Magnetizing Current, Inductance, Inrush, etc.

Hey, this could be a great thread...I'm a great believer in the power of analogies, but I'm stuggling here, and waving my paws about a fair amount.
The whole megnetism thing is extremely difficult to find an analogy for...the 'look upon it as a flow of water' starts to falter.

I shall return hopefully with a bucket full of analogies...

Del

Ok....
Try this for a start...lets assume we are connecting the primary of a regular transformer (say a 50w 24v one just for the sake of some numbers and to avoid confusion with those guys who deal with the umpteen KV MW jobs...)

Maybe if we consider the molecules of the core material (or maybe small domains of it at a macro level) to be freely pivoted little bar magnets (Yes they are painted red with shiny ends ).

When power first comes on there is the resistance of the wire and some limited inductance. As the current rises it starts to align all the little bar magnets, they take a finite time to line up as they interact with each other have inertia..whatever..I don't know I'm a cat and running out of steam...

Hope this has helped to start the discussion.
Look upon my contribution as a sort of 'Inrush'
Eagerly awaiting the thoughts of the others....

Del

Righ on Del. As the electrons keep rushing in, they get more and more of those pretty little red/shiny magnets lined up until eventually they're all in a nice neat row. That's when the electrons coming from behind can rush right through. I'm visualizing a similar thing with a bunch of rowboats anchored haphazardly across a river. Getting across is quite tiring and slow. If those da**ed boats would just line up in a nice row, we could run across with little effort. Similarly, once the magnets line up, resistance to electron flow drops right down and current increases.

Then what happens ? Next ....

Arrrgghhh no! I think youv'e fallen in the river!

Once they are lined up they create a nice magnetic field which opposes the current flow which lined 'em up... and this slows up the inrush down to the quiescent level.

The current always makes the magets line up in a way that opposes the current that lined 'em up in the first place...dunno how we explain why though .

As the AC flows it is constantly re-aligning the magnets, this takes a finite time and maybe we can use this to explain voltage current phase relationship? Or maybe I'll go up the pub.

That's the problem with analogies...I spose it's as easy to find one which gives the opposite result.

Where's everyone else? Help us out here guys...the next generation of engineers is waiting to be inspired

Del

Ooops - must be having a senior moment. I was thinking dc not ac. Where's that pub?

Hello oldeng

Some music to listen to, just for you, and as you requested: No Maths.

When a voltage is first applied to an inductor, it commences the alignment of an invisible magnetic field, and part of that field may include magnetic materials such as soft iron.

Because the magnetic field has to receive energy from somewhere to enable that magnetic field to be established, the energy can only be supplied from the source electric supply. (For the work to be done, energy must be supplied)

At the same time the magnetic field and any magnetisation of included magnetic materials is happening, because the magnetic field is not constant but increasing, a reverse EMF is generated into the wire/coil.

This reverse EMF opposes the supply EMF, thus reducing the supplied current, but as the reverse EMF energy actually is originally supplied from the original supply voltage, it is of lesser amount than the original voltage supply, (Due to resistance and magnetisation losses).

The supply voltage does all the work, being opposed by the reverse EMF until a balance point is reached, and no further increase in the magnetisation occurs. (Note that this never happens, just that the Supply current dwindles very rapidly, almost to zero).

That is why the Supply Current lags behind the Supply Voltage, in an Inductor.

The proportion of Supply Current lagging behind the Supplied Voltage depends on the actual amount of magnetisation in the magnetic circuit, and for an Alternating Current Supply, the frequency of the applied AC and waveform shape.

So, a large transformer with 50 tonnes of soft iron core is going to take much energy to establish the magnetic field - then run it back through zero, and then reversing the field - In US Supply area 60 times each second, in UK Supply area 50 times each second. This effect is called hysteresis loss, and ends up mainly in the form of heat loss.

As the AC supply frequency rises higher and higher, the effect becomes greater.

Extra bonus explanation:

A radio transmitter antenna uses this above effect, causing an electromagnetic wave to be radiated from the antenna each half cycle of the AC waveform.

Because the transmitter antenna is resonant or nearly so with the transmitter frequency, as the first electromagnetic wave is starting to collapse, because the transmitter has passed the waveform through the zero point, another waveform arrives just in time, but the same polarity as the collapsing electromagnetic waveform.

The newly supplied waveform to the antenna has the full power of the transmitter behind it, while the collapsing electromagnetic waveform is much less in strength.

Thus the law of repulsion takes place, and the latest waveform from the transmitter forces the collapsing waveform away from the antenna.

A constant succession of the above events means that there is a succession of "radio or TV waves" radiating out from the transmitting antenna, becoming weaker as they progress.

As far as knowing how electricity works, what light, gravity or magnetism really are, the short answer is that there are theories, but nobody on Earth really knows, theorists just try and work it out, but no true answers.

Kind Regards, from far away....

This last posting is getting closer to providing a good analogy. I've always had difficulty with understanding magnetism.

It is not necessary to think of little dipole magnets to understand the magnetic effect. In fact they are a complication. The basic magnetic field is established in space around a conductor carrying current. A back pressure to the flow of current exists in the form of a reverse electromagnetic force or voltage which is induced in the wire where, and this is remarkable, the current flow is either decreasing or increasing. There is no reverse EMF once the current is constant.

Energy is stored in the magnetic field. This is hard to appreciate. But a partial analogy might be a balloon that is being filled with air. The irony is that the back pressure/reverse voltage from the balloon is not proportional to the amount of air in the balloon, but is proportional to the rate at which the balloon is being filled with air. In other words the effective voltage in a circuit containing an inductance is the driving voltage less the reverse voltage created by the inductance, where the reverse voltage is proportional to the rate of change of the current flowing within the wire in the loop.

This reverse EMF is modest when the driving voltage is endeavoring to build-up current at a slow rate. But if the voltage is rapidly changing and is endeavoring to build up current at a high rate, then the reverse EMF is substantially higher. This is why electronic chokes are more effective at blocking current flow when exposed to higher frequency voltage fluctuations.

Think of Electro-magnetism in terms of a single, straight wire which is generating a magnetic field around the wire. If you don't believe that such a magnetic field exists, then string two extension cords across your basement approximately 3 inches apart. Then plug the far ends together and short the near ends momentarily across a 12 V battery. (One way to limit the current is to brush a single strand of multistrand copper wire over the terminals of the battery. Wear gloves and safety glasses.) The electric cords will then either swing together or swing apart. I'll let you do the experiment to find out which one is correct. The reason why that the hanging cords will move is that when an electric current is established in the wires, then the wires have magnetic fields surrounding them that interact with each other.

A magnetic field has a direction. The right hand rule says that if your thumb points along the wire in the direction of current then the direction of the magnetic field surrounding the wire is in the direction that your fingers are pointing.

<>We like, in order to help us visualize what is happening, to imagine that a magnetic field is similar to lines in space that are either close to each other or spread apart. When the lines are close to each other, the force field is more intense. When the lines are far apart, the force field is less intense. In fact, the force field is not limited to or existing in the form of lines; we only use the lines to depict the presence of the force field.

Magnetic lines arising from two objects that are forced to occupy the same space will tend to cause the objects to draw together if their respective magnetic lines are directed in opposite directions. If their respective magnetic lines are both oriented in the same direction, then a force will rise that will tend to separate the two objects. This is the source of the old expression that, in respect of magnets marked respectively with the North and South poles, like poles repel and opposite poles attract. This rule is indirectly referring to the interaction of the directed magnetic lines that we imagine to constitute the magnetic force field. Again, it is the force fields that interact and not actual lines in a literal sense.

When you coil wire you combine the magnetic fields of the individual adjacent wire segments. But this is creating a mechanical complication which is distracting to understanding the basic phenomena which is present even in the case of a single straight wire. The experiment of hanging two extension cords in the basement demonstrates the phenomena in its most basic sense.

I'm not sure that I understand much more than what I've just set out, but if you want an analogy to inductive reactance, I suggest that it is somewhat akin to a balloon that is being filled with air arising from establishing a flow of electric current through space. If you remove the voltage source that is creating the flow of current, then the magnetic field, like compressed air in a balloon, would tend to keep the current flowing through a completed circuit in the same way that the compressed air from the balloon will tend to flow out of the balloon until the balloon, or its equivalent the electromagnetic field around the wire, has totally collapsed. But the balloon analogy is not perfect.

Good luck in your search for better analogies. It's an honorable pursuit.

Ho Sparky,Graebeard,Del and the rest.

OK so your in the sun in that wee island off Oz,but we're heading for Spring.

Unfortunately so far your explanations match up with my thoughts,I had assumed I was on the wrong track somewhere as I couldn't take it any farther towards inrush explanation.

But I think I've got a handle on it now with your help.Its the twisting motion of the magnets that produces back emf,so when the twisting slows the back emf reduces and the current rises.At saturation they stop twisting so the current is only limited by resistance. This is where the maths helps perhaps,as its not the twist but the rate of twist that matters,or even the rate of rate of twist.

Now we come to the dreaded inrush,I dont think you've had a go at that yet.

What confuses me is what's the difference between switching on and the sinusoidal changes and reversals.Perhaps a red herring but I read in a manufacturers blurb that the biggest inrush occurs if switching is at zero crossing,that really threw me.

Looking forward to your further thoughts.

Del note boring blade sharpening machine photos now posted.

If you measured the DC resistance of the motor at rest, just using a simple Ohmmeter, this is the R component at switch on of the motor in the simple Ohm's law of

I = V/R. Where I is current and V is the applied voltage.....

Most motors have extremely low DC resistance!!! Which is why the current at start up is so high!

Once the motor starts turning, you start to get back EMF produced which opposes the inrush current more and more and this reduces it eventually to the Running current value when the motor reaches its designed RPM!! The difference between inrush and running current can be up to 8:1, though a lower figure is generally more true 5:1 or so....

I hope this assists you in understanding the problem and will allow you to personally check this out in a simple manner using a DC Ohmmeter....hands on is always better!!!

Hi

an excellent method to visualize magnetic circuits is described in the following paper, which you can find on the internet: Hamill, "Gyrator-capacitor modeling: a better way of understanding magnetic components"

Max

Hi all, Perhaps Andy is confusing inrush with startup current?

I've read the gyrator capacitor proposals and certainly the model includes the energy storage properties of magnetic circuits but there's a bit of maths involved.

The IEEE disinformation service can be circumvented as the proposals can be found for free at www.hamill.co.uk.pdfs/lecomctg.pdf if you're interested.

It's difficult to visualize the functions so I'm still looking for inrush explanation/analogy as in the energization of transformers.

Why does the average tool transformer go 'boing' at switch on,sometimes tripping mcbs,with a transient lasting apparently a lot more than a mains cycle?

Thanks to all for their valued contributions so far.

They are one and the same thing!!! Did you not realize that? Two names for the same thing!!! (there could be more but I cannot think of a further name at this time!!

I wish you well with your further education!

Now,now, Andy,keep the heid!

I'm not going to sit at the back and keep quiet 'cos thats how I got into this mess about 50 years ago.

It was when the elec eng lecturer missed out a whole blackboard of matrix manipulations without comment,but avid copying, by all, I realised that the rest of the class were lost too.

No,they aren't the same,motor startup involves the inertia of a rotor,and its load.This is evidenced by the startup conditions lasting longer depending on the type of load,high inertia,longer startup,more of a mechanical concept.Are you suggesting that there is some sort of virtual high inertia rotor in a highly inductive magnetic circuit?This may help the visualization.

Keep the suggestions coming please,we'll beat this yet and confound the physicists.

But don't tell them,they think we're just rude mechanics.

Inrush and startup current are exactly the same thing. When a motor is first connected across the supply, the motor is NOT turning, therefore no back emf is being generated and the effective resistance of the motor is low. You can measure this with a simple Ohmmeter and this will tell you using simple Ohm's law, what the inrush current will be approximately.

But as soon as that current is there, the motor starts to turn, this also causes back emf to be producedp7 (generator effect), which opposes the inrush/starting current and starts to reduce it.

As the motor speeds up, back emf gets greater and greater, opposing the inrush/starting current until the motor reaches its top speed. This is the point at which starting/inrush current is now running current.

Generally the design of the motor puts normal running current as the lowest current usage of all, but overloading (undersized motor) or bad design could make that statement invalid....

I did NOT mention anything to do with "virtual high inertia rotor in a highly inductive magnetic circuit?" That was you......!

Please stop making such statements, they have nothing to do with starting or inrush current!!, you are really confusing yourself......!!! and maybe some others with you!! Perhaps the professor was right?

Simple visualisation (not reality).

The electrons flowing in a wire are like water in a pipe. The inertia is similar to the inductance. When you turn on the tap, the water flow is delayed by the need to accelerate the water. Similarly, when you turn the tap off, the water wants to keep flowing and can cause water hammering. The same is used in switching power supply to increase or decrease the voltage (pressure). In an AC circuit, imagine that you are alternatively pushing the water in and out of the pipe. The water flow will be delayed with respect to the force applied.