Why the inductor voltage blows up for a sudden current change?

[Abhishek Dwivedi]

Dear All,
This is my first question to the forum.
"Assume there is a + dc voltage across a inductor. So current will continue to increase. Now if the current through the inductor is forced to stop using some means ( eg. Current was flowing in inductor through a ON switch and then switch turns off in no time.) . I know the mathematical reason but I want to understand the physical intuitive reason about why the voltage across the inductor blows up.

Thanks
Abhishek

 

[cnh1995]

Magnetic field in the inductor collapses very fast. This gives rise to high voltage spikes as E=dΦ/dt. You may see sparks across the switch. Energy stored in the magnetic field is suddenly released.

 

[Rive]

...intuitive...

Well. If it'll be taken seriously then I'll deny that it's from me, but the way I think about this (before starting with the calculations) is something like 'this inductor wants to keep this current flowing, so it'll provide enough voltage on the terminals to make it possible - till the energy stored runs out'.

 

[Abhishek Dwivedi]

Magnetic field in the inductor collapses very fast. This gives rise to high voltage spikes as E=dΦ/dt. You may see sparks across the switch. Energy stored in the magnetic field is suddenly released.

Thanks Helper. I also think the same. Please give me your feedback about what I think. " So Magnetic flux can't collapse in no time or current can't change in no time and to achieve it what inductor does is to generate a potential ( Faraday's Law) so as to maintain its current. But the current can't flow so inductor keep on increasing its potential in a hope to get the current go on . Current never flows as there is no path for it. So Inductor keeps on generating its voltage as high as it can before sparking.

 

[cnh1995]

So Magnetic flux can't collapse in no time or current can't change in no time and to achieve it what inductor does is to generate a potential ( Faraday's Law) so as to maintain its current. But the current can't flow so inductor keep on increasing its potential in a hope to get the current go on . Current never flows as there is no path for it. So Inductor keeps on generating its voltage as high as it can before sparking.

Your reasoning sounds convincing, but I am not sure experts here will agree that it's the right approach.
If you want to know what happens "exactly" at the moment you open the switch or the sequence in which things happen, you'll have to go one level deeper i.e. Maxwell's equations or Electromagnetic wave theory and work with extremely short time scale.
As far as circuit theory is concerned, I find @Rive's reply spot on.

See if this old thread of mine helps.
https://www.physicsforums.com/threads/discharging-of-an-inductor.892992/.

 

[jim hardy]

I want to understand the physical intuitive reason about why the voltage across the inductor blows up.

This should help
https://www.khanacademy.org/science...fields/magnetic-flux-faradays-law/v/lenzs-law

There's nothing like feeling something to make it intuitive. Have you ever been around an electric fence?

I'd suggest take an analog ohm meter and a small transformer not a big one.
Set to lowest ohms scale, RX1 if it has that.

Measure resistance of primary winding with secondary shorted, ie its wires twisted together.
Note where needle settles, and more importantly how quickly it moves to rest position.

Now untwist the secondary and repeat. Be careful hold the two primary wires in same hand on two different fingers . That'll keep any shock going through just that hand not through your body.
Needle should settle at same place but it'll get off to a slow start.
That slowness is the magnetic field building up inside the transformer.

You will notice a slight shock when you lift the test lead (or when it slips off) .
That is the transformer's magnetic field collapsing , Lenz's Law says it'll try to maintain the current that your ohm meter established.
Only place that currrent can flow is through your fingers.

That's why i said a SMALL transformer - running this experiment on ones over about a pound can be painful.
That's why i said hold the two wires in one hand , never subject yourself or anybody else to a whole body electric shock however mild.

old jim

 

[tech99]

Dear All,
This is my first question to the forum.
"Assume there is a + dc voltage across a inductor. So current will continue to increase. Now if the current through the inductor is forced to stop using some means ( eg. Current was flowing in inductor through a ON switch and then switch turns off in no time.) . I know the mathematical reason but I want to understand the physical intuitive reason about why the voltage across the inductor blows up.

Thanks
Abhishek

This often come up and I offer a mechanical analogy. The inductor has inertia, like a moving mass, because it stores energy (in its magnetic field). To bring it quickly to a stop requires a large force. The force must be in the direction to provide braking, so must oppose the original accelerating force.

 

[jim hardy]

Please give me your feedback about what I think. " So Magnetic flux can't collapse in no time or current can't change in no time and to achieve it what inductor does is to generate a potential ( Faraday's Law) so as to maintain its current. But the current can't flow so inductor keep on increasing its potential in a hope to get the current go on . Current never flows as there is no path for it. So Inductor keeps on generating its voltage as high as it can before sparking.

You have the idea just right.

"Anthropomorphizing" often helps us talk through a concept. You should refine your mental image to visualize the lines of flux surrounding the inductor, and how they'll move through the windings as the field collapses.
images here will help
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/indcur.html

 

[fcouto]

So Inductor keeps on generating its voltage as high as it can before sparking

You're very close to saying: " charge buildup with opposite signs on the switch's open terminals, quickly increase the electric field". This electric field, which simultaneously reduces the current (and the mag.flux), also generates a spark when above a certain threshold. Integrating the electric field around a closed loop at any given instant will give you not zero, but exactly the induced fem (Faraday-Lenz), which in turn equals -dΦ/dt. The equations are coupled so as to transfer energy from the mag. field to the dissipative path of the sparking electrons.

 

[tech99]

You're very close to saying: " charge buildup with opposite signs on the switch's open terminals, quickly increase the electric field". This electric field, which simultaneously reduces the current (and the mag.flux), also generates a spark when above a certain threshold. Integrating the electric field around a closed loop at any given instant will give you not zero, but exactly the induced fem (Faraday-Lenz), which in turn equals -dΦ/dt. The equations are coupled so as to transfer energy from the mag. field to the dissipative path of the sparking electrons.

If we think of the inductor as having inertia, it will try to keep the current flowing. If a perfect short circuit was applied to the inductor, the current would flow for ever, as in a superconducting magnet. To stop the current quickly we need a high reverse voltage, which could conceptually be applied from a reversed battery or by inserting a high resistance - the open switch.

 

[fcouto]

I know the mathematical reason but I want to understand the physical intuitive reason

Mechanical analogy is always nice, and may be the best answer to this "intuitive reason" type of question. It'll satisfy for most electric circuit basics. But the charge buildup is the only correct microscopic physical phenomenon explaining these rapid voltage surges, sparks, and so on. The flux inertia drives the inductor, but a simple resistor has also increased voltage across its terminals due to charge buildup and the resulting additional electric field: a high resistance path slows down charges inside, but is quickly offset and a current is set up.