Engineering magnetics -- practical introduction to BH curve-----make money online

A practical introduction to understanding magnetic devices such as transformers and motors.  This video covers BH curves, reluctance, permeability, DC and AC magnetic circuits, and some applications.

CORRECTION:  at 13:48, I  say that permeability can be negative.  This is not true.  All permeabilities are positive.  Diamagnetic materials have permeabilities that are lower than empty space (eg .95 relative permeability).  There is another quantity which is called susceptibility, which describes the ratio between flux carried by a material, and the flux carried by the space which the material occupies.  This quantity can be negative.

Please put questions in the comments, and I'll do my best to answer them!

I apologize that I didn't mention inductance.  This video focuses only on the magnetic side of things.  Inductance is how the electrical circuit interacts with the magnetics.

today on Applied Science we're gonna

take a practical look at engineered

magnetics I've actually wanted to make

this video for a long time because it's

common to teach electronics with

batteries and wires and make it very

understandable but for some reason when

it comes to magnetics it's typically

taught heavy on the math and the theory

so after watching this video you'll have

a really good conceptual grasp of

magnetics and if you want to design your

own transformers or motors this is a

great place to start it so I mentioned

batteries and light bulbs and this is

actually a really great place to start

over here we've just got a d-cell some

wire and an incandescent light bulb and

this is even taught in grade school

level I mean it's pretty clear what's

happening here we've got an electric

current that's flowing through the wire

making the light bulb glow and flowing

back into the battery and then you know

a few years later we introduce the

concepts of there there's a voltage here

and there's a resistance in the light

bulb and then a current flows through

here that's determined by the voltage

and the resistance and it's actually

almost exactly the same thing for

magnetics let me show you so I have a

little magnet between these two pieces

of steel and then we've got a steel

track here making a circuit and a

compass just to show what the field is

in this gap here and then I've got

another magnet here just to pull the

compass sideways so that we can see when

there's no field and so if I put the

thing in here you can see the compass

needle moves of course and if I turn it

around it goes the other way

nothing too shocking just yet but what

to notice is that if I hold the magnet

out here and turn it the compass moves a

little bit because the magnetic field

can even go through the air but if I put

it into this steel piece the magnetic

field has a much easier time making its

way out to the compass and so to notice

the same exact quantities that we get in

electric circuits apply to magnetic

circuits we essentially have something

like a voltage source we have something

like a resistance and there's actually a

flow through here a quick note about

terminology there's actually quite a lot

in the field of magnetics and it makes

it hard to understand because there's

just so many new terms and so I've tried

really hard in this video

do not introduce terms that aren't

necessary but unfortunately when you go

off and search for more information on

the internet they're gonna crop up so

we're gonna have to cover some like for

example in electric circuit we call it

resistance when there's an opposition to

a flow of current and in a magnetic

circuit we call it reluctance and so the

trouble is that if you go searching for

stuff like magnetic resistance you won't

really find it on the internet you have

to search for reluctance so I'll do my

best to describe each term as it comes

up and keep in mind we're trying to keep

it minimal and I'll always do my best to

describe them first it helps a lot to

visualize what's going on in here so I

took the magnet with the two steel

pieces out of that circuit and I'm just

going to sprinkle some iron filings on

here and you know you've definitely seen

this demo before but the thing that's

important to note is that there's sort

of field lines flowing out of the ends

in making a circuit even in air right

like everything is an electric circuit

everything is a magnetic circuit and the

trick is that electricity really really

doesn't want to flow through air so we

can usually neglect it in thinking about

electric circuits it's actually very

fortunate because all of our circuit

boards require air and insulators to

work properly but with magnetics even

though the field doesn't really want to

go through air we'll talk about what the

exact numbers are later it can still

make it and so there's no there's a

magnetic field flowing around here and

we could do the analysis and figure out

just how much is flowing through there

based on the reluctance of air so let's

put this into the circuit so you can see

how that changes the field lines okay

now we'll try the same thing with this

in circuit notice that there is quite a

bit less activity here and the reason is

that these steel pieces are pulling away

the magnetic field it's easier for the

magnetic field to flow through these low

reluctance pieces of Steel rather than

flow through the air so you know it's

there's two camps on whether you want to

anthropomorphize physical phenomenon but

you could say the magnetic field would

rather flow through the steel than it

would flow through the air or you can

just think of it in terms of reluctance

the steel has a lower reluctance

and so it conducts more of this

magnetism than the air around it and if

we took these gaps out and made it

perfect you would have even less field

leaking out into the air so we've shown

that a magnet acts like this source of

magnetic force it's kind of similar to

voltage in an electric circuit but

there's an important difference between

a magnet and a battery a battery is an

electrical energy source

I mean it's converting chemical energy

into electrical energy and we're

actually pulling electrical energy out

and burning it up in this tungsten

filament lamp but a magnet is not an

energy source a magnet is much more like

a compressed spring so at the factory

they started with a piece of material

that was not magnetic and put a lot of

energy into it to make it into a magnet

so think of this as kind of like a

compressed spring so it's providing

force for us because it's a compressed

spring and we can squeeze it harder or

you can uncompress the spring and get

the energy output only once so it's much

more like a small storage vessel that's

filled up at the factory and you really

just can't get you know forever energy

out of it of course ok let's try

something else so instead of the

permanent magnet with the two pieces of

steel I have just a single piece of

steel here with about 50 turns of wire

wrapped around it and when I close the

switch here about 3 amps flows through

there and you can see the compass needle

turns when I put current through there

so it's as if this piece of steel is

becoming a magnet when we put power

through and when we stop putting power

through it is no longer a magnet you

could almost say it's an electromagnet

right okay so we'll try it with a

different material let's try a piece of

brass okay I'm turning it on and off and

you can see it's the same three amps and

we'll zoom in on the compass so you can

see this a little more closely but

there's barely any movement okay now I'm

going to switch to another third type of

material this will be a piece of

stainless steel

it may be hard to see that the compass

is actually moving a little bit more

than the brass but less than the steel

so let me switch to a meter so we can

actually put some numbers on this

instead of just looking at a compass

moving so I've got this magnetic meter

that I got from Amazon and it reads off

in units of military and this is kind of

the sensor on the end of this probe here

a Tesla is a unit of magnetic flux

density so it sounds complicated but

really if you think about these field

lines connecting one side of the magnet

to the other the flux are the lines

themselves and the density is just how

close the packed they are together so

the Tesla is used as a unit of

describing kind of how you know

intimidating your magnet is you know

what met up an MRI machine is like a 1.5

Tesla or a 3 Tesla machine a large

neodymium magnet could be something like

half a Tesla at the surface and so

here's a couple of them stuck together

with a little space in between and if we

put the probe right on the surface we're

getting about you know 460 millet has

load so about half a Tesla at the

surface now the interesting thing is if

I turn the probe 90 degrees we get

almost zero in fact I can turn it to a

point where in fact it is zero and I'm

just turning the probe on the surface

here and this makes sense because this

probe only measures magnetic flux in one

direction this way you can kind of see

there's like a little dot on the on the

surface there and that's that's the

sensor itself and on the other two faces

there's no dot so it only measures in

this way and since the magnet is

polarized so that the North Pole is this

way if we put the probe so that it's

cutting through all those so that it's

lined up with the magnetic field then we

get the full reading but if it's 90

degrees off we get nothing so anyway so

about half a Tesla at the surface but if

we move away from the surface the

reading goes down really fast really

fast so for this far away and the reason

is that air has high reluctance to this

magnetic flux so even though the magnet

is trying to push a lot of magnetic flux

through the air the air is not very good

at conducting it and that's why this

drops off so quickly there's just not a

lot flowing so when you measure a magnet

you

have to specify where this flux

measurement is happening okay so let's

measure the little electromagnets that

I've made here I'll go back to the

sensitive scale adds a decimal point and

we'll put the probe here and it's

already measuring a teeny bit but now

I'll turn the current on and with the

current on it's doing about you know

10.5 Millett aslan okay let's try the

stainless steel little bit of residual

there with the current on about 1.4 mil

otezla and with it off about 0.5 and

with the brass 0.4 with almost no

residual so 0-2 0.4 so we can tell that

there's something about the material

itself that determines how good of a

magnet it becomes when we put it into a

coil of wire and run some current

through the coil so if we were gonna

graph that out it might look something

like this I should also mention that if

we put more current through the coil we

get more Tesla's out of the end of our

electromagnet so and instead of clicking

the button I'm going to turn it up

slowly on the supply and so if you watch

the current going up over here you can

see as we put more current through we

get more Tesla's out the magnet so we're

getting 30 mil of Tesla at about 8 amps

and we're getting about 20 mil of Tesla

at 5 amps and so on okay let's kind of

graph out what's going on here so the

the input to our system here is the

current through the wire and the output

of our system are the number of Tesla's

that we're getting out of our

electromagnet so when we put the probe

here and we turn the current off or

getting more Tesla's out okay it's

pretty good as it turns out the input to

this electromagnet is the number of

turns times the current that we're

putting through it divided by the length

of the coil and the reason that this you

know is the case is that one turn of

Electress of one turn at a certain

current is the same thing as two turns

at half the current right

you think about it if you're if we drew

a black box around this coil you could

not tell the difference if you were

inside the middle of the coil between

this two turns at one amp and one turn

at two amps it's the same thing so the

unit is actually pretty convenient it's

just amps times turns divided by meters

and the meters is like the length of the

coil and the reason that we throw that

in there is because this is actually

like the intensity of this magnetizing

field and so if this was spread out

among a whole meter yeah that makes

sense that's less intense than if we're

all crunched up over into one little

centimeter like this one is so we

include this length there because this

is actually the magnetizing intensity so

you've probably heard of this BH curve

before it's it's actually not that

mysterious but for some reason if folks

don't explain it sort of simply so I

hope this is making sense I mean it just

think of it as the y axis b which is the

symbol for this magnetic flux density is

dependent on how much current and how

many turns were basically putting into

it so how good of an electromagnet and

how much work are we putting into this

electromagnet is basically what this

graph is going to show so we saw these

three materials we had brass stainless

steel and steel and we saw that each one

of them has a different characteristic

so even with the same number of turns

the same current and the same length of

the coil on each one of these pretty

much the steel performed really well the

stainless steel was kind of intermediate

and the brass was was basically nothing

and I should point out that brass really

is pretty close to nothing the only

reason that we saw any fields coming out

at all is because because there's a

little bit of air actually in here I

mean the brass is not really playing

apart but let's let's not worry about

that too much just at the moment okay so

if we were gonna plot out the

performance of these coils we might have

a really good performer like this so

this is maybe the steel and then we've

got kind of an intermediate performer

and that's the stainless steel and then

we've got a really low performer and

that's maybe the brass

and what we want to do is basically come

up with a description you know a number

that describes how good these materials

are at conducting at becoming

electromagnets when we put them in a

magnetic field and as it turns out

that's basically the slope of this of

this graph and we call that mu so mu is

a description that it's one number that

describes how good these materials are

and actually is a fun side note there is

such a thing as a negative material the

set graph actually keeps going and these

are diamagnetic so air itself or even

empty space is actually on this graph

it's let's let's consider it basically

brass or air is pretty much the same

thing air an empty space or true vacuum

are basically the same for this for this

argument and it has a very very low

permeability but it's there it's

actually a universal constant it's just

a number and so to make things easy when

we're talking about steel or stainless

steel what we're doing is describing how

much better they are at conducting

magnetic fields than air or empty space

is so a lot of times you'll see mu with

a subset R meaning relative so mu

relative of steel may be like a thousand

and mu relative of air is one I mean

that's that's what we're measuring

relative to however when we do the

numbers later on we'll actually plug in

what the real value for air or empty

space is so just keep in mind that when

you hear permeability of a thousand or

two thousand or a hundred or whatever

that's just the relative permeability

compared to air or vacuum the actual

number is something like times 10 to the

minus 7 it's just not convenient to

throw around even though we're starting

to get further away from this electronic

analogy when understanding magnetics it

helps to keep a few things in mind one

is that in electronic circuits we're

almost always using copper as the

conductor so if you don't really care

about the material properties so much in

electronics because it's always copper

and so copper has a certain resistivity

to it it's a material property

and then if you want to flow you know an

amp through a wire you basically look up

on a table to see how big the wire

should be because we're not really

caring about you're not gonna switch to

a different material basically whereas

with magnetics choosing the material is

actually a pretty important part of it

and they each have different

permeabilities like we just said each

one of these has a different slope on

the graph another thing to keep in mind

is that there's multiple unit systems

unfortunately for magnetics but there's

a couple of things that will help you

out one is that the English system is

almost never used so you can't even

blame this on us Yankees because the you

know the American units or whatever are

almost never used so you can ignore

these then unfortunately it seems to be

about half and half split between the

CGS system which is centimeter Graham's

second and true SI units which is metre

kilogram second and if you look through

here you'll probably recognize some of

these different units for this video I'm

only going to use SI units but if you're

searching around on the internet and you

come across let's say Gauss just keep in

mind that there's a conversion factor

and a Gauss is the same as a Tesla

there's just a conversion you have to do

an over sted is the same as amp turn per

meter so in our previous chart here H

could be amp turn per meter and the

vertical axis is B and Tesla this could

be gouged over here and it's the same

concept the same everything it's just

you know just different units

unfortunately the conversion factors get

to be kind of weird because the geometry

comes into effect so 1 Tesla is 10,000

Gauss that one's pretty easy but the

conversion between some of these other

things are not so straightforward I

would recommend if you're cruising

around the internet reading about this

kind of stuff just focus on one unit

system and I would recommend SI units

but it's up to you and then when you get

really comfortable with what all these

different things are then you know just

do the conversion in your head or just

when you see something like Oersted you

just know that's amp turns per meter

with a conversion factor but until you

get a really good conceptual grasp of

all these different things I would

recommend sticking to one system

okay so I keep saying that the magnetic

circuit is just like the electric one

and we can do the same type of

resistance ohms voltage kind of analysis

on it so let's let's actually do it

we've got the simple one up here to

start with let's say we're given that

the voltage is 1.5 the EMF or

electro-motive force and we're also

given that the resistance of the bulb is

5 ohms pretty easy

the current is equal to the voltage over

the resistance 0.3 amps no problem now

the same kind of thing will happen for

the magnetic circuit one of our Givens

is that the MMF or the magneto motive

force is 250 amp turns and in this case

our coil has 50 turns and we're going to

put 5 amps through it so 250 next our

circuit has kind of two resistive

elements one of them is this steel bar

that makes a circuit through here and

the other element is the gap and the

total amount of reluctance because it's

that's basically magnetic resistance

will equal the sum of those two things

it's actually very easy just add them

together so we can calculate the

reluctance of the core or the steel part

and then the reluctance of the air-gap

add them together and that's our total

reluctance

so our Toth our core plus our gap and if

we look over at the unit's here

reluctance is actually denoted by this

stylized R and I really don't like the

the weird symbols and stuff because

again it just makes the material harder

to understand but there aren't too many

this is really it and this is as

advanced as the math is going to get in

this video so the reluctance for any

kind of material is the length of it

divided by the permeability times the

area and this is actually exactly the

same as in electronics it's just again

we always use copper and so we're not

really starting with the resistivity of

copper but if you have a really thick

copper wire that can carry a whole lot

more current it has a lower resistance

or if you have a short copper wire as

opposed to a long one that also has

lower resistance even though the

resistivity the intrinsic property of

doesn't change the same in both cases or

any case same thing here if you know the

permeability of your material the length

and the area are what determined the

reluctance are like the total resistance

to magnetic flux so let's do the corer

first by the way a core is just a

material that's good at conducting

magnetic fields so if you're building a

transform or a motor or something the

core is basically the metal that's

carrying this magnetic field so in this

case the core has a length of 324

millimeters and when you're doing the

math always use the right unit system I

would strongly recommend MKS so

everything in here is metre kilogram

second so instead of 324 it's point 3 to

4 which is the length from from here to

here and then the permeability of the

steel I'm estimating at 500 remember we

talked about relative permeability the

actual permeability of empty space

happens to be this number so 4pi times

10 to the minus 7 is just a universal

constant it's just the way it is it's

one of those very few universal

constants that defines how the whole

universe works and it's just there and

then the area is pretty straightforward

it's an 8 millimeter diameter and if you

calculate it out it's 5 times 10 to the

minus 5 so if you calculate all this out

it comes out to be 1 times 10 to the 7

and the units are a little weird if we

look over here the units for reluctance

our amp turns per favore or I'm gonna

say Weber because that's a little bit

more comfortable don't worry too much

about the unit's here just remember that

the reluctance is 1 times 10 to the 7

for this core ok now we'll do the same

thing for the gap but we're already

running into a problem what's the area

of the gap like if we were to cut the

gap with a plane through it

perpendicularly I mean it's it's

infinite area there's it's air I mean

there's no there is no yeah there's no

area to it so what we use is a

approximation and the way to do it is to

add the length of the gap the distance

to each of the dimensions in space to

calculate it out so since this is a

circle I'm going to use point zero zero

four four millimeters is the radius plus

0.03 which is the length squared times

pi to get the area if it were a square

cross-section you would add the length

to both the width and the depth and

multiply them together

and that's actually a very good

approximation and the reason we do that

is because the field lines sort of bow

out through this gap right like you've

you've seen in that with the iron

filings that the magnetic flux kind of

tends to bow out from these air gaps and

so we use this area approximation so if

we do the math for that one we get six

point six times ten to the six for the

reluctance of the gap and then just like

with the electric circuit the total flux

and the unit of this is the Weber and

you can remember this one because it

sounds like ampere so amperes vapors and

it's equal to the magneto motive force

divided by the total reluctance and we

know all these numbers and if we plug

them in we get 1.5 times 10 to the minus

5 this is great but it's it's actually

very tough to measure this in an

electric circuit it's easy to measure

the current we can just put an ammeter

in circuit and measure it or we could

even use a clamp meter but in magnetic

circuit it's very tough to measure the

total flux so instead what we want to do

is measure the flux density that's

something that I can measure directly

with the meter over here and to figure

out the flux density all we do is divide

by the area again so if we know the

total flux is 1.5 times 10 to the minus

5 and we want to know what the density

is we just divide by the area that's

pretty straightforward so be the flux

density

we'll use the area from the gap again

it's the same the same area here is here

and we got out for milites low not too

bad right let's try it out

so if we come over here we'll take the

compass out and put the probe in and we

can see there's a little bit of residual

it's coming up at about point four and

if I turn the current on lo and behold

we get about four mil otezla that's

pretty cool now it's true that if I move

the probe around like put it over here

it drops precipitously I mean it's only

about 0.7 in the middle if we go to the

other side it's about three so what this

is telling us is that our approximation

for the area is close but not great but

actually it's pretty good considering we

just went from Universal constants

measuring the geometry to a value that's

within you know even within 20 or 30

percent or even 50 percent is pretty

good pretty cool let's do another one

okay so in this one we've got a steel

ring and I put a hundred turns of copper

on there so this time we have the

magneto motive force of 500 because it's

a hundred turns at five amps so 500 the

total reluctance is still the reluctance

of the core plus the reluctance of the

gap and it's the same math as in the

previous one I'm still estimating the

permeability of the steel here at 504

for pi e negative seven is the

permeability of free space we end up

with a reluctance for the core and a

reluctance for the gap and one thing

that's interesting here this is 2.1

times 10 to the 7 this is three point

one times 10 to the 6 so that tiny

little air gap in this system actually

has 10 times the magnetic reluctance of

this whole steel core it's a really

important thing to keep in mind air is

500 times less permeable than steel than

a lot of steels and for some decent

materials used in transformers it's even

several thousand times so if you put an

air gap into your magnetic system most

of the reluctance and in a lot of cases

all of the you know reluctance that you

have to worry about is in the air gap so

anyway we do the the flux this is the

total amount of

magnetism flowing through the circuit

2.1 times 10 negative 5 vapors and since

we can't measure that we want to measure

B which is the flux density divided by

the area and we use that same trick to

estimate the area of the gap we get 188

Militello

so let's try it out now one thing you'll

notice is that we're already measuring

40 Millett as 'la and I'm not even

flowing any power through here so we'll

turn the power on and we're getting

about a hundred so you know it's it's

it's a little more than half of what we

estimated but like I say the point of

this is not to you know the video is not

to give you a lot of practice doing the

math or showing how rigorous this can be

if it's within a factor of two I'm very

happy since we're starting with

universal constants and geometry and so

actually getting even this close I think

it's pretty good however there is

something to notice here even without

any current flowing it mean the meter is

pretty close to zero it's reading you

know point six now and if I go into the

air gap without any current flowing it's

actually reading forty Militello so that

means that our graph here can't be quite

right because H is amped turns per meter

and if it's zero then that would mean we

shouldn't be getting any B at all so

this graph is actually slightly

inaccurate there's another couple of

factors to take into account here this

residual magnetism we keep seeing is

something you've probably experienced

yourself if you just take a brand new

plain old nail out of the package it has

a very slight influence on the compass

but not much whereas if we magnetized

the nail by putting it into a strong

magnetic field now it's quite strong or

much stronger than it was and so clearly

the steel nail has the ability to retain

magnetism and so if we were going to put

this back on the graph instead of just

having a linear relationship here the

actual relationship has this hysteresis

to it and so what this means is that

let's say we start off in the middle

here at zero magnetism so zero applied

field and also zero magnetism coming out

of our electromagnet

when we apply power it starts moving up

like we said it would and it eventually

saturates which we'll talk about in a

minute but then if we start reducing the

power we can come back to this point

here so that H is zero we're not

applying any magnetic field we're not

trying to magnetize it anymore but we

still have being we still have magnetic

flux coming out of the device then if we

start going negative so we basically

switch the polarity of our coil so if H

is amp turns per meter now we're giving

it negative amps or basically flip the

polarity it actually requires us to

apply this negative polarity field to it

in order to get back to zero B and then

we can keep going it's saturates again

and then comes back to this point or

over here where it's zero power and now

it's a magnet in the other direction and

so you've probably seen this BH curve

and by now in the video we've gotten to

this point where we're talking about it

maasai click hysteresis to it and all

magnetic properties have this there is

no such thing as a perfect magnetic

material as far as I know that only is a

straight line up here forever

well error is what I mean paramagnetic

diamagnetic materials have this linear

relationship that goes on forever but

any ferromagnetic core is going to have

this more of a shape and so I mentioned

saturation what this means is that we

can keep trying to make a better and

better electromagnet by adding more

turns or pushing more current through

our coil but what ends up happening is

we only get a certain amount of B out so

for you know measuring this

electromagnet with our meter here like

this and we keep putting more and more

current in eventually the graph will

slide down into a flatness here and no

matter how much current we put through

we don't actually make it a better

electromagnet and this is a material

property as it happens for a lot of

normally encountered magnetic materials

like electrical steel and all this the

saturation point is about 1.5 tesla so

getting above 1.5 tesla is difficult

very difficult because we just don't

have materials that can carry that much

magnetic flux

whenever you encounter a magnet that's

higher than 1.5 tesla like a MRI machine

is commonly 3 Tesla you have to use

other technology like superconducting

coils or or just quote a lot of current

with coils close together

remember if we put if the coils are

close to each other like this and we put

our probe in the middle here even if

this is air remember air still has some

amount of permeability it's just that we

start losing the the help from the steel

so if we're trying to make a really good

magnet using steel works up to 1.5 tesla

and then beyond that we have to use just

air or superconducting coils basically

okay in this setup we have an isolated

variac with its output connected to

these clips and it goes through a clamp

meter this measures the current going

through here and then there's a hundred

turns on this steel core with an air gap

and the air gap has the probe for the

magnet meter the Tesla meters stuck in

there and I've modified the Tesla meter

so that its output I added this jack to

the side so its output actually goes

into the oscilloscope and the output of

the current meter also goes into the

scope so basically we're just going to

plot magnetic field in the gap versus

current and let's see how that looks

okay so I'm going to start increasing

the current and at this low level you

can see we've got current versus time

here the magnetic field versus time here

and then this is the XY plot so we've

got milites 'la and the y axis and amps

on the x axis this is pretty much the BH

graph so we're gonna turn it up and you

can see something starts to happen I'm

gonna stop the scope so that I can turn

the power off and we'll retain it here

you can see that there's definitely this

saturation effect right at about 120 mil

of Tesla so the thing behaves linearly

up to a point and then breaks over and

becomes pretty flat and when a current

is 0 we actually have quite a bit of

retained magnetic field in fact it's

about 80 Millett as low in this set up

and you can see the current is pretty

higher this is topping out at about 17

amps

if we scroll through here you could

figure out exactly how many amps it

takes to saturate this material another

very important thing to keep in mind is

that the this whole system is dominated

by the air gap remember how we said that

the reluctance of the air gap is much

higher than the reluctance of the

material itself if we wanted to use this

set up to measure the material itself

without the influence of the air gap we

have to come up with another scheme of

doing it

if we use the air gap to basically

insert our Tesla meter in there then we

affect the whole system and what we're

actually measuring is the saturation of

the whole system including the air gap

so steel saturates much higher than 120

milli Tesla the only reason that it's

showing up this way on our graph is

because of that air gap so let's go back

to the set up and see if we can figure

out another way to do this if we add

another little coil to this metal ring

we've kind of made ourselves a simple

transformer so we've got the primary

over here putting the magnetic field

into this steel ring and then we've got

another coil over here that we're using

as a sensor and we just said that this

whole thing saturates it you know 150

militares or something like that what

that would mean is that when we get up

to the saturation point we shouldn't get

much additional output out of this

secondary coil this is why saturation is

typically bad in your transformer design

so we're putting more and more current

in here but we're not getting any more

magnetic field out because the the

system has been saturated so what we're

going to do is add an oscilloscope probe

to the second coil and then look at the

voltage while we do the same experiment

and see what we can get with that okay

so we're still looking at B with the

Tesla probe here and then current going

into the primary coil of our new little

transformer and then this sense voltage

is the output of our transformer so you

can see I can scale up and down and

everything is still pretty much the same

so I'm going to scale up and then stop

just sweet so I don't have to run

current through there since we're doing

you know 17 amps that thing heats up

pretty quick so I don't like to leave it

on for too long we can see the same

phenomenon same saturation and

everything and this is the voltage that

we're getting out

the transformer are secondary basically

and it's horribly distorted the voltage

we're putting in is pretty close to a

sine wave and what we're getting out is

this horrible thing here now the trick

is how do we use this to figure out what

the magnetic flux is in the coil so up

until now we've only been talking about

currents in magnetic fields this is the

first time that voltage is cropped up

and as it turns out the voltage is

proportional to the change in magnetic

field with respect to time and that's

just sort of another universal law to

sort of deal with but it's convenient

that we can just measure the voltage and

then figure out what the magnetic field

is doing in there and since I said that

the voltage is proportional to the

change in magnetic field what we want to

do is mathematically integrate this now

we aren't going to do the math ourself

we're actually going to use the

oscilloscope to do it so I'm going to

turn on the math trace and we can edit

it and with a formula that I'm going to

use is the integral of channel eight

which is the voltage that we're picking

up from that sense coil plus the voltage

on channel one and channel one is just

this very simple voltage divider just a

potentiometer and the reason that I need

that is without this offset control the

integrator is going to drift and I'll

show you what I mean in a minute

so we'll say that's fine okay and

currently this XY plot is plotting

channel three on the x-axis which is

current going into the transformer and

channel 2 on the y-axis which is the

direct measurement of the magnetic field

with the Tesla probe however if we

change the y-axis to be a mathematical

calculated field look at that it's

almost exactly the same will flip back

and forth so that's the actual measured

field and chattin the math channel which

is the integral of the sense voltage is

this almost the same thing it's actually

working it's always nice when things

work it's kind of surprising sometimes

now the scale is not set up properly

this has units of micro volts micro volt

seconds but the cool thing is that since

we have the magnetic field probe in

there and we know that the flux is the

same at all points in the circuit

because it's a circuit so the flux is

necessarily the same kind of everywhere

and the area is kind of almost the same

in the gap as the core we can use this

to calibrate our system and then we can

actually use this thing to measure the

real magnetic permeability of any

material we want if we can make it into

a ring or make it into a transformer

here's what I mean about the integrator

drifting so I've just got this

potentiometer here that's feeding a tiny

little voltage into channel one we can

put it up there it's it's just a flat

looks looks varying it's very small that

you can see it goes up and down when I

turn the potentiometer and it's the the

integration that we set up is basically

adding channel one to this and so if it

integrates a constant it basically just

splays out but we don't want this and so

we kind of tune the pot in to give us a

really decent looking eh curve I thought

there might have been a way to do this

in the scope itself but as it turns out

this is actually easier because I have

really fine control over it and it's

it's easier to just dial it in like that

and then we can go up and current again

you can see it's it's starting to

separate a little bit here so you can

use this to kind of dial it in just

right and then another interesting quirk

is that to get the to get the plot

centred on 0 I'm using the level trigger

and triggering off the sense voltage

that comes back and so we want this

thing to trigger it basically just the

right level so that it's plotting the

zero at just the right way okay let's

take a look at another material this is

a ferrite transformer it's two separate

coils found on there and previously we

were looking at this steel ring that I

made myself but this is the real ferrite

transformer that's purpose-built now we

can't put the probe into the gap anymore

because there is no gap so we just

connected the input power to here and

we're going to use our sense and

integrate technique on that side to see

what this looks like okay here we go I'm

gonna start increasing the power and

then I'm gonna stop the oscilloscope so

I can turn the power down and let's see

what we've got here first you'll notice

that the BH curve looks quite a bit

different than

it did for the steel some things to

notice the remain the retained magnetism

when the power goes to zero is actually

quite a bit less than a steel so the

last vocabulary word for the video is

the coercivity of a material which

describes how much magnetism it retains

when the field has been turned off

so this BH curve actually gives us quite

a lot of information about the material

1 the permeability which is the slope of

the line likely you know what we saw in

the earlier part of the video the

saturation which is where this thing

levels off and you just can't get any

more magnetism out of it and the

coercivity which is how much magnetism

remains when you cross 0 another

interesting thing that you can get from

this graph is the area inside this curve

if you add up all the area inside here

this describes how lossy the transformer

is based on the core losses you have to

basically spend energy to make this core

into a magnet and then you flip back the

other way each time each cycle so if

you're feeding this thing AC and it's

taking a lot of energy to create a

magnet then you have to switch

everything around and undo everything

you did see for a transformer you

typically want something with a low

coercivity a high permeability and are

really high saturation the problem is

that you can't always get everything

that you want but you can get what you

need and so for example in low frequency

transformers the best material choice is

typically steel it has a high saturation

it the coercivity is reasonably high but

that's kind of ok because it only flips

polarity 60 times a second which is

relatively low so even though we waste a

little bit of energy with this

coercivity problem the fact that the

saturation is really high and the

permeability is also quite high that

ends up being the best material choice

for a transformer that operates at 50 or

60 Hertz now if for higher frequency

transformers like this little toroid

this could be designed to work it may be

10 to 100 kilohertz or something like

that

and in this case that high coercivity

would be a really big problem at at 10

or 100 kilohertz we'd be wasting so much

power that you wouldn't really be a good

material choice and so instead these

little toroid x' are often made of

ferrite and ferrite has a lower coercive

'ti as we saw on the oscilloscope so

even though the saturation is also lower

when you do all the math and figure out

how much material would I need how much

does it cost how hard is it to process

ferrite ends up being a better choice

for high-frequency transformers and

steel ends up being the best choice for

low frequency transformers also don't

confuse the shape with the material

choice like for example this is a toroid

but it's actually a steel core so is

this right this is a thing that I cut up

myself but they make toroidal

transformers out of Steel

it just happens to be that most of these

smaller high frequency transformers are

toroidal and made out of ferrite there

are also ferrite transformers that are

not toroidal for example this one so

what's the difference between a toroid

and Ecore or something that's not a

toroid if these are both ferrite

transformers why would I pick one or the

other

again it comes down to more of a

manufacturing cost balance type thing

winding a toroid is actually relatively

expensive if you if you're holding a

roll of wire try to think about the

difficulty and sort of winding it around

the Tauri it's actually not that easy

you need a specialized winding machine

the benefit is that a toroid has no air

gaps and it's essentially self shielding

like all the fluxes contained within

that core

whereas this Ecore has some sharp edges

and you have to join it together and so

there might be a gap and again it's it's

really kind of more of a manufacturing

thing don't think that toroid x' and

other types of transformers are all that

different it's just a different way to

accomplish the same thing there's also

quite a few different kinds of ferrite

teenies are marked 43 and then these

have a little sticker on there and this

one is painted oftentimes the toroid

that are painted like this one are

actually not ferrite it's powdered iron

so they take

iron powder and mix it with epoxy and

form it into a toroid and you know these

things are all relatively similar

usually the differences involved how

well it works at a specific frequency

range so for example maybe this is

really great at a megahertz and this

other one is really great at a hundred

kilohertz but really it all comes down

to the BH curve diagram so they have

differing amounts of coercivity and

differing saturation points and

different permeability but it has this

additional problem of the BH curve being

dependent on the frequency at which you

test it and so not only do all these

variables exist they also change

depending what frequency you're putting

through this thing which is why

magnetics get to be fairly complicated

and then of course if you're using a

square wave that's a combination of

frequencies and so you can get

complicated right in a hurry let's

finish up by taking a look at a couple

unusual magnetic things that you might

find in everyday items this is a flyback

transformer from an old CRT television

set and you'll notice that there's a

little gap in here it's a ferrite core

but there's an air gap in here and at

first you might think well it's you know

it's just manufacturing tolerance they

obviously snapped this thing together

and didn't quite make it but actually

this is intentional you can see they

even put some glue over here to set the

gap thickness very precisely and the way

that they designed this thing they

actually want there to be some extra

reluctance in the core at first you

might think that's pretty dumb why

didn't they just use a smaller core that

would achieve the same thing right as it

turns out not quite because the air gap

the reluctance in this air gap helps

this thing store power on each cycle

that goes through and the way that a

flyback transformer works is that you

want to like charge up the magnetic

field and then you stop charging it up

and the magnetic field collapses and

goes into the other coil that's in here

I think the purists would claim this is

actually a coupled inductor not really a

transformer because of the way it's used

but the trick is that the air gap

actually allows it to store energy

between cycles and that's how it works

you might find other transformers that

have careful

controlled air gaps as well not the most

common thing in the world now here's one

so this is actually an iron core in fact

this is an inductor this is not a

transformer because it's only got two

leads but you can see they put some wood

or some cardboard or something in here

to make an air gap even in this iron

core again because they didn't want too

much magnetic field in here this adds

reluctance and prevents the field from

getting too high another way to achieve

this which is pretty weird this is a

microwave oven transformer and this is

the primary and this is the secondary

and if you look in there you can see

there's actually metal that's shorting

out the magnetic field so if we were

going to draw the magnetic field circuit

diagram for this thing you would

actually have a short in there and if

you knock those pieces out this is what

they look like

it's a shunt a magnetic shunt and it

serves the same purpose of putting a gap

in these transformers you can sort of

control how much magnetic field goes

through the transformer and the point of

these shunts is to limit how much

current you can get out of the secondary

the way that a microwave oven works is

it tries to pull a large amount of

current out of this coil and it relies

upon the transformer doing the current

limiting and that current limiting is

achieved by bleeding off some of the

magnetic field and making the

transformer less effective as it gets up

into higher currents even though it

seems like the ideal material would have

no power loss sometimes that's actually

exactly what you want in an application

like this these little ferrite beads are

clipped on to this USB cable with it

here this one clips on but these are

molded on and the idea is that the

ferrite will prevent high frequency

noise from going through here and that

and actually if this is lossy that's

even better because we want to get rid

of this high frequency noise we don't

want to couple it to anywhere so

sometimes you actually want a material

that is purposefully lossy because we

want to get rid of that high frequency

signal which is interference in this

case

in the case of magnetic storage media

like this we actually want a material

with a really high coercivity right

because we want to set the magnetic

information in here and then have it not

change until we're ready to set it again

so these typically have higher

coercivity and the you know the

permeability and that sort of thing

doesn't really matter that much in this

case so the point is that depending what

you're doing with magnetics you can

search for a BH diagram that sort of

meets your need and then make sure that

the frequency that you're running at you

get that BH characteristic at that

desired frequency

IFS of course a whole lot more to cover

but I think this actually does cover all

the aspects of magnetic circuit design

and I hope some of this has made sense

and made it easier for you to get

started in the field once you start

reading on the internet you'll find that

people try to dump the math on you and

it's not necessary I think it's really

much better to sort of keep things

conceptual until you really really need

the math for a specific reason and then

you can dive in and do the heavy duty

analysis okay see you next time byefree money earning sites, money earning websites, get money online, online earning tips, online earning without investment, earn money online without investment for students, earn money by clicking ads, earn money online without investment, online earn money website, online jobs to earn money, best online income site, top 10 online money earning sites, easy income online,