Could someone point me to where I can research, measure, calculate, and/or size these ferrite rings if voltage and rough max amps are known?

It's for 5, 12 and 24VDC low power system pulling up to 2 to 3 AMPs per run. Some amps are far lower.

Also would like advice about bundling several loops in one ring, or what to stay away from, how small of AMPs when it doesn't matter, etc.

/tr;dr

I have a problem. I've been collecting a lot of used 3D printers and repairing them. It all started when our daughter won a 3D Printer. And then COVID hit, so we've been doing 100s of PPE since. I am sitting on 7 3D printers now, with 3 functional (repaired two, and working on the others).

I noticed that all of these printers suffer from sever EMI. All of them, except the 3 very expensive Dremels. If you move any axis, extruder, anything with a motor, then the LCD display will light up, cooling fans start to spin, and even some other other axis may move if you do it fast enough.

The 3D printing community doesn't seem worried. "They all do that."

However, I noticed these Dremel 3D45s and 3D20 she won all don't suffer from many EMI.

Pulling the covers off, I've noticed the key difference is that the Dremels all use ferrite magnet rings around almost every cable. Some cables are bunched up into a single harness with a single ring, most others are isolated with their own rings.

I recall EMI and how these ferrite work from my electronics classes 30+ years ago. This is what I want in my first DIY build.

However, I have no idea where to start calculating and sizing them, what models are better than others, and even if there is a specific direction.

Thanks for your time!

Below are pics of the Dremel 3D20 and its different-sized and types of ferrite rings. How do they calculate which ones to use?

 

I don't think you will need anything special for this application. Standard 5 and or 7 mm clamp shell units should work. They work by energy dissipation in the selected unit frequency range so sizing them is much the same as selecting a power resistor.
https://resources.altium.com/p/how-do-ferrite-beads-work-and-how-do-you-choose-right-one

https://gistgear.com/industrial/industrial-electrical/ferrites

 

If you move any axis, extruder, anything with a motor, then the LCD display will light up, cooling fans start to spin, and even some other other axis may move if you do it fast enough.

The 3D printing community doesn't seem worried. "They all do that."

Although the ferrite beads can't hurt, they won't solve the "problem" of the LCD lighting up, cooling fans, etc. This is because your stepper motors are being turned into generators when you move the axis manually. You are now the prime mover and the motor is just doing it's generator thing. Stepper motors are really good at this because they typically have many poles and as such generate many pulses of induced voltage when spun. The voltages produced are going to be proportional to the force you are moving the axis. If the induced voltages are high enough, they can cause strange effects in your circuitry. I avoid this issue by not manually moving my axis unless I have to, and then only moving them slowly when I must. The reason your other printers are not doing this, is there is most likely a relay/switch that is removing the motor from the circuit when it is powered down or the steppers are disabled.

I don't know if this is helpful or not, just my experiences with the issue.