerful, permenant magnets. They especially want to largely eliminate China's 97% monopoly, on the entire worlds supply of rare Earth materials (magnets).

Powerful magnets are used in so many (usually electronic) things, like cordless drills, Fighter Jets. That (The US especially), wants China's dominance with them, defeated.

So it is a big technological battle (For MRI), between ever more powerful permanent magnets, (possibly) higher temperature super conducting magnets and possibly future upcoming/invented improved techniques for scanning (especially medically) stuff.

My guess, would be that sooner or later, we can crack, the quantum level stuff. When this has been done. We will open up a huge Pandora box, of phenomenal new inventions, relatively unimaginable, by today's stuff.

A bit like another machine/steam/metal/wheel/electricity/transistor/valve-tube/computer-Microprocessor invention age.

Presumably this will lead to unimaginably powerful computer chips, with phenomenally capacities with hard disk drives (SSDs) etc.

Decently engineered quantum devices, should/might lead to much better scanning technologies and/or much improved existing ones.

So maybe quantum level permanent magnets and/or super-conductors are possible, in the future, allowing much better MRIs to be produced.

Huge weight (25-50 tons) and size

If super strength permanent magnets, can/are invented, I guess that would reduce/solve that problem.

Or maybe one day, room temperature super-conductors, will be invented. From your description of them, they would probably have a large/huge range of applications.

It sounds like they could conveniently power electric vehicles. Except that if you crash the car, you would have to run like crazy, away from the rapid "melt down" and worse, of the massive super-conducting, inductance storage device. (Partial joke).

Another possibility, might be if, rather than real super conductors are used, they invent super low resistance conductors (wires), which could be used to make very powerful (conventional) electro-magnets, powerful enough to be viable for MRIs. Or at least basic, not brilliant focus ones (like the photos, you kindly linked to).
E.g. Nano technology, which "MAY" end up inventing highly conductive materials, compared to what is available today (excluding super conductors).

I'm NOT familiar enough with the calculation figures, to work out if that is a possibility, or NOT ?
Also they could/may still use way too much power, leading to too much expensive power consumption and cooling issues and/or only usable for very short periods of time. (Due to the huge heat build up).

 

[William Gaatjes]

It sounds like they could conveniently power electric vehicles. Except that if you crash the car, you would have to run like crazy, away from the rapid "melt down" and worse, of the massive super-conducting, inductance storage device. (Partial joke).

It sure is an interesting idea, but some means would need to be found to reduce the charging time for the superconducting inductor.
After reading about, that would be a challenge using a super conducting inductor as a power storage device for mobile use. Mark R wrote about how long it takes to safely discharge such a coil. To charge it, takes just as long, if not longer. The inductor would oppose any current change with a massive emf. The dI/dt. It cannot be charged fast.

What is needed is a superconducting inductor that does not act like an inductor when being charged. But acts like one at the moment when the inductor carries the nominal current, as if we could turn as witch. What we need is a device that acts like a capacitor when charged, and then starts acting as a super conducting inductor. But that would need science capable of changing a pure electrical field into a pure magnetic field. I do not think that exists, even in the lab. Of course there are resonance circuits with L-C combinations but that is not what i had in mind. Although that would be fun, a tank circuit with a superconductor. But i think we would also need an ideal capacitor, i wonder if these exist ? :hmm:
But still, An AC resonant circuit with a superconducting inductor would not run forever, because the charge of the electrons need to be changing directions all the time because it is AC.

 

[Red Squirrel]

Best way is to get an isolation transformer for your scope to get galvanic isolation. You have to make sure you connect the ground first since your scope will be floating(ESD) but this way, you can never create a ground loop or fry your scope when connecting your probe ground to something that has potential with respect to the circuit ground. I use an isolation transformer permanently for the scope on my work. For my scope at home i have two old transformers where i connected the secondary sides together to get 230V in and out. Make sure the earth connections are not connected.

Yeah I've thought of that, been told you should not do that, but really if working with mains by nature I wont touch anything related to it anyway including the ground probe. I'm fairly new to electronics so kind of building up a small lab, isolation transformer is on the list though probably a good real bench floating psu first (and not the ATX one I'm using now). If anything I'd want to float whatever I'm testing and not the scope but floating the scope works well if I need to probe something that's "in place" such as a ground referenced battery bank or equipment.


On the subject of MRI machines what happens during a power outage at a hospital? Like where they have to actually shut down power for a while for maintenance or what not? Do MRIs have auxiliary power that keeps everything like the coolent running? Guessing you can't just cut the power to one without taking some steps. For unplanned outages the generator still takes some time to kick in as well.

 

[SOFTengCOMPelec]

It sure is an interesting idea, but some means would need to be found to reduce the charging time for the superconducting inductor.
After reading about, that would be a challenge using a super conducting inductor as a power storage device for mobile use. Mark R wrote about how long it takes to safely discharge such a coil. To charge it, takes just as long, if not longer. The inductor would oppose any current change with a massive emf. The dI/dt. It cannot be charged fast.

What is needed is a superconducting inductor that does not act like an inductor when being charged. But acts like one at the moment when the inductor carries the nominal current, as if we could turn as witch. What we need is a device that acts like a capacitor when charged, and then starts acting as a super conducting inductor. But that would need science capable of changing a pure electrical field into a pure magnetic field. I do not think that exists, even in the lab. Of course there are resonance circuits with L-C combinations but that is not what i had in mind. Although that would be fun, a tank circuit with a superconductor. But i think we would also need an ideal capacitor, i wonder if these exist ? :hmm:
But still, An AC resonant circuit with a superconducting inductor would not run forever, because the charge of the electrons need to be changing directions all the time because it is AC.

I think they CAN be charged VERY quickly, in general (Inductors), but I'm not sure if super-conducting MRI (in Hospitals), have other limitations, for other reasons e.g. Safety/cost/fire risks etc.

Basically, the higher the voltage (charging), the quicker the inductor charges. But if around 1000 Amps, is already flowing. A huge number of kiloWatts, would be required, as the charging voltage is increased.

The existing charge rate may be limited, because mains sockets, only give a few kiloWatts of electricity, and more powerful sockets, may not be available, such as 3 phase and/or the power supplies may be too expensive. Given that it (presumably), only infrequently needs charging.

Given a powerful enough energy source, inductors can be charged VERY quickly. (There are limits though, such as the insulation breakdown voltage and maybe other factors). Any heating effects, could/will limit the charging rate.

There could also be other factors, limiting the charge rate. I'm NOT too knowledgeable about super conductors and safety requirements, in hospital environments.

Example/analogy. Super fast battery chargers CAN be created. But their use is often restricted and/or banned, because super quick ones, can easily set fire to the batteries, if things go wrong. Especially Lithium based batteries, in fast radio controlled car competitions.

tl;dr
I.e. Safety (e.g. fire risks) and/or costs and/or limited available mains (kiloWatts) sources.

For cars, they could use exchangeable power packs and/or VERY powerful charging stations.

EDIT: Correction: I've NOT seen the circuit diagram for the MRI stuff. So I'm NOT sure of the specifications of the "Back emf Protection Diodes". They could also be limiting the maximum charge rate, depending on exactly what they are, and how they are wired up.
E.g. They may have a poor maximum reverse voltage specification, due to their extremely high current capabilities, or be designed like that on purpose.
For reasons that I CAN'T remember. I've seen stuff like that conduct (at somewhat lowish voltages), in BOTH polarity directions. Maybe it was to protect other circuitry, connected to the system, I can't remember.

 

[Brainonska511]

On the subject of MRI machines what happens during a power outage at a hospital? Like where they have to actually shut down power for a while for maintenance or what not? Do MRIs have auxiliary power that keeps everything like the coolent running? Guessing you can't just cut the power to one without taking some steps. For unplanned outages the generator still takes some time to kick in as well.

Cooling is passive. The superconducting magnet is in a Dewar of liquid helium and the liquid helium Dewar is surrounded by a Dewar of liquid nitrogen, to slow the boil off of helium.

There are some magnets (mainly older ones) that use a pumped-helium system that keeps the coldest helium near the magnet, but I'd bet that even in a power-outage, the passive cooling will be more than enough to sustain superconductivity.

Of course, in the event of an outage, you wouldn't be able to run experiments, as the consoles that supply the RF pulses and record data wouldn't be operational.

I've seen power outages in our own lab (only once or twice in the last 5 years), with its 14.1 T NMR magnet. The only thing that happens is the console system needs to be reset when the power comes back on. There is no damage to the magnet at all (and in fact, there are actually no electrical lines going to the magnet anyway; the only cables going to the magnet are really going towards secondary things that are removable, like temperature control, the room temperature shim stack, and the probe).

 

[William Gaatjes]

I think they CAN be charged VERY quickly, in general (Inductors), but I'm not sure if super-conducting MRI (in Hospitals), have other limitations, for other reasons e.g. Safety/cost/fire risks etc.

Basically, the higher the voltage (charging), the quicker the inductor charges. But if around 1000 Amps, is already flowing. A huge number of kiloWatts, would be required, as the charging voltage is increased.

The existing charge rate may be limited, because mains sockets, only give a few kiloWatts of electricity, and more powerful sockets, may not be available, such as 3 phase and/or the power supplies may be too expensive. Given that it (presumably), only infrequently needs charging.

Given a powerful enough energy source, inductors can be charged VERY quickly. (There are limits though, such as the insulation breakdown voltage and maybe other factors). Any heating effects, could/will limit the charging rate.

There could also be other factors, limiting the charge rate. I'm NOT too knowledgeable about super conductors and safety requirements, in hospital environments.

Example/analogy. Super fast battery chargers CAN be created. But their use is often restricted and/or banned, because super quick ones, can easily set fire to the batteries, if things go wrong. Especially Lithium based batteries, in fast radio controlled car competitions.

tl;dr
I.e. Safety (e.g. fire risks) and/or costs and/or limited available mains (kiloWatts) sources.

For cars, they could use exchangeable power packs and/or VERY powerful charging stations.

EDIT: Correction: I've NOT seen the circuit diagram for the MRI stuff. So I'm NOT sure of the specifications of the "Back emf Protection Diodes". They could also be limiting the maximum charge rate, depending on exactly what they are, and how they are wired up.
E.g. They may have a poor maximum reverse voltage specification, due to their extremely high current capabilities, or be designed like that on purpose.
For reasons that I CAN'T remember. I've seen stuff like that conduct (at somewhat lowish voltages), in BOTH polarity directions. Maybe it was to protect other circuitry, connected to the system, I can't remember.

Those diodes are most likely transzorbs or tvs. They can be unidrectional or bidirectional. They are like a combination in qualities of very robust zenerdiodes and voltage dependent resistors. They do nothing until the clamping voltage is reached, then they start conducting, in a very fast breakdown manner.


But i still think that charging inductors is a tricky situation. The superconducting state allows for very high currents to flow through the inductor. But it is the inductance that determines the rate of change you can use. A 100uH inductor charges much faster then a 100H inductor.

I think the superconducting inductors used for MRI are air coils without a core. That seems to me, is done because it is not easy to get magnetizable material that would not saturate at such high magnetic flux as used in MRI. A normal air coil inductor has a very low inductance.
To get more inductance, you either add a core that is made from magnetizable material to increase the magnetic flux or you add more windings. In both cases, increasing the inductance. But adding more windings with a normal inductor also increases the resistance and thus increases the resistive dissipation for a given amount of current flowing.
Since adding a core is not an option, the windings are increased. And since the windings are made of superconducting material, the resistance is no longer an issue and the current can be increased to a point below the point where the superconductor stops super conducting(Has to do with the current density).

But the whole point was not because it is superconducting, but because the very large inductance is opposing the current change, makes it slow to charge. For example a 100H inductor will counter a 1A/sec increase with a -100V emf. I normally work with in the uH range inductors. And sometimes in the mH range with steppermotors, and it is fun to see the current flow rising linear. At such a small scale it is only in the microseconds or milliseconds with only a few milliamps to a few amps.

 

[SOFTengCOMPelec]

Those diodes are most likely transzorbs or tvs. They can be unidrectional or bidirectional. They are like a combination in qualities of very robust zenerdiodes and voltage dependent resistors. They do nothing until the clamping voltage is reached, then they start conducting, in a very fast breakdown manner.


But i still think that charging inductors is a tricky situation. The superconducting state allows for very high currents to flow through the inductor. But it is the inductance that determines the rate of change you can use. A 100uH inductor charges much faster then a 100H inductor.

I think the superconducting inductors used for MRI are air coils without a core. That seems to me, is done because it is not easy to get magnetizable material that would not saturate at such high magnetic flux as used in MRI. A normal air coil inductor has a very low inductance.
To get more inductance, you either add a core that is made from magnetizable material to increase the magnetic flux or you add more windings. In both cases, increasing the inductance. But adding more windings with a normal inductor also increases the resistance and thus increases the resistive dissipation for a given amount of current flowing.
Since adding a core is not an option, the windings are increased. And since the windings are made of superconducting material, the resistance is no longer an issue and the current can be increased to a point below the point where the superconductor stops super conducting(Has to do with the current density).

But the whole point was not because it is superconducting, but because the very large inductance is opposing the current change, makes it slow to charge. For example a 100H inductor will counter a 1A/sec increase with a -100V emf. I normally work with in the uH range inductors. And sometimes in the mH range with steppermotors, and it is fun to see the current flow rising linear. At such a small scale it is only in the microseconds or milliseconds with only a few milliamps to a few amps.

My knowledge of MRI scanners and super conductors, is way too limited, for me to try to comment here. Hopefully, anyone else, who does have such knowledge can/will, if necessary.

Even inductors, can confuse me to an extent. Especially when the more complicated, magnetic properties of the core are discussed.

 

[William Gaatjes]

My knowledge of MRI scanners and super conductors, is way too limited, for me to try to comment here. Hopefully, anyone else, who does have such knowledge can/will, if necessary.

Even inductors, can confuse me to an extent. Especially when the more complicated, magnetic properties of the core are discussed.

It took a time for me as well to grasp how inductors work. And even now, i have to look it up sometimes in my old school books.
These guys and gals do a good job explaining :
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indcon.html#c1

As does the wiki about inductors.
https://en.wikipedia.org/wiki/Inductor

I tried to simulate a100H inductor with 1microOhm resistance with ltspice but i did not see anything happening. I used a 1 Ohm series resistor and a 1V pulse source. No emf. Strange.

 

[William Gaatjes]

An inductor works just like a capacitor, only with currents instead of voltages.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indtra.html#c1

 

[William Gaatjes]

When you try to change the voltage over a capacitor quickly, it will prevent(opposes) that change by letting a lot of current flow, as long as it has enough electrical charge. The direction of the current depends if you either increase the voltage over the capacitor or decrease it.

EDIT:
A capacitor of 100F is not easy to discharge or charge. The only thing is that a capacitor will take all the current you give it as long as it ESR (equivalent series resistance, internal ohmic resistance) does not prevent the current flow or the finite amount of current your power source can provide.

 

[SOFTengCOMPelec]

An inductor works just like a capacitor, only with currents instead of voltages.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indtra.html#c1

Exactly, I'm pleased you know this.

That is why A HIGH Charging Voltage, charges an inductor, much quicker. In much the same way that a capacitor, can be charged VERY quickly, by using ever higher currents.

Obviously, eventually physical limits, such as internal resistances, break down voltages, heat effects and other stuff, can also limit the maximum rates.

EDIT:
That's why stepper motor drivers, which want to achieve VERY high stepping rates, use ever higher drive voltages (subject to limits, such as the maximum allowable voltage on the winding's).

You may have seen this on your scope, when you watched the stepper motor current build up. The higher the voltage (available to the stepper coil), the quicker the current rises. Good controllers, reduce the voltage, after the desired current has been reached. To avoid burning out the stepper motor and/or wasting power.

 

[SOFTengCOMPelec]

When you try to change the voltage over a capacitor quickly, it will prevent(opposes) that change by letting a lot of current flow, as long as it has enough electrical charge. The direction of the current depends if you either increase the voltage over the capacitor or decrease it.

EDIT:
A capacitor of 100F is not easy to discharge or charge. The only thing is that a capacitor will take all the current you give it as long as it ESR (equivalent series resistance, internal ohmic resistance) does not prevent the current flow or the finite amount of current your power source can provide.

I think that I worded my earlier post, badly.

In many senses, I know a lot about inductors, sorry.

I meant that more complicated concepts, such as talking about the changes in hysteresis of the permeability of a coil between different core materials vs temperate, and magnetic strengths, and saturating the core. Does indeed confuse me. (I may have got the terminology wrong here, as it partly confuses me).

But I'm fine with basic Inductor Physics.

 

[William Gaatjes]

Exactly, I'm pleased you know this.

That is why A HIGH Charging Voltage, charges an inductor, much quicker. In much the same way that a capacitor, can be charged VERY quickly, by using ever higher currents.

Obviously, eventually physical limits, such as internal resistances, break down voltages, heat effects and other stuff, can also limit the maximum rates.

EDIT:
That's why stepper motor drivers, which want to achieve VERY high stepping rates, use ever higher drive voltages (subject to limits, such as the maximum allowable voltage on the winding's).

You may have seen this on your scope, when you watched the stepper motor current build up. The higher the voltage (available to the stepper coil), the quicker the current rises. Good controllers, reduce the voltage, after the desired current has been reached. To avoid burning out the stepper motor and/or wasting power.

It is true that the higher the voltage, the higher the current and the faster the build up of magnetic field and as a result more mechanical strength, but the inductance is relatively low.
High voltages are not the only thing needed to get high stepping rates. You also need to get that magnetic field out of the coils again as fast as possible again. How do you that the best way ?
Well, the coil is connected to a H bridge of top mosfets and bottom mosfets.
By cleverly switching the right mosfet on in the opposite direction and using the body diodes of the other mosfets, you effectively get a form of synchronous rectifying. The coil of the steppermotor starts to function just as the inductor in a switch mode power supply, the mosfets and body diodes in the H bridge are the (synchronous) rectifiers and the bulk capacitor of the power supply takes in all the current. The fun part of using the body diodes is that once the current through the stepper motor coil reaches almost zero(the voltage over the body diode drops below the forward conducting voltage), the body diode stops conducing and prevents current flowing in the opposite direction. This prevents a magnetic field build up that opposes the desired magnetic field in the other coil.
This is basically how all high performance stepper controllers work.

 

[William Gaatjes]

i had to research that for work once.

:blushing:

 

[William Gaatjes]

I should note in my example above, it is not really synchronous rectifying since the body diodes of the mosfets are used in my example.
But there are steppermotor controllers that really do synchronous rectifying and track the current through the coil. Once the current reverses direction, the mosfets are switched off. Using the body diodes, no sophisticated current tracking is needed that can measure both polarities.

 

[SOFTengCOMPelec]

It is true that the higher the voltage, the higher the current and the faster the build up of magnetic field and as a result more mechanical strength, but the inductance is relatively low.
High voltages are not the only thing needed to get high stepping rates. You also need to get that magnetic field out of the coils again as fast as possible again. How do you that the best way ?
Well, the coil is connected to a H bridge of top mosfets and bottom mosfets.
By cleverly switching the right mosfet on in the opposite direction and using the body diodes of the other mosfets, you effectively get a form of synchronous rectifying. The coil of the steppermotor starts to function just as the inductor in a switch mode power supply, the mosfets and body diodes in the H bridge are the (synchronous) rectifiers and the bulk capacitor of the power supply takes in all the current. The fun part of using the body diodes is that once the current through the stepper motor coil reaches almost zero(the voltage over the body diode drops below the forward conducting voltage), the body diode stops conducing and prevents current flowing in the opposite direction. This prevents a magnetic field build up that opposes the desired magnetic field in the other coil.
This is basically how all high performance stepper controllers work.

Where my knowledge, gets weak/poor, is when you try to change the magnetic field so quickly, that it is NOT just the basic inductance issue, but the "speed" of the magnetic transfer, itself, which is limiting things. I've heard/read about this, but don't understand it very well.

With many capacitor types (excluding electrolytics and similar), you can charge/discharge them at extremely high/fast rates.

But (my limited understanding), has heard that magnetic fields, can have "OTHER" delay terms, involved. As magnetism can be/is relatively slow (sometimes), compared to other things in Physics.

tl;dr
The magnetic dynamics (speed etc) of cores, is something which I do NOT currently understand very well. So once the inductor has exceeded the limits of normal/simple inductor physics, I am increasingly unsure of what is going on.

 

[William Gaatjes]

Where my knowledge, gets weak/poor, is when you try to change the magnetic field so quickly, that it is NOT just the basic inductance issue, but the "speed" of the magnetic transfer, itself, which is limiting things. I've heard/read about this, but don't understand it very well.

With many capacitor types (excluding electrolytics and similar), you can charge/discharge them at extremely high/fast rates.

But (my limited understanding), has heard that magnetic fields, can have "OTHER" delay terms, involved. As magnetism can be/is relatively slow (sometimes), compared to other things in Physics.

tl;dr
The magnetic dynamics (speed etc) of cores, is something which I do NOT currently understand very well. So once the inductor has exceeded the limits of normal/simple inductor physics, I am increasingly unsure of what is going on.

Well, that comes down to the physics level what happens in a material, how electrons move through that material and how they are aligned. I remember something about Weiss areas. These are magnetic domains of a group of atoms where all electrons are aligned. Giving a certain preferred angle of the magnetic field. When exposed to a external magnetic field, these domains align themselves according to the external field. But they prefer their angle they got when the material was going from a fluid state to a solid state. That is all i know.
There exists ferro, para and dia magnetics for cores. But i have to look that up too for details.

 

[SOFTengCOMPelec]

Well, that comes down to the physics level what happens in a material, how electrons move through that material and how they are aligned. I remember something about Weiss areas. These are magnetic domains of a group of atoms where all electrons are aligned. Giving a certain preferred angle of the magnetic field. When exposed to a external magnetic field, these domains align themselves according to the external field. But they prefer their angle they got when the material was going from a fluid state to a solid state. That is all i know.

You're are making me feel a bit better, by saying that. To me it's a bit like understanding classical Physics well, but NOT knowing much about Quantum level Physics, Quantum Mechanics.

I have to own up, to preferring to use standard stepper motor driver integrated circuits, rather than designing my own (driver/control circuitry). If I was to use any stepper motors.
But if I was ever involved with something going into large scale production (with stepper motors), then the cost savings of designing your own circuitry to do it, would probably make it viable. Depending on the bulk cost of the driver chips, etc.

Slow/simple stepper motor requirements, can often work with just simple transistor/Fet on/off circuits. Such as the ULN2003 series. (Stepper motor types vary, so that would NOT be suitable for ALL types. Unipolar/bipolar etc etc).

Example datasheet

 

[William Gaatjes]

You're are making me feel a bit better, by saying that. To me it's a bit like understanding classical Physics well, but NOT knowing much about Quantum level Physics, Quantum Mechanics.

I have to own up, to preferring to use standard stepper motor driver integrated circuits, rather than designing my own. If I was to use any stepper motors.
But if I was ever involved with something going into large scale production (with stepper motors), then the cost savings of designing your own circuitry to do it, would probably make it viable. Depending on the bulk cost of the driver chips, etc.

Slow/simple stepper motor requirements, can often work with just simple transistor/Fet on/off circuits. Such as the ULN2003 series. (Stepper motor types vary, so that would NOT be suitable for ALL types. Unipolar/bipolar etc etc).

Example datasheet

In all honesty, i am not as smart as i seem. I did not figure it all out myself.
That was done long before i even started working.
I read a lot about all the issues with stepper motors and came across a novel datasheet from ST. Although TI also has very nice dc motordrivers like the DRV8801 that do the same, using the rectifying technique to get rid of the back emf of the motor. Very high performance motordriver (2.8A) in a tiny smt package.

If you look for this datasheet from ST electronics: L6208age14, it will explain in detail how they do it for bipolar stepper motors. It is possible and fun to program a mcu to do the same way of controlling the mosfets in a H bridge.

datasheet :
http://www.st.com/web/catalog/sense_power/FM142/CL851/SC1794/SS1554/PF63235?s_searchtype=partnumber