Rated current of a common mode choke

[Mathhhew]

TL;DR

I'd like to know the maximun value of current that can go through my coil

Hello!

I've obtained common mode choke from detached pieces, so I don't know anything about them, exept for their inductance/resistance.
I'd like to know how could I determine the rated current of these coilsthank you in advance for your help

 

[berkeman]

Are you having problems with your previous account where you asked about this? The Mentors can try to help you fix any account problems...

I've seen on the internet modellisation of the perturbation with Norton equivalent sources, but it seemed complicated... Is your method enough to generate enough common mode current?
For the ferrit toroid, I'm asking arround what we have in stock

 

[Mathhhew]

Are you having problems with your previous account where you asked about this? The Mentors can try to help you fix any account problems...

Yes and no because that's my friend's account, I can't connect to it ...

 

[berkeman]

Oh, so you are two people with the two accounts? That would be okay, but still confusing that you are asking similar questions. Are you both working on the same project?

 

[Mathhhew]

Oh, so you are two people with the two accounts? That would be okay, but still confusing that you are asking similar questions. Are you both working on the same project?

yes we work together. I've asked because I'm the one who collected the coils, so it's easier for me to interact and conduct the measurments

 

[berkeman]

Okay, fair enough.

Summary:: I'd like to know the maximun value of current that can go through my coil

I've obtained common mode choke from detached pieces, so I don't know anything about them, exept for their inductance/resistance.
I'd like to know how could I determine the rated current of these coils

The current rating will depend on the saturation flux of the cores. Do you have any datasheet on the cores, or even know what material they are made out of? What is the frequency range where you want this CM choke to work? What is the level of DM excitation?

 

[Mathhhew]

Okay, fair enough.
The current rating will depend on the saturation flux of the cores. Do you have any datasheet on the cores, or even know what material they are made out of? What is the frequency range where you want this CM choke to work? What is the level of DM excitation?

Well like I've said, these coils are from detached pieces, so no i haveno datasheet. The range of frequency is between 1kHZ to 5-10 MHz. And for DM excu=itation, I'm sorry but english isn't my native language, so I don't know the meaning of the abbreviation. Is it differential mode?

 

[berkeman]

Yes, Differential Mode versus Common Mode. Anything that creates a flux in the core can saturate it. What size are they physically? How many turns? (pictures?)

What current levels are you anticipating? What level of CM impedance do you need in your application?

 

[Mathhhew]

Common mode current will be created with a step-down chopper powered with 15V and from what I've read, we can expect a peak at 0.1 A. The coil has around 30 windings.
Here's some pictures (the ruler is graduated in centimeters)

 

[DaveE]

This is a complex question. Frankly, it requires more effort (plus some judgement and risk) than you'll get for free on social media. So, I'll jump right to the most practical answer. This is for normal common mode chokes, not necessarily for other inductor applications.

Build a prototype. Subject it to worst case conditions, mostly I²R losses plus the hottest ambient. You can usually ignore core losses in common mode filtering applications. Measure the hot spot wire temperature. Not just the outside of the coil if it's multilayer. You may want to bury a thermocouple into the windings when you make it. Then compare that data to the magnet wire insulation specs. With some derating; good designs always have some margin built in. Also consider the effect of very high temperatures on nearby stuff. I've seen SMPS inductors get hot enough to melt the solder on their leads that, of course, is way too hot.

https://temcoindustrial.com/product-guides/wire-cable-and-accessories/magnet-wire/magnet-wire-faq
https://www.mag-inc.com/Design/Design-Guides/Designing-with-Magnetic-Cores-at-High-Temperatures

 

[DaveE]

Wait, maybe I'm confused. Are you asking about how much differential current you can have, which is about heating? Or are you asking about how much common mode current you can have, which is about inductance?

0.1A peak CM current sounds ridiculously high for the standard EMC filtering applications, is this for something else like GFCI?

 

[Mathhhew]

Wait, maybe I'm confused. Are you asking about how much differential current you can have, which is about heating? Or are you asking about how much common mode current you can have, which is about inductance?

0.1A peak CM current sounds ridiculously high for the standard EMC filtering applications, is this for something else like GFCI?

The question was about how much differential current.
I'm not really satisfied with your answer because the measure will be hard to conduct. Is there some kind of order of magnitude of current related to the section of the wire it is going through or a way to estimate it if I know from what kind of equipment it comes from? I'm not trying to have a precise value, only an estimation

 

[berkeman]

I'm not trying to have a precise value, only an estimation

Find similar common-mode chokes with a Google search (same size, same frequency range, similar looking number of turns) and use those datasheets to give you a ballpark idea.

Edit/Add -- Also, do you have access to an LCR meter that you could use to measure the inductance(s) of that choke?

 

[DaveE]

Is there some kind of order of magnitude of current related to the section of the wire it is going through or a way to estimate it if I know from what kind of equipment it comes from? I'm not trying to have a precise value, only an estimation

Yes. You'll first make some assumptions about the design, a preliminary design. Then you'll calculate the worst case, losses. You'll want to divide these into copper losses and core losses. The copper losses are essentially just the resistance of the winding, for which you need the resistance per turn, number of turns, and the mean length per turn. Don't forget to adjust for the elevated temperature of the wire. As I said, core losses don't matter much for DM currents, since the H field cancels. Then your left with a difficult thermal problem for whatever structure you have. Most likely you will use an iterative approach to fix the stuff you don't like in the results with a modified design.

In short, you have to learn how to design magnetics like the EEs that do that for a living. There are some papers out there on the net, but I don't think you'll find much like a tutorial or book. It's just a bit esoteric for that. This is one of many reasons you may be better off buying a CM choke than designing your own.

Back in the day, I learned this from this book (an earlier version, actually), which has some very practical approximations. Ultimately, good designs end up being measured anyway. It's often easier and more accurate, since there are a lot of variables in the construction.

One way to measure copper temperature is to measure the hot vs. cold resistance of the windings and make a model (because the temperature isn't equally distributed, of course) based on the geometry and the TCR of Cu. It is the hot spot temperature of the wire insulation that matters most.

 

[DaveE]

There is some design info online at mag-inc.com and at micrometals.com. Look into that and search for similar things from other core manufacturers.

 

[Tom.G]

Measure the diameter of the wire used.

When you know the wire size, you can look up in a "Wire Table" the maximum current it can safely carry. Then divide that current by 4 to get a safe current for the choke.

This is not an exact answer but a 'safe' answer, and it is "close enough" because you do not have exact information about the choke.

Here is a "Wire Table" with the current rating for different wire sizes.
https://www.powerstream.com/Wire_Size.htm

If the wire is too small to measure, unwind several turns then wind 10 to 20 turns on a pencil or something else. Make the turns touching each other, measure, then divide by the number of turns to get the wire size.

(please let us know what you find)

Have Fun!
Tom

 

[Mathhhew]

According to the table, and because I have wires with a section of 0.5mm2 (approx. 24 AWG), I should be able to use up to 3.5 A. That should be enough for what I'm going to do, even after dividing by 4.

But I was thinking of another method. I assume the main problem with current is how high the heat inside the coil will be, so what if I use a current generator to power the coil and follow the evolution of the heat

Build a prototype. Subject it to worst case conditions, mostly I2R losses plus the hottest ambient. You can usually ignore core losses in common mode filtering applications. Measure the hot spot wire temperature. Not just the outside of the coil if it's multilayer. You may want to bury a thermocouple into the windings when you make it.

I think that's kind of what you said. Is there a temperature from which it is considered that the use of the coil is dangerous or its efficiency is impaired?

 

[DaveE]

Is there a temperature from which it is considered that the use of the coil is dangerous or its efficiency is impaired?

Well, to start with, don't exceed the temperature rating of the magnet wire insulation, as I said before. Note that we don't know what kind of wire you have specified. Definitely don't exceed the Curie Temperature of the core, which would be pretty hard to do.

Also, don't exceed other temperatures of nearby things (PCBs, solder, plastics, nearby components, etc.) you must determine those as the engineer designing the component/circuit/system. Basically, everything you buy to put in your design has temperature specifications, don't exceed those.