# Curious which electromagnet core material will switch polarity best

#### Holes Flow

Joined Oct 26, 2017
10
Experts,

I'm rolling my own EMs, and thought a core material would be a snap-soft iron. I was reading on this U of Surrey web site how different materials perform. Iron is naturally a good magnifier of the field produced, but then I wondered about how a coil would retain a field if switched in polarity, and whether hysteresis of an iron core would make it worse.

Here is the application: Simple 'levitation' of neodymium magnet by 3 coils (triangle underneath). In this case, the fields in the same polarity can be adjusted in strength. But the intention is to have the same electromagnets (EMs) be pulled away from each other, and have the permanent magnet (PM) be suspended between the 3, on the same plane. So if the PM is level with the other 3, there can be both repulsion and attraction switching quickly.

I've made years past 'shock boxes' that utilized the collapsing of the DC magnetic field across an audio transformer to zap the holder via a mercury switch, so I know the field of a DC EM will retain the field for a time-even when the polarity is switched.

So one question is: Which core material will allow a high degree of magnification of the EM's field, while still allowing the polarity-even the magnitude-of the field changing very quickly?

I'm driving these coils with an STM32 processor at 72MHz, using hall sensors, batteries, hand-rolled coils that are about 1" long and about 1/2" at their widest in diameter, so these won't be huge. But if the nearest pole of the PM presents itself to one coil as another polarity (spinning/flipping in place), even if the magnet is being levitated/suspended, the nearest EM would have to switch polarity fast. It is entirely feasible that the PM could spin rapidly.

Since the PMs are from 2mm to 8mm in size, does this really sound like it will be an issue? I'm stocking up on spools of various gauges and core materials, but I lack an o-scope Thanks in advance for your thoughts...

#### GopherT

Joined Nov 23, 2012
8,012
For low frequency switching (up to hundreds of hz) Plates of silicon steel are commonly used (like transformer laminated cores and motor). Each layer is insulated from subsequent layers with paper, or chemical etching.

Higher frequencies are done with iron powders. Larger particle size and softer (annealed/hydrogen reduced) iron for lower frequencies and smaller particles (down to 5 micron) for up to a few hundred megahertz. Ferrite beyond that.

Log on to eBay and craigslist to find a used o-scope for $40 to$250

#### BR-549

Joined Sep 22, 2013
4,938
You gotta have a scope. Even if you have to sell a kid.

They have been looking for that kind of material for years. It's still a trade off.

What frequencies are going thru the coils?

#### Holes Flow

Joined Oct 26, 2017
10
I messed up, reading back, not mentioning DC for levitating neo magnets. Well, pwm from the stm32, amplified. I don't think ac would work to repel/attract, right? I only used xformers in the day to, well, transform. i come from the days of formal soldering school and 4-banded resistors.
My dear teacher, Mr. Lehr:
"How can i tell if this 5u4 is still good?"
"Drop it. If it breaks, it was good."

For low frequency switching (up to hundreds of hz) Plates of silicon steel are commonly used (like transformer laminated cores and motor). Each layer is insulated from subsequent layers with paper, or chemical etching.

Higher frequencies are done with iron powders. Larger particle size and softer (annealed/hydrogen reduced) iron for lower frequencies and smaller particles (down to 5 micron) for up to a few hundred megahertz. Ferrite beyond that.

Log on to eBay and craigslist to find a used o-scope for $40 to$250
You gotta have a scope. Even if you have to sell a kid.

They have been looking for that kind of material for years. It's still a trade off.

What frequencies are going thru the coils?
I have the only kid away already! Something involving a blood contract, i think... still a little fuzzy on how all that went down. They sell these cute 400\$ digitals...

BR-549

I wonder how many don't get this Samples, Junior...

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