Author Topic: Help me understand inductance.  (Read 707 times)

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Online hamster_nz

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Help me understand inductance.
« on: October 17, 2018, 08:32:39 am »
I'm feel quite happy about capacitance, resistance, BJTs, MOSFETs and so on, but one thing I struggle with is inductance. I just don't 'grok' it. SMPS sort of make sense, and snubber/flyback diodes seem to be the wrong way around based on intuition. It is just me?

I know the math (more or less), and I know that it could be approximately summed up as:

Quote
When current to flows it induces a magnetic field, and that stores energy. Once the the source of current flow removes, the energy stored in the collapsing field induces current and voltage in the conductor. The ratio of current to voltage depends on the remaining impedance within the circuit - so a field collapsing into an open circuit can create relatively high voltages, and into a short circuit can create very high currents.

So is that pretty much it for inductors, ignoring coupled magnetic fields?

Maybe the important bit that I am missing that the current from the collapsing field is in the opposite direction to the current that created it?  It is not trying to 'consume' the energy stored by sustaining the flow of current (like a capacitor does), but it tries to 'return' the energy stored in the magnetic field, so the current is reversed.

As you can see, I just don't grok it. Help! Hopefully discussing it will make give me a lightbulb moment...
 

Online chris_leyson

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Re: Help me understand inductance.
« Reply #1 on: October 17, 2018, 08:59:49 am »
It's not like that with inductors. If you remove the voltage source that is causing the current to flow in the inductor then the magnetic field will try to keep the current flowing in the same direction and the voltage will reverse in order to try to keep the current flowing. You can instantaneously change the voltage across an inductor but not the current. Opposite for a capacitor, you can instantaneously change the currrent flowing in or out of a capacitor but not the voltage. In an inductor the energy stored in the magnetic field tries to keep the current constant, whereas in a capacitor the energy stored in the electric field tries to keep the voltage constant.
 

Online hamster_nz

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Re: Help me understand inductance.
« Reply #2 on: October 17, 2018, 12:55:09 pm »
If you remove the voltage source that is causing the current to flow in the inductor then the magnetic field will try to keep the current flowing in the same direction and the voltage will reverse in order to try to keep the current flowing.

So this is the bit that troubles me. How would the "voltage reverse in order to try to keep the current flowing". In that statement is the cause and effect the wrong way round?

Would this be better said as "the voltage at the terminals reverses BECAUSE the energy in the magnetic field keeps the current flowing".

That seem to work while looking at a boost converter's topology. The switch goes off, the end of the inductor goes + enough for current to flow through the diode and into the filter cap. For them the reverse voltage spike is a feature.

Do buck converters use the inductance differently? They seem to use the inductor's ability 'moderate' current flow (prevent quick changes in current) to produce the low voltage on the output, and the voltage spike from switching is steered to ground by the diode - for them the spike it is a 'problem' (energy lost in the diode) and not a feature?


 

Offline Wimberleytech

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Re: Help me understand inductance.
« Reply #3 on: October 17, 2018, 01:04:17 pm »
Maybe just focusing on the math will help.
V = L (di/dt)
When a source V is applied across an inductor, the current increases, so di/dt is positive.
When the source V is removed, there is no source for additional current, so the current decreases, thus di/dt is negative.
Because di/dt is negative, the voltage across the inductor switches polarity.
 

Offline rfengg

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Re: Help me understand inductance.
« Reply #4 on: October 17, 2018, 02:21:51 pm »
I'm feel quite happy about capacitance, resistance, BJTs, MOSFETs and so on, but one thing I struggle with is inductance. I just don't 'grok' it. SMPS sort of make sense, and snubber/flyback diodes seem to be the wrong way around based on intuition. It is just me?
Maybe the important bit that I am missing that the current from the collapsing field is in the opposite direction to the current that created it?
Quote

The snubber/flyback diodes across a large inductor similar to the energising coil of a relay prevents breakdown of the active device which drives it. When a relay is switched off, the inductor /coil winding produces a e.m.f in reverse in  order to keep the current across it the same direction prior to the event of the switchoff happening. (V=L di(/dt)) .
Since the current is interrupted abruptly , the V induced is large which may exceed the breakdown voltage of the device (for, e.g. the VCE of a BJT).
Since the voltage produced  is reversed to keep current flowing the same way , the diode across the relay windings, clamp this to roughly 0.7V which prevents the active device from breaking down.
If you sketch out the schematic , this should be obvious.
Better still, think of this in another way..........the diode is put the "wrong way around" as you state........in this "wrong way" around, think of how the voltage needs to be polarised to make the diode conducting.......and then try to infer how this voltage is being produced and how it protects the active device.

Also, the current from the collapsing field is not in the opposite direction to the field that created it........the voltage across the inductor reverses to keep the current in the same direction as it was, before it was interrupted .
« Last Edit: October 17, 2018, 02:27:32 pm by rfengg »
 

Offline bsfeechannel

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Re: Help me understand inductance.
« Reply #5 on: October 17, 2018, 03:54:21 pm »
So this is the bit that troubles me. How would the "voltage reverse in order to try to keep the current flowing". In that statement is the cause and effect the wrong way round?

Voltages can induce currents as currents can induce voltages. They are the two sides of the same phenomenon.

Since inductors tend to maintain their currents, when you switch from a voltage source to a load, the voltage has to be inverted so as to maintain the current.

You find an analogous behavior with a capacitor. In order to maintain its voltage, it inverts its current when you switch from a source to a load.

 
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Online IanB

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Re: Help me understand inductance.
« Reply #6 on: October 17, 2018, 04:08:56 pm »
As you can see, I just don't grok it. Help! Hopefully discussing it will make give me a lightbulb moment...

Maybe it is easier to think about what inductors do, rather than how they do it.

Functionally, a capacitor resists any change in voltage. In a very similar way, an inductor resists any change in current.

There's a good analogy with hydraulics here. Once water is flowing in a pipe, it resists any change in the flow. If you try to stop the flow, high pressures can result. This is just like the high voltages that result if you try to stop the current flow through an inductor.

In the video below you can see the hydraulic analog of a switch-mode boost converter. Once the water is flowing through the downhill pipe it doesn't like to stop. By suddenly stopping the flow, enough pressure is generated to boost the water way uphill, far higher than it started.


I'm not an EE--what am I doing here?
 

Offline BravoV

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Re: Help me understand inductance.
« Reply #7 on: October 17, 2018, 04:49:38 pm »
I always thought inductor analogy is like this hydraulic wheel below, when the wheel gained the rotating momentum caused by the initial current, and if the current stopped or even decreased, the rotating momentum will keep trying to maintain/move the current (liquid), hence the polarity reversal (pressure reversal) at the input & output pipes.



Cmiiw.
 

Offline bsfeechannel

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Re: Help me understand inductance.
« Reply #8 on: October 17, 2018, 05:21:52 pm »
I think that the problem lies in the fact that OP thinks that voltages are causes and currents are effects. How can a voltage be at the mercy of a current?
 

Online Mechatrommer

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Re: Help me understand inductance.
« Reply #9 on: October 17, 2018, 06:15:01 pm »
think of it when the power source is removed, inductor will become a "charger", not "source". in charging mode, where the current goes will become more positive and theoretical infinite of inversed potential in open space time, while capacitance similarly will become a "charger", but instead of voltage, current will change direction infinitely in theoretical shorted space time. it always amazed me how these two work in pair complementing each other nicely, its one living proof that the law of this universe is not "random" as many claimed how it was created. in fact "random" is just another translation for our lack of understanding of the fact everything in the universe is governed by laws in one way or another, lumped in the world of knowledge of "statistics and stochastics". so "something" must have "selected" this kind of non random law, hence my signature is somewhat delusive, for the purpose of it to being delusive and for the sake of harmony and peace in this little forum :P otoh we can keep capacitance practically indefinite in time in few cents of component, but not that easy at keeping the magnetic field around (by applying pure short once its created).
if something can select, how cant it be intelligent? if something is intelligent, how cant it exist?
 

Online hamster_nz

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Re: Help me understand inductance.
« Reply #10 on: October 17, 2018, 07:05:50 pm »
So this is the bit that troubles me. How would the "voltage reverse in order to try to keep the current flowing". In that statement is the cause and effect the wrong way round?

Voltages can induce currents as currents can induce voltages. They are the two sides of the same phenomenon.

Since inductors tend to maintain their currents, when you switch from a voltage source to a load, the voltage has to be inverted so as to maintain the current.

You find an analogous behavior with a capacitor. In order to maintain its voltage, it inverts its current when you switch from a source to a load.



Yes! This! It (kind of) makes perfect sense. I'm going to have to ponder/think/sleep on this one, but I can sort of feel the understanding starting to form....
 

Offline hugo

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Re: Help me understand inductance.
« Reply #11 on: October 20, 2018, 11:56:34 am »
Just go to: http://www.falstad.com/circuit/

The "Circuits" menu contains a lot of sample circuits for you to try.  ;)
 

Online T3sl4co1l

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Re: Help me understand inductance.
« Reply #12 on: October 20, 2018, 01:01:00 pm »
If you (think you?) understand capacitors:

Just swap V and I.

It's that simple. ;D

To really cook your brain -- and drive home your knowledge of circuits! -- demonstrate the equivalence of parallel-series circuits where V and I are swapped, L and C, and series and parallel.  The node voltage equations of one will be exactly the current mesh equations of the other (with V replaced with I, of course).

(Uh, you will have to be far enough into intro EE to know what those things are, and the basics of how to do them.  I don't know how "beginner" that is in this case...)

Tim
Seven Transistor Labs, LLC
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
 

Offline kosine

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Re: Help me understand inductance.
« Reply #13 on: October 20, 2018, 10:41:07 pm »
If you read the following carefully (maybe twice!), the fundamentals should hopefully make more sense:

1. Electrons have momentum:
Think of inductance as length (L).
Say a short wire and a long wire both carry 1A; Both have the same current (I), BUT the longer wire contains more electrons in motion.
It therefore has more electrical momentum. (Compare this with pipes carrying water.)
Electrical momentum is termed "magnetic flux" and is given the Greek symbol phi.

Since longer wires contain more electrons, the total momentum (phi) increases with length.
But a greater current also means more electrons, so momentum (phi) also increases with current.
Taken together, momentum (flux, phi) is proportional to L, and also to I.
So phi = LI. (The units are chosen such that the proportionality constant is 1.)


2. Momentum accumulates over time:
Now, to get a current flowing at all, we need to apply an electromotive force. This is the voltage (V).
Increasing this applied force/voltage will increase the momentum, but so will applying it for a longer period of time (t) because momentum accumulates.

Therefore we can also deduce that phi is proportional to V, and also to t.
So phi = Vt = LI
This is the textbook inductor equation, and is analogous to classical momentum:
Momentum = Ft = dm/t (since F=ma and a=d/t^2. Think of "mass per second" as the current.)


Inductors (as a component) are essentially a very long wire wound into a coil to make them more compact. (The coiling also increases the effective length due to magnetic effects, but that gets a lot more complex to explain.) Inductors allow you to build up momentum in the circuit, thereby storing the electrical equivalent of kinetic energy. If you suddenly break a circuit with lots of momentum in it, all that energy has to go somewhere. It'll either be converted into potential energy (i.e., a voltage), which is how a SMPS works*, or you can safely dissipate the energy as heat by adding a diode. That allows current to circulate back through the inductor until all the stored energy is used up.

* If you break the circuit on the high voltage side of an inductor, all the electrons flow away from the break resulting in a negative voltage spike. Break the circuit on the low side, and all the electrons pile up and cause a high voltage spike. How large these voltages get depends on many things, including the length of the coil. At it's simplest, that's its inductance (L).
 


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