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That's a lot of back EMF

M

Mark R

Diamond Member
ErILJ3g.jpg


Magnet power supply unit (front), energy dump unit (left), connecting wires and magnet (background).

One of the problems when decommissioning a high field magnet is how to discharge the current from the magnet coils. Due to the high energy stored in the magnetic field, simply opening the circuit will cause bad things to happen.

Under normal operation, once the magnet is charged, it is short circuited by a superconducting switch. Once this is closed, the current will continue to flow and the PSU can be disconnected.

To de-energise the magnet, the PSU must be reconnected. The PSU ramped up until it is taking the magnet current, and there is no current through the superconducting switch. Once now current is flowing through the switch, the switch is opened by heating, and the PSU is set to a gradually reducing current.

The energy dump unit is a big box of resistors intended to absorb energy and allow a faster reduction in current without overheating the PSU.

Dumping the energy from the magnet took most of the day. Not sure what the magnet current was, but probably around 1 kA, judging by the size of the wires and connectors.
 
Mark R said:
ErILJ3g.jpg


Magnet power supply unit (front), energy dump unit (left), connecting wires and magnet (background).

One of the problems when decommissioning a high field magnet is how to discharge the current from the magnet coils. Due to the high energy stored in the magnetic field, simply opening the circuit will cause bad things to happen.
...
Click to expand...
But just think of all the Youtube views you might get.....

:awe:
 
In my field, CTs exhibit the same problem. They usually have shorting blocks to take the energy out of the coil and do work on the transformer. The windings of a CT have a laughable ratio compared to what you are dealing with, but can still shock or arc badly if not taken out carefully.
 
Mark R said:
ErILJ3g.jpg


Magnet power supply unit (front), energy dump unit (left), connecting wires and magnet (background).

One of the problems when decommissioning a high field magnet is how to discharge the current from the magnet coils. Due to the high energy stored in the magnetic field, simply opening the circuit will cause bad things to happen.

Under normal operation, once the magnet is charged, it is short circuited by a superconducting switch. Once this is closed, the current will continue to flow and the PSU can be disconnected.

To de-energise the magnet, the PSU must be reconnected. The PSU ramped up until it is taking the magnet current, and there is no current through the superconducting switch. Once now current is flowing through the switch, the switch is opened by heating, and the PSU is set to a gradually reducing current.

The energy dump unit is a big box of resistors intended to absorb energy and allow a faster reduction in current without overheating the PSU.

Dumping the energy from the magnet took most of the day. Not sure what the magnet current was, but probably around 1 kA, judging by the size of the wires and connectors.
Click to expand...

Can you draw a schematic diagram? I don't see how you can reduce or stop the current through the superconducting switch by introducing a power supply in parallel to the superconducting switch.
 
Is there a sacrificial switch you could use with a large fuse? It could even have a large variable resist to ease the transition. Shunt voltage slowly to the resistor as heat, throw the switch once you're below what it's rated for.
Also, what about disconnecting power on the low side of the transformer?
Finally, why not correct the power factor with a capacitor bank until the load is barely inductive, then just use a regular disconnect rated for the voltage used?
I'm just spit-balling...
 
Hello Mark. I can think of a method to de-energise the magnet, but it is different from the sequence of events you describe.
 
I remember it like it was yesterday, when they energized our 14.1 T magnet in the lab. 250 L of liquid helium to cool the magnet down and top up the helium dewar, followed by the slow energizing with a gigantic power supply. The tech made a big deal about having a very clear path as adding or removing current from the magnet are the most likely times for something to go wrong (eg: magnet quench, where the cryogenic helium rapidly boils off and you need to get the hell out of the room).
 
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So how much energy is "in" one of those coils anyways? It was my understanding that the residual magnetism is kind of low, and the field is much stronger when the coil is on.

Last electromagnet I messed with was flowing about 4A @ 12vDC, or ~48W. I had no way to accurately measure the field strength, or number of turns, but the wire was 20 AWG enameled copper.

I have bought more wire to experiment further with Lenz's law, especially with AC currents. Have not figured out my core design yet, but the ultimate goal is to bring a cylindrical permanent magnet through the core to do some kind of work (launch projectile is the easiest thing I can think of.)
 
natto fire said:
So how much energy is "in" one of those coils anyways? It was my understanding that the residual magnetism is kind of low, and the field is much stronger when the coil is on.
Click to expand...
The problem is, the coil is always on once it's charged. It's a superconducting electromagnet - they charge it, then disconnect the supply. This leaves current flowing in the magnet, keeping it energized. When you want to move it or take it out of service, you need to reverse the process. These types of magnets don't really have on/off switches like a typical electromagnet.

On a related note, there are a few working prototypes of non-superconducting, high-field magnets for MRI and NMR applications. I've seen work on a 9.5 T magnet which could be turned on and off, no liquid helium required (I think it had a standard-sized bore for NMR work and MRI development) and a commercialized version of a 2T MRI magnet for imaging mice.
 
Brainonska511 said:
The problem is, the coil is always on once it's charged. It's a superconducting electromagnet - they charge it, then disconnect the supply. This leaves current flowing in the magnet, keeping it energized. When you want to move it or take it out of service, you need to reverse the process. These types of magnets don't really have on/off switches like a typical electromagnet.

On a related note, there are a few working prototypes of non-superconducting, high-field magnets for MRI and NMR applications. I've seen work on a 9.5 T magnet which could be turned on and off, no liquid helium required (I think it had a standard-sized bore for NMR work and MRI development) and a commercialized version of a 2T MRI magnet for imaging mice.
Click to expand...

Thank you. I guess I should have assumed that when I saw the video of someone playing around with one of those. While I am sure the localized field on one of those is very strong, gonna guess it takes even more power to drive electromagnets in a crane at a scrap yard, which can pick up several tons of metal. not even going to count an arc furnace,which makes the best short circuit ever.
 
Never realized electromagnets actually stored a considerable amount of energy, how does this work, does it basically act like a huge capacitor because of so many wires close together creating a huge capacitance? Or does it have to do with the collapsing of the magnetic field? What kind of bad things happen when you let unshort it, does it basically create a huge EMP and arc between the two ends?

I've played a bit with electromagnets but never got that far into it. Would be fun to build a coil gun some day though or just a ridiculously huge electromagnet that runs off 120v and uses a full 15 amp circuilt. Still nothing compared to MRIs and stuff though.
 
Red Squirrel said:
Never realized electromagnets actually stored a considerable amount of energy, how does this work, does it basically act like a huge capacitor because of so many wires close together creating a huge capacitance? Or does it have to do with the collapsing of the magnetic field? What kind of bad things happen when you let unshort it, does it basically create a huge EMP and arc between the two ends?

I've played a bit with electromagnets but never got that far into it. Would be fun to build a coil gun some day though or just a ridiculously huge electromagnet that runs off 120v and uses a full 15 amp circuilt. Still nothing compared to MRIs and stuff though.
Click to expand...
My semi-educated guess: It's a big superconducting inductor, and inductors in circuits store energy in a magnetic field. Capacitors store energy in an electric field.

When charge begins to move through a wire, or a coil of wires, it puffs out a magnetic field that remains there as long as the charge flows continuously and consistently. When the charge flow (current) stops, the magnetic field collapses. A moving magnetic field then causes charges in the wire to move - the magnetic field, or lack of one, effectively opposes changes in current.
So if you've got an inductor and try to push charge through it, there is opposition. It doesn't "want" to make a magnetic field. But push hard enough and you'll still end up with that field.
Now you've got a static field with DC through it. The magnetic field now "wants" things to stay that way. So when the external source of current stops, the magnetic field collapses and effectively tries to maintain that current.

That's why you normally put a diode across the coil of a relay when it's being controlled by a transistor: When the coil is de-energized, its magnetic field collapses, and you have a sudden burst of voltage as the magnetic field's energy gets deposited into the wire. The diode provides a quick way of providing a short circuit for the energy, preventing it from damaging the connected semiconductors.


It sounds like a large superconductor's powerful magnetic field also doesn't want to go away quietly.
 
Jeff7 said:
My semi-educated guess: It's a big superconducting inductor, and inductors in circuits store energy in a magnetic field. Capacitors store energy in an electric field.

When charge begins to move through a wire, or a coil of wires, it puffs out a magnetic field that remains there as long as the charge flows continuously and consistently. When the charge flow (current) stops, the magnetic field collapses. A moving magnetic field then causes charges in the wire to move - the magnetic field, or lack of one, effectively opposes changes in current.
So if you've got an inductor and try to push charge through it, there is opposition. It doesn't "want" to make a magnetic field. But push hard enough and you'll still end up with that field.
Now you've got a static field with DC through it. The magnetic field now "wants" things to stay that way. So when the external source of current stops, the magnetic field collapses and effectively tries to maintain that current.

That's why you normally put a diode across the coil of a relay when it's being controlled by a transistor: When the coil is de-energized, its magnetic field collapses, and you have a sudden burst of voltage as the magnetic field's energy gets deposited into the wire. The diode provides a quick way of providing a short circuit for the energy, preventing it from damaging the connected semiconductors.


It sounds like a large superconductor's powerful magnetic field also doesn't want to go away quietly.
Click to expand...


Oh that makes sense, so with a ridiculously large coil that effect is so big you probably get a jolt of like thousands of volts flying back at the end of the two wires, probably killing anything connected to or near it.
 
If you recall an electric field that is changing in time induces a magnetic field, and conversely a magnetic field changing in time also induces an electric field.
 
Mark R said:
ErILJ3g.jpg


Magnet power supply unit (front), energy dump unit (left), connecting wires and magnet (background).

One of the problems when decommissioning a high field magnet is how to discharge the current from the magnet coils. Due to the high energy stored in the magnetic field, simply opening the circuit will cause bad things to happen.

Under normal operation, once the magnet is charged, it is short circuited by a superconducting switch. Once this is closed, the current will continue to flow and the PSU can be disconnected.

To de-energise the magnet, the PSU must be reconnected. The PSU ramped up until it is taking the magnet current, and there is no current through the superconducting switch. Once now current is flowing through the switch, the switch is opened by heating, and the PSU is set to a gradually reducing current.

The energy dump unit is a big box of resistors intended to absorb energy and allow a faster reduction in current without overheating the PSU.

Dumping the energy from the magnet took most of the day. Not sure what the magnet current was, but probably around 1 kA, judging by the size of the wires and connectors.
Click to expand...

That is an interesting story Mark. But how do you charge such a coil in an MRI device ? I mean, how does the opposite function ?
Remove the PSU when the desired current is reached and then close the super conducting switch within a few micro seconds ?

Or just charge the coil with a PSU, short the PSU (works like a current source which switches off when the current drawn from the PSU reaches a set level) with the super conducting switch and then remove the PSU ?
 
You can demonstrate the principle of back EMF really easily.
All you need is a small induction motor (any small appliance such as a box fan works perfectly). Make sure it's turned on (of course).

Instead of plugging it into the wall, grab a 9V battery and touch the prongs of the plug on the battery terminals. Notice how the spark seems much bigger than it would be if you simply shorted out the terminals?

Now to show higher voltage without getting shocked, grab a NE-2 "pilot lamp" style neon lamp, like this one,
images
wrap each wire from the tiny glow lamp to a prong on the plug and repeat what you did earlier with the battery. Notice how the lamp flashes brightly? These require about 90V to reach point of ionization (giving off light) and (again) with the 9V battery alone, not possible.

Finally, if you want to actually feel the effects of this, grab the plug by the prongs while repeating what you did before. Nice shock and (again) no way you would feel this from the 9V battery alone.

Now imagine dealing with 100s of kVA of power and a huge electromagnet...
 
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William Gaatjes said:
That is an interesting story Mark. But how do you charge such a coil in an MRI device ? I mean, how does the opposite function ?
Remove the PSU when the desired current is reached and then close the super conducting switch within a few micro seconds ?

Or just charge the coil with a PSU, short the PSU (works like a current source which switches off when the current drawn from the PSU reaches a set level) with the super conducting switch and then remove the PSU ?
Click to expand...


You just connect the PSU to the coil terminals. Open the shorting switch (which is just a piece of superconducting wire mounted to a heater) - so you turn the heater on to break superconductivity, leaving it a regular high resistance wire.

Then you turn the current up on the PSU gradually. The magnet will produce a back EMF as the current rises, so you typically aim for 1-2 volts.

Once you reach nominal magnet current, you set constant current mode on the PSU, and as current is no longer changing the terminal voltage will fall to zero. Once this happens, there will be no leakage current through the superconducting switch (if V=0 then I=0). You can then turn off the switch heater, and allow superconductivity to recover in the switch.

You then start reducing current on the PSU, and the current from the magnet will start circulating through the switch. Once PSU current is zero, you disconnect.

The total amount of energy stored in the magnetic field varies with magnet design, but on a high end MRI, like the one just decommissioned it is around 5 kWh.

Some of the new higher field research MRI scanners going in have stored magnetic energy of about 30-50 kWh, with some of the extreme field scanners having up to 200 kWh of stored energy.
 
Mark R said:
...
The total amount of energy stored in the magnetic field varies with magnet design, but on a high end MRI, like the one just decommissioned it is around 5 kWh.

Some of the new higher field research MRI scanners going in have stored magnetic energy of about 30-50 kWh, with some of the extreme field scanners having up to 200 kWh of stored energy.
Click to expand...
😵


Any estimates on the peak current surge capacity out of something like that? Say I were to have a remote-control robot short out the ends of it with a length of a solid 3x3" aluminum billet....
 
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