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Inductor saturation current

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Dr. Dark Current

Tue Jul 08 2008, 09:36AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Lets say I have a gapped ferrite inductor choke, I know core material, gap size, core cross-section and number of turns. How do I calculate saturation current of the inductor?
[this is for choke design of current fed inverter]

Dr. Slack

Tue Jul 08 2008, 02:14PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

ignore the cross section (not needed, as long as airgap W and H are >> airgap length)
and assume the core has infinite permeability (for a good first approximation)

find the Bsat from the material specification (varies with frequency and acceptable power loss, choose an appropriate value)

compute the Hsat.air needed to get the Bsat across the airgap using u0 = 4piE-7

divide by the length of the gap to get ampereTurns (a proportionally large measurement error can come in here)

divide by the number of turns to get amps

As a minor refinement, you can add the Hsat.core to the Hsat.air needed to get Bsat through the length of the core, but it will be small compared to the airgap, and ignoring it gives an error on the safe side

tesla500

Tue Jul 08 2008, 07:25PM

Registered Member #347 Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106

In equation form:
B = H * u0
B = I * n / l * u0

where
B = flux density (Tesla)
H = magnetizing force (Amperes per meter)
I = current through winding (Amperes)
n = number of turns on winding
l = total length of core gap (meters)
u0 = permeability of free space (Henries per metre) = 4*Pi*10^-7

Solving for I:

I = B * l / (n * u0)

So, knowing the maximum flux density B for your core material, and the winding specs, you can compute the maximum current.

David

Dr. Dark Current

Tue Jul 08 2008, 08:25PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thanks

Edit- my core is of unknown material from SMPS, from what I found online I used Bsat=0.3T, does this sound about right?

GeordieBoy

Tue Jul 08 2008, 09:26PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

If you are not sure of Bsat I would err on the cautious side, because gapped ferrite tends to saturate more abruptly than the gradual loss of permeability that you typically see with iron-powder.

Also remember to use the peak current in the calculations. That is the worst case DC bias plus the positive AC ripple contribution if it is for something like a buck choke or DC link choke that has a +/- ripple component.

By-the-way if you are totally unsure of the material properties, dimensions and the air-gap, there is a test that you can use to find the approximate onset of DC saturation for "junk box chokes". I'm sure the power electronics guys on here can describe it in more detail but it basically involves discharging a large capacitor across the choke and monitoring the rising current using a CT, current probe or Rogowski coil. Current initially rises linearly at a rate determined by the applied voltage divided my the inductance, then the current takes off at a much steeper slope as saturation sets in and inductance falls. You can can approximate the position in the current ramp where the gradient changes.

-Richie,

Dr. Slack

Wed Jul 09 2008, 07:00AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Bsat=0.3T sounds fairly reasonable for low freuqency operation.

I was a bit inaccurate in my first post saying that Bsat varied with operating frequency, it's the manufacturer's recommended Bmax that varies, and it's to do with the power dissipation due to going round the hysteresis loop. For reasonable ferrites, you should be able to work all the way up to Bsat at frequencies to 100kHz without the core getting too warm, derate Bmax at frequencies above that.

Steve Ward

Wed Jul 09 2008, 07:15AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

For reasonable ferrites, you should be able to work all the way up to Bsat at frequencies to 100kHz without the core getting too warm, derate Bmax at frequencies above that.

Most power ferrites ive looked at suggest derating above 20khz, but these were strictly big power ferrites, where you are less likely to run really high frequencies.

Ive tried the ramping input current to watch for gapped ferrite saturation. The current was switched from a big lytic with an IGBT. I personally have done this with special high bandwidth hall-effect type current sensors (so i can do DC and AC up to 300khz BW).

Dr. Dark Current

Wed Jul 09 2008, 07:29AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Thanks guys. Though i have no way of looking at single "transient" events with my tube scope

I think I'll start a new thread as I'm lost in the dc link inductor design...

Steve Conner

Wed Jul 09 2008, 09:26AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Just use a square wave instead of a single pulse, then. I've used a square wave from a signal generator boosted by a 100w audio power amp for this purpose before. And a low value resistor in series with the choke works fine for current sensing. You can use the same setup to measure ESR of capacitors.

uzzors2k

Tue Jul 15 2008, 10:21AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Quick question: The total air gap length in a U or E core set is the summed length of every air gap in the core, not just the one air gap on the arm which the winding is placed on?

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