A Practical Look At Chokes For EMI Control | Hackaday

Radio frequency electronics seems to be a magic trick even for those interested in delving into the field. Unfortunately, the poor soul can only deal with it accidentally, for example when seeking to minimize electromagnetic interference.

This is a good way to explain the theory from a practical and results-oriented perspective.

As a hobby mechanic and machine tool manufacturer, [James Clough] has encountered many cases of EMI raising his head. Variable frequency drives are where EMI may cause problems, and chokes on the motor phase output are usually specified. He used an expensive choke coil sold specifically for VFD applications on a machine, but he wanted to know whether a cheap ferrite core would do the job, and he began to look for answers.

Facts have proved that borrowing a vector network analyzer to scan some ferrite cores is not satisfactory, so [James] used a function generator and an oscilloscope to conduct simple experiments. His demonstration demonstrated how the impedance of the choke coil increases with the frequency of the test signal, and this is exactly the behavior you want in a VFD—passing relatively low frequency phase signals while blocking high frequency EMI. He took a good measure to connect it in parallel with the choke and showed that the performance of a low-pass filter is much better.

We like demonstrations like this. These demonstrations will not only scratch the itch of intellectuals, but also have a practical goal. [James] not only showed (at least in some cases) that a $13 ferrite can do the work of a $130 VFD choke, but he also showed how they work. This is the basic stuff, but this is what you need to know to move on

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No comment? In my opinion, this is a good practical use case and laboratory test method. It is always interesting to observe the simple test method and then the method of the rack-mounted version...even for desktop devices, because the devices are getting smaller and smaller. Maybe I am studying filters in depth for the time being, and my first SMD 0603/0805 welding is practicing on these filters. The following is my first attempt:

Yes, interesting video. I actually watched it to understand how putting the lid in parallel with the ferrite would improve its performance (well, I admit that I just want to watch it and make a comment, because it is obviously incorrect), but when watching the video I realize the misunderstanding. The capacitor is parallel to the source, not parallel to the ferrite.

For those who want to learn about ferrites and filters, it is good practical information. Phasor circuit analysis will enable you to go further in the noise range.

Useful, thank you!

Yes, it is strange, these replies are few. I actually watched the video and liked it because I have never done any experiments on these cores...now I want to test them to see how they perform in xD. Thank you!

Larger is not always better, try using smaller ID ferrite. Fewer IDs are more effective for the same cross-sectional area, and are cheaper and easier to find from reputable sources (for example, Allied or Digikey's Laird products). The relative efficiency from one ferrite to another ferrite with the same composition: = number of turns * (cross-sectional area) / ID, IIRC.

I have been catching up with the EMI problem several times and have learned a lot in the process. Include the knowledge/noise ratio characterization of some colleagues who often talk about EMI, like this is a mysterious beast, it will be everywhere and cause trouble for your clumsy design. It's not. On many machines that I had to redesign, I never thought about minimizing EMI. Within 10 minutes, a grounding terminal block was added near the end of the cable in the cable trough and fixed in terms of cutting a 25-foot shielded extension cord shorter than the new TB. There are a bunch of GRN/YLW under my feet. This angered all residents who were "investigating" the issue. No need to investigate, this is the basic content I told him. It's too late now, I cut all the wires, goodbye! walked away. I don't want to hear another story about the memory of water or how many Italian villas he owns. I'm not kidding...

In addition to ferrite, any phase cable must be shielded, and the shield must be grounded with a shield clamp such as Icotek 37620. I like these clips very much. Do not solder the wire extension wire to the braided shield, nor to the terminal or other ground, no matter how short the length is. You need surface area. The best results can be obtained by using only one common mode ferrite and winding the wires together instead of individually. As close as possible to the VFD/drive. Cylinder is better than doughnut, you want extra cross-sectional area and smallest ID. Be sure to perform a temperature test afterwards. After finding that the ferrite became too hot, I had to reduce the number of turns or increase the size. If you focus on drives and servos, you can eliminate a lot of noise, but don't ignore good shielding practices for other cables. In one instance, I changed from a noise of about 300v to 40mV. Not sure if I completely believe in 300V, but this is the range of the diff probe.

*Line-side EMI filter is used in the driver he showed

*Properly shield all cables

*Ferrite on phase cable

* Noise bit isolation

* Ground all the sheet metal and ensure that the incoming PE is good.

* Ground circulation awareness

Thank you for your lengthy post. I have a question about "Cylinders are better than donuts": Do you have experience with other shapes and their relative effectiveness in a given size? (Such as double E or pot type)

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