inductor sizing

[Jamie Morken]

Hi,

For a filter inductor in an LC filter, does the inductor size (core
volume) scale linearly with inductance * current rating^2, all other
variables the same, ie. core material, frequency etc.

Also I need some toroid cores for 200kHz LC filters, 250uH and 50Amps
peak current, are these going to be huge cores and what core materials
would be best to use, ferrite or powder cores?

I've downloaded the www.magneticsinc.com "Magnetics Inductor Design
Using Powder Cores" program:

http://www.magneticsinc.com/software/inductor.asp


cheers,
Jamie

 

[John Larkin]

Hi,

For a filter inductor in an LC filter, does the inductor size (core
volume) scale linearly with inductance * current rating^2, all other
variables the same, ie. core material, frequency etc.

That's stored energy, which should be proportional to core volume.

Also I need some toroid cores for 200kHz LC filters, 250uH and 50Amps
peak current, are these going to be huge cores and what core materials
would be best to use, ferrite or powder cores?

I've downloaded the www.magneticsinc.com "Magnetics Inductor Design
Using Powder Cores" program:

Iron powder cores tend to burn their paint off in situations like
this. Permalloy powder is very good but expensive. The "Kool-Mu" cores
are almost as good, but cheaper. Gapped ferrites are OK here, too.


John

 

[D from BC]

Hi,

For a filter inductor in an LC filter, does the inductor size (core
volume) scale linearly with inductance * current rating^2, all other
variables the same, ie. core material, frequency etc.

Also I need some toroid cores for 200kHz LC filters, 250uH and 50Amps
peak current, are these going to be huge cores and what core materials
would be best to use, ferrite or powder cores?

I've downloaded the www.magneticsinc.com "Magnetics Inductor Design
Using Powder Cores" program:

http://www.magneticsinc.com/software/inductor.asp


cheers,
Jamie

Huhhh..dunno about that 1st question.
I'm no magnetics expect...but will take question #2 for $2 million..

Let's say you're filtering a DC supply with an LC filter.
You have 50 amps DC passing through the 250uH inductor.

By Ampere's law. H= 0.4*Pi*N*I/Le
CGS units
Le = mid circumference of the core
N = turns
I = amps

The field intensity H * ur = B flux in gauss
ur = relative permeability
(Note: ur varies, ~2000 for some ferrites)

If B>>Bmax. core spec (around 4000 gauss for some ferrites) ...that
inductor acts like a paper weight (resistor + an air core L).

My guess (without doing the math) is that you'll DC saturate a ferrite
and will end up choosing a powdered core.

As for powdered size, the Magnetics Inc program might select a size
you like.


D from BC
British Columbia
Canada.

 

[John Fields]

That's stored energy, which should be proportional to core volume.



Iron powder cores tend to burn their paint off in situations like
this. Permalloy powder is very good but expensive. The "Kool-Mu" cores
are almost as good, but cheaper. Gapped ferrites are OK here, too.

---
ISTM that, after the spec's were defined and the questions were:
"are these going to be huge cores and what core materials would be
best to use, ferrite or powder cores?", your response addressed
neither question and, while seemingly designed to assert your
guruness in matters magnetic, did little to help the OP...

 

[John Larkin]

---
ISTM that, after the spec's were defined and the questions were:
"are these going to be huge cores and what core materials would be
best to use, ferrite or powder cores?", your response addressed
neither question and, while seemingly designed to assert your
guruness in matters magnetic, did little to help the OP...

Get a life, Fields.

John

 

[D from BC]

Huhhh..dunno about that 1st question.
I'm no magnetics expect...but will take question #2 for $2 million..

Let's say you're filtering a DC supply with an LC filter.
You have 50 amps DC passing through the 250uH inductor.

By Ampere's law. H= 0.4*Pi*N*I/Le
CGS units
Le = mid circumference of the core
N = turns
I = amps

The field intensity H * ur = B flux in gauss
ur = relative permeability
(Note: ur varies, ~2000 for some ferrites)

If B>>Bmax. core spec (around 4000 gauss for some ferrites) ...that
inductor acts like a paper weight (resistor + an air core L).

My guess (without doing the math) is that you'll DC saturate a ferrite
and will end up choosing a powdered core.

As for powdered size, the Magnetics Inc program might select a size
you like.


D from BC
British Columbia
Canada.

Oops..have to fix a few things..

H=(0.4*Pi*N*I)/Le

And...
ur decreases especially on the knee to saturation.

Like biasing a transistor, best not to over bias.
There has to be useful permeability for your target inductance.
I've seen some app sheets suggest operating a ferrite toroid at
1/2Bmax.


D from BC
British Columbia
Canada.

 

[John Larkin]

Hi,

For a filter inductor in an LC filter, does the inductor size (core
volume) scale linearly with inductance * current rating^2, all other
variables the same, ie. core material, frequency etc.

Also I need some toroid cores for 200kHz LC filters, 250uH and 50Amps
peak current, are these going to be huge cores and what core materials
would be best to use, ferrite or powder cores?

I've downloaded the www.magneticsinc.com "Magnetics Inductor Design
Using Powder Cores" program:

http://www.magneticsinc.com/software/inductor.asp


cheers,
Jamie

Why do you need that much inductance? 250 uH at 200 KHz is an
impedance of 300 ohms, which seems a lot for a 50 amp circuit.

John

 

[John Fields]

Get a life, Fields.

---
There once was a fellow named Larkin
who thought that to him all should hearken.
Without thinking twice,
accept his advice,
not criticize, lest his thoughts darken.

 

[Jim Thompson]

---
There once was a fellow named Larkin
who thought that to him all should hearken.
Without thinking twice,
accept his advice,
not criticize, lest his thoughts darken.

JF is a poet but doesn't know it ;-)

...Jim Thompson

 

[John Larkin]

---
There once was a fellow named Larkin
who thought that to him all should hearken.
Without thinking twice,
accept his advice,
not criticize, lest his thoughts darken.

Stick to designing 555 circuits; your career as a poet doesn't look
promising.

John

 

[Harry Dellamano]

Hi,

For a filter inductor in an LC filter, does the inductor size (core
volume) scale linearly with inductance * current rating^2, all other
variables the same, ie. core material, frequency etc.

Also I need some toroid cores for 200kHz LC filters, 250uH and 50Amps
peak current, are these going to be huge cores and what core materials
would be best to use, ferrite or powder cores?

I've downloaded the www.magneticsinc.com "Magnetics Inductor Design
Using Powder Cores" program:

http://www.magneticsinc.com/software/inductor.asp


cheers,
Jamie

To properly design that inductor more info is needed, such as; Iac (For
core, proximity and skin effect losses), Idc (For I^2R losses), Ipk (for
core max flux density) and E*T (core saturation) along with the operating
frequency already given. Max ambient temp, inductor temp rise and size
constrains would be helpful.
Given that we can then offer an opinion.
Cheers,
Harry

 

[John Fields]

On Wed, 05 Mar 2008 09:00:36 -0600, John Fields


JF is a poet but doesn't know it ;-)

 

[Tom Bruhns]

That's stored energy, which should be proportional to core volume.





Iron powder cores tend to burn their paint off in situations like
this. Permalloy powder is very good but expensive. The "Kool-Mu" cores
are almost as good, but cheaper. Gapped ferrites are OK here, too.

John

My question here would be something along the lines of "why on
earth do you need 250uH at 200kHz and 50 amps??" To me it implies
either a huge amount of power (i^2*L*f/2; at rather high voltage), or
a poorly/strangely designed filter system. If it's lots of power, you
should of course expect to be using relatively large parts, and it's
worth thinking very carefully about the design of the inductor and the
system into which it fits--probably not something to just jump into
without experience at lower power first. -- I wonder what it is that
Jamie is actually trying to accomplish.

Cheers,
Tom

 

[Jim Thompson]

Yep ;-)

...Jim Thompson

 

[Jamie Morken]

To properly design that inductor more info is needed, such as; Iac (For
core, proximity and skin effect losses), Idc (For I^2R losses), Ipk (for
core max flux density) and E*T (core saturation) along with the
operating frequency already given. Max ambient temp, inductor temp rise
and size constrains would be helpful.
Given that we can then offer an opinion.

Hi,

The inductor is for generating 120VAC at 60Hz:

http://rocketresearch.nekrom.com/ne...inverter/240VAC 60Hz split phase inverter.jpg

I have a simulator of the PWM controller for this circuit, if I set it
to output 3.6kW at 120VAC, with a 250uH inductor and 10uF cap I get this
waveform: (THD + noise = 1.7%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 1.7percent THD.jpg

With an inductor RMS current of 28.5amps and peak current of 45Amps
and capacitor RMS current of 1.8Amps and peak of 5.2Amps.



If I change the inductor to 50uH and the cap to 100uF,
I get this waveform: (THD + noise = 2.4%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 2.4percent THD.jpg

With an inductor RMS current of 32amps and peak current of 86Amps
and capacitor RMS current of 17Amps and peak of 46Amps.

This larger 100uF cap is required with the smaller inductor or else
there is way too much ripple. Also this 100uF cap will be expensive
as it has a large current rating requirement.

It seems like a tradeoff, if going for the 250uH/45Amp inductor,
what toroid material would be good and still economical to use?

I am planning on ordering some various toroids so I can experiment with
winding them and stacking them if necessary, so am looking for the best
core types to be using for this.

cheers,
Jamie

 

[D from BC]

Hi,

The inductor is for generating 120VAC at 60Hz:

http://rocketresearch.nekrom.com/ne...inverter/240VAC 60Hz split phase inverter.jpg

I have a simulator of the PWM controller for this circuit, if I set it
to output 3.6kW at 120VAC, with a 250uH inductor and 10uF cap I get this
waveform: (THD + noise = 1.7%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 1.7percent THD.jpg

With an inductor RMS current of 28.5amps and peak current of 45Amps
and capacitor RMS current of 1.8Amps and peak of 5.2Amps.



If I change the inductor to 50uH and the cap to 100uF,
I get this waveform: (THD + noise = 2.4%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 2.4percent THD.jpg

With an inductor RMS current of 32amps and peak current of 86Amps
and capacitor RMS current of 17Amps and peak of 46Amps.

This larger 100uF cap is required with the smaller inductor or else
there is way too much ripple. Also this 100uF cap will be expensive
as it has a large current rating requirement.

It seems like a tradeoff, if going for the 250uH/45Amp inductor,
what toroid material would be good and still economical to use?

I am planning on ordering some various toroids so I can experiment with
winding them and stacking them if necessary, so am looking for the best
core types to be using for this.

cheers,
Jamie

F type ferrite from Magnetics Inc starts into saturation around 1/2
oersteds.
You can't even get 1 turn on it with 30 amps DC!

0.5 = (0.4*Pi*1*30)/le

The math gets horrid..
A 75 centimeter magnetic pathlength... Ewww

However, I'm just learning this stuff myself..
Maybe there's still some useful inductance in saturation.


D from BC
British Columbia
Canada.

 

[John Fields]

Stick to designing 555 circuits; your career as a poet doesn't look
promising.

 

[D from BC]

I saw Beowulf last night..cool graphics, Jolie is hot but I did wayyy
too much fast forwarding.

Here's my version of Beowulf

King makes noisy party.
Next door neighbor Grendal gets pissed, shows up, eats people.
Grendal doesn't eat the King even when taunted. Mystery.
Hired gun Beowulf shows up.
Beowulf gets to boink the kings wife if he kills Grendal.
Beowulf kills Grendal.
Beowulf boinks kings wife.
Grendal's mom gets pissed, shows up, kills people.
Beowulf visits Grendal's mom.. What a hot milf!
The deal. Beowulf boinks Grendal's mom and her magic makes Beowulf
'Super King'.
(Grendal's mom likes popping out human/monster hybrid kids.)
All's well until some jerk steals the horn from Grendal's mom.
Her new son goes postal.
Beowulf kills that one too.
Beowulf dies.
Successor to the throne wants some xxx with Grendal's mom. What a hot
milf!
And the cycle continues..


D from BC
British Columbia
Canada.

 

[Harry Dellamano]

Hi,

The inductor is for generating 120VAC at 60Hz:

http://rocketresearch.nekrom.com/ne...inverter/240VAC 60Hz split phase inverter.jpg

I have a simulator of the PWM controller for this circuit, if I set it to
output 3.6kW at 120VAC, with a 250uH inductor and 10uF cap I get this
waveform: (THD + noise = 1.7%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 1.7percent THD.jpg

With an inductor RMS current of 28.5amps and peak current of 45Amps
and capacitor RMS current of 1.8Amps and peak of 5.2Amps.



If I change the inductor to 50uH and the cap to 100uF,
I get this waveform: (THD + noise = 2.4%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC with 2.4percent THD.jpg

With an inductor RMS current of 32amps and peak current of 86Amps
and capacitor RMS current of 17Amps and peak of 46Amps.

This larger 100uF cap is required with the smaller inductor or else
there is way too much ripple. Also this 100uF cap will be expensive
as it has a large current rating requirement.

It seems like a tradeoff, if going for the 250uH/45Amp inductor,
what toroid material would be good and still economical to use?

I am planning on ordering some various toroids so I can experiment with
winding them and stacking them if necessary, so am looking for the best
core types to be using for this.

cheers,
Jamie

Jamie,
Your Vout + PWM waveforms confuse me. The frequency of the output and PWM
is about 0.35Hz. There is no PWM just a square wave and the filtered output
is shifted 0 or 360d and at that amplitude and phase cannot be the filtered
output.
Please advise,
Harry

 

[Tom Bruhns]

Hi,

The inductor is for generating 120VAC at 60Hz:

http://rocketresearch.nekrom.com/new/split phase 240VAC inverte...

I have a simulator of the PWM controller for this circuit, if I set it
to output 3.6kW at 120VAC, with a 250uH inductor and 10uF cap I get this
waveform: (THD + noise = 1.7%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC wit...

With an inductor RMS current of 28.5amps and peak current of 45Amps
and capacitor RMS current of 1.8Amps and peak of 5.2Amps.

If I change the inductor to 50uH and the cap to 100uF,
I get this waveform: (THD + noise = 2.4%)

http://rocketresearch.nekrom.com/new/120VAC THD test/120VAC wit...

With an inductor RMS current of 32amps and peak current of 86Amps
and capacitor RMS current of 17Amps and peak of 46Amps.

This larger 100uF cap is required with the smaller inductor or else
there is way too much ripple. Also this 100uF cap will be expensive
as it has a large current rating requirement.

It seems like a tradeoff, if going for the 250uH/45Amp inductor,
what toroid material would be good and still economical to use?

I am planning on ordering some various toroids so I can experiment with
winding them and stacking them if necessary, so am looking for the best
core types to be using for this.

cheers,
Jamie

?? Seems to me a reasonable starting point would be about 5uH and
10uF (comprising perhaps ten 1.0uF or twenty 0.47uF high quality
polypropylene caps) as a first stage out of the FET bridge, followed
by perhaps 1uH and another 10uF (which doesn't have nearly as high a
ripple current), to yield a fraction of a volt p-p 200kHz ripple at
the output, assuming a 3 ohm load and 150V out, input switching to
+200 for 4.3us and -200 for balance of the 5usec cycle. Looks like it
does OK through the range of duty cycles needed to generate the output
60Hz sinewave. Though the first-stage capacitor has to handle a lot
of ripple current, I'll bet you'll find that doing it this way, with a
couple stages, will be cheaper by far than a 250uH inductor that meets
your needs. I believe Wima MKP10 caps would do OK in this
application. If you have frequencies lower than 200kHz on the output,
I'd recommend solving that with a better control system.

Perhaps I don't understand just what you are doing, but at least the
way that's obvious to me to generate a "clean" 60Hz wave from the
partial schematic you provided suggests to me that you don't need
anywhere near 250uH to do what you want.

Cheers,
Tom