Eddy Currents

Look up the definition of "wasted" energy.

decent answer

Thank you, I will need to read about the Carnot Cycle.

I assumed everyone knew it was deg K.

Assuming is nearly always dangerous... I always try to avoid it, but every so often, I catch myself doing it.

Waaay back, I remember a college professor in a lecture saying "... and therefore, obviously ...". Whatever "it" was, "it" was NOT obvious to me, but I was too timid/stupid to say so. I swore I would never say that in any lecture I gave. Ever since, I have at least modified it to say "...it should be obvious that..."

Having worked with a few university professors in the situations you mention, I found out that usually it was not obvious even to the lecturer!

Sorry, my formula is in error. It should be:

Eff = (TempHot-TempCold)/TempHot.

(All temperatures in degrees Kelvin)

If TempCold = 0 (absolute zero), then Eff = 1. Otherwise Eff < 1.

There's one common device that does utilize the waste heat that a motor produces, that's the blower motor in the plenum of a hot air heating system. Of course that's a detriment when it is in cooling mode! Fortunately, as losses go, eddy current losses are fairly small, almost always exceeded by bearing, windage, and copper losses.

Sorry I am currently unable to attach anything to my replies.

Here is a fairly good physical display on eddy current.

https://www.kjmagnetics.com/blog.asp?p=eddy-currents

Seems like eddy currents are of so little concern any more that we should be calling them ed currents.

Q #1:

Doping of metal will also affect eddy current circulatory patterns as well as decrease the amount of current flow thereby decreasing heat production.

This method often is used in electromagnetic design as well as in audio speaker magnet design.

Q # 2:

Application of metal doping in the past was very limited in motor and transformer design.

There has been significant breakthroughs in technology and manufacturing processes allowing several motor manufacturers using metal doping to maximize motor and transformer efficiency.

Presently the cost of implementation far outweighs any realized short term savings benefits however it is reasonable to expect 100% payback of investment within 5-7 years with the length of payback time period depending on the application and load characteristics.

Last year I commissioned my large centralized air compressor project which utilized seven each, 33000 SCFM, high-efficient 98.7% rated, constant pressure, centrifugal fan units.

I specified motors with maximum efficiency rating available of 98.7% and was able to justify initial investment cost by illustrating 100% payback within 54 months (4-1/2 years) from date of commissioning.

We are in the 13th month of operation and have already realized an energy savings of 30.5% over previous air compressor equipment operation for the same time period.

Barring any unexpected major equipment failures and maintaining current realized energy savings rate we will recapture 100% of our investment expense well before the 4 year anniversary

The motors are manufactured using advanced proprietary metal doping techniques in the rotor shaft, rotor laminations, and stator laminations.

The ratio of payback is favorably skewed due to the old compressor units being Circa 1940, worn out, piston type, reciprocating, units with synchronous motors which does not allow fair comparison of investment payback as if we just replaced lower efficient motors with higher efficiency rated motors leaving the air compressors units the same.

As far as justifying difference in cost for using 98.7% efficiency motors verses standard 96% efficiency rated motors the system will have to be in operation for 7 years before we could recover our initial investment if based solely on energy savings.

In this case the 2.7% improvement in motor efficiency meets long term investment payback but does not meet the expectations of a 5 year investment recovery however the realized savings in lower motor maintenance costs allowed decreasing the expected payback time period from 7 years down to 4-5 years.

I am sure the cost difference of using higher rated 98% + efficiency motors will decrease with time and competition for sales.

As this is a general question, there are at least 2 methods used to reduce eddy currents not previously mentioned:-

1. Employ non-magnetic materials (e.g. aluminium, brass etc) near high currrents.

2. Move magnetic materials as far away as possible from the high currents.

You can also sometimes reduce the temperature rises (but not the wasted energy) by using thicker materials where you can't move the materials away (e.g. switchgear or trunking).

How does one make an electro-magnetic machine like a motor buy using non-magnetic materials?

The question was about eddy currents in general, not just motors etc.

Non-magnetic does NOT mean no eddy currents! In fact, since most non-magnetic metals are better conductors than steel alloys, the eddy currents will be significantly greater in the aluminum or brass you mention.

Try dropping a good Nd disk magnet vertically through a section of copper or heavy-walled brass or aluminum tubing. If the tubing ID is only a little larger than the OD of the magnet, it will take a couple of seconds per foot of tubing for the magnet to fall through. This also will work in the corner of an angle or channel extrusion held at an angle. Depending on several factors, the magnet may slide, roll, or tumble as it falls, held back by the magnetic field of the eddy currents

Many years ago, I had a Bogen™ 33¹/₃ RPM phonograph turntable whose rollers were sized to make it rotate a bit faster than correct speed, and had a spinning aluminum disk with a magnet to slow it down to fine-tune it to correct speed. Moving the magnet towards the center of the disk, where the speed of the aluminum relative to the magnet was slower, would reduce the eddy-current losses and allow the table to rotate faster. Moving the magnet towards the outside of the disk, where the speed of the aluminum relative to the magnet was faster, would increase the eddy-current losses and slow the table to the desired speed, as indicated on a stroboscopic ring..

"Non-magnetic does NOT mean no eddy currents"

Good point. I was referring to magnetic fields produced directly by electricity (e.g. in a high current switchboard) not a moving magnet. Tricky subject!

Even there, it could be a problem. If you were to use aluminum sheet to make trays to support/guide high-current AC cables, there could be significant eddy-current losses.

Just switching to aluminum won't accomplish much in the way of decreasing 'eddy currents'.

https://www.youtube.com/watch?v=RBN_cYEgeMA

Thanks everyone for their answers, this helps to fill in the gaps for a better understanding of the subject matter.

My next question deals with reversing a formula to determine an unknown value.

Tony

1. Is there a method employed ( other than lamination ) to reduce the heat generated or to utilize the developed heat for its working potential ?

Induction cooking uses this effect VERY efficiently to make saucepans hot, with no laminating, with a far lower usage of power than any other system used and FAR less danger to the user. No possible burning of oil and fat for example.

Only a wood fire, cut from your own trees by yourself, dried for a season or two by yourself, can really compete better in cost!!

2. If #1 is accomplished, will that alter the lines of force between the N & S poles and in what way ?

You need to define that better as the "problem" is only seen with AC and the need for Laminations in chokes and transformers and AC solenoids and the like.

The poles will generally be switching either 50 or maybe 60 times a second, depending upon where you live....but if we are talking about poles, I personally would expect them to be in the same position for either AC or DC....if you simply ignore the switching of course!!

Whether this helps or not I cannot say!!

"Induction cooking uses this effect VERY efficiently to make saucepans hot". You've definitely made me an enthusiastic fan of induction cooking, but the cause(s) of the heating in induction cooking must be something besides eddy currents. Otherwise it would work with copper and aluminum pots, and the induction elements I've tried will not work with non-magnetic pans. (I've tried on multiple occasions) I suspect it has to do with magnetic domains "rubbing against each other" (magnetic friction). I'd sure like to know more...

"2. If #1 is accomplished, will that alter the lines of force between the N & S poles and in what way ?" I believe you are probably correct that the pole positions will be essentially unaffected, but the number of lines of force (the field strength) will definitely be affected. Conservation of energy dictates that the magnetic field of eddy currents must oppose the field that generated those currents. This is equivalent to saying that the presence of eddy currents weakens the magnetic field strength, thus reducing the efficiency of motors, transformers, etc., so anything that can be done to reduce those eddy currents should increase the efficiency of the devices, as long as whatever is done has no other side effects.

With regard to pots and pans of any metal, the frequency of the oscillator needs to be changed to suite them. I believe they use 50 KHz for steel and iron.

The usual "Hobs" generally only use the frequency for iron and steel....but I have read somewhere that specialist hobs are around for other metals.....but don't quote me on that.

Its probably also related to the way that a high quality metal detector uses the speed of decay of the induced voltage in the (underground?) target metal, to determine the type of metal and possibly its size.

Gold and Aluminium decay really fast, steel and iron really slow.....(from memory only!)

Not that most people will have gold cooking pots, but certainly aluminium is often used. Nowadays, with a steel slug in the base to allow induction cooking to work!!

As I've posted somewhere in CR4 previously, the frequency of my induction cooktop changes with the setting, in the general range from around 20kHz to around 52kHz. I don't know if the changes in frequency are the principal controlling factor, or if they are a consequence of some other change.

I had missed that post of yours.

I was assuming that the 50 KHz marked on the base of my Hob, was the frequency all the time, never checked it out....

I was simply assuming that the voltage was varied, not the frequency....

But, as I don't know over what frequency range steel and iron get hot, that could be a strong possibility....

I looked at the wiki page here:-

Induction_cooking

and found this:-

Varying the heat

Less sophisticated induction cookers regulate the heat delivered by switching the field on and off relatively slowly; if a pot with a thin bottom is used, the temperature may fluctuate markedly due to the low thermal inertia of the small amount of metal. This does not occur with cookware that has a thicker bottom, or with induction cookers with more fine-grained control.

Which could mean that if you were measuring the frequency by a counter, switching on and off would possibly reduce the total number of cycles seen.......Just a thought!!!

But I really don't know how it is done exactly.

Here's the waveshape of the field produced by my cooktop at maximum power:

The digital display indicates the frequency of 52kHz

And here is the waveshape at roughly half power:

where the frequency is down to 33kHz. I see that the peak voltage is actually higher at the lower power setting, but that may have been due to how and where I held the probe. I did NOT have it mounted.

Well done.

I certainly cannot explain it adequately......

Thanks for the great pictures too....

Thanks guys for your help in understanding the material. Because of your explanations and the way you explain it, with the examples shown, I now have a better understanding of these concepts.

I do have one more question and after this one, I promise I won't bother you for awhile.

Determining Impedance:

Figure (a) shows a schematic, there is a horizontal line at the top and one at the bottom, an arrow ( like in geometry / line segment ) is placed vertically, with letter z to represent Impedance, to the right is a symbol for single winding transformer ( XL) , below that is a symbol for resistor (r) .

Figure (b) shows graph paper, in which each square is 1/4", inductive reactance is 4 ohms vertical & resistance is 3 ohms horizontal. A line is drawn diagonally across the graph paper. A ruler is placed alongside the diagonal and the measurement is 1 1/4". The book calls this " right angle method of addition " .

It almost looks like the slope of a line in a linear equation .

Is this an accurate method to use ? Or is there a formula that can be substituted for greater accuracy ?

Thanks in advance, tony

It would be much better if you could take a photo of the illustration and post it here.

If I hear you correctly, they are illustrating that impedance is the vector sum of resistance and reactance. Mathematically, you can use the Pythagorean theorem to calculate impedance: Z=√(R²+X²). X_L is the inductive reactance of the transformer.

Other method of reducing the eddy current is by way of addition of Silicon as an alloying element. This results in increasing the resistivity of the material. So (eddy) voltage practically remaining constant, eddy loss which can be written as eddy voltage square divided by R(eddy). So eddy loss reduce when silicon is added.

Similarly by making laminations thin, the path length of of eddy current is increased which in turn increases resistance resulting in reduction of eddy current loss.

It is certainly true that using laminated core material reduces eddy currents significantly compared to a solid core, because the insulating coating on the laminations prevents eddy currents from flowing from one lamination to the next, and that thin laminations are better than thick ones...

But I fail to understand how making thinner laminations leads to an increase in eddy current path length. Please explain.

Above figure explains how reduction in lamination thickness (keeping total thickness same) results in reduction in Eddy current loss.

My point exactly! Look at those eddy current loops. Since the vertical portion of each loop is smaller in the laminations than in the solid core, the eddy current paths are shorter, not longer.

In fact, I suspect (but am not certain) that the loops are, as shown in the illustration, also shorter in the horizontal direction, making them even shorter still, NOT longer.

The Story of Eddy Currents

Eddy was no roamer
With magnetic fluxes did he yield
Followed paths of least reluctance
And never strayed from field to field

Now Eddy had a girlfriend
Her name was Millie Watts
He warmed her gently with his love
Her laminates tied him in knots

But Eddy had a bully
Big Hugh Surge was his name
And with his brother nasty Spike
Put an end to Ed's love game

So Eddy found a rougher gal
Frau Kyla Volt was his ruination
She showed him where to flash his cash
And shorted out his lamination

And now Eddy is a wild boy
He parties hard on gin and rum
Turns up wasted all the time
As his harmonics thrum and hum

- BabyGuinness

Liked it, never seen it before either, thanks.

As of now there is no technology using other than laminated core to reduce eddy current to satisfactory limit.

Really? Perhaps you should look into Amorphous (Glass) Metal ribbon wound transformer core technology, approximately 70-80% lower core losses versus standard layered sheet laminated cores.

Correct, and there are also powder metallurgy cores, although I'm not sure if any of them work well at line frequency.