AC electromagnet - eddy current heat, lamination orientation?

[peskywinnets]

So I made a guitar sustainer (basically an AC electromagnet, with a permanent magnet on the bottom top pull the strings down in the absence of any AC signal going through the coil) - it works ok but the driver coil/core gets extremely hot ...especially at high frequency (frequency range is up to 5khz). I'm presently using a mild steel core (which isn't optimum, but easy to source!), but really should be using electrical steel laminations for the coil core.

For the least eddy current induced heat, which way should the laminations be orientated, I've done this (bad) drawing to help....

Bearing in mind where most of the magnetic direction force/flow is, which option would be best for the lamination orientation? Option 1, option 2 or option 3?

Also all related tips welcome! (type of laminations, supply source (I'll be ripping upa transformer for my immediate source!)

 

[IRstuff]

Fine, whatever, I'm done with this, but your heat problem is in no way related to eddy currents, and that requires no quessing

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[rothers]

What's the current density in your coil windings ?

 

[EdStainless]

No 1 is how it is done in real life if you are using grain oriented steels.
They carry field best in that orientation.
Use thin lams, though it is a trade off, less loss but also less steel.
Don't clamp them too tightly.

Are you just looking to build one of these?
I may have a small stash of 0.004" thick V Permandure lams around......

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Plymouth Tube

 

[peskywinnets]

Re current density - dunno...how do I work that out?! There's about 120mA flowing through the coil & about 200 windings.

re am I looking to build?. Well, I'm just dabbling but this has become a bit of an obsession! I've made a lot of circuits, wound a lot of coils, tried a lot of different core types (blade core, pole pieces etc)....and really this heat thing is the last main issue to conquer...I really like the funky harmonics that a sustainer can generate, but it's when in harmonic mode that the thing gets hot...too hot. I've no idea what V Permandure lams are? (they sound cool though!)

My issue is that if using lams ...where would I get them made? (they'd need to be a custom size...and pretty small at that!) Or if using something other than lams...where do I source the material - this is pretty niche stuff & most suppliers aren't interested unless there's an order placed by the tonne!

 

[EdStainless]

You need a military surplus outlet, a real one.
You need to find some old Air Force electronics that has a transformer in it.
Make sure that it says 400Hz on it (in stead of 60Hz).
Then take the transformer apart and cut the pieces you need out of those lams.
You will wan to epoxy the stack together to minimize buzzing.
These lams will be thin and they will be designed for higher frequency than typical stuff.

Maybe ebay.....

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Plymouth Tube

 

[peskywinnets]

Does anyone hold a view on how well an alnico magnet rectangle (ie like a blade ina guitar pickup) would work as the core/centre for a sustainer?

I was thinking about getting some magnets made for guitar pickups anyway & perhaps it might be worth investigating getting a few made for a sustainer to try.

My main worry is thatthe alnico will demagnetize due the alternating AC magnetic field (which has a similar profile to a degausser....a strong AC signal fading out to zero - albeit I'd imagine far less strength than a degausser)

 

[peskywinnets]

Ok, next dumb question(!)

When I fitted & played the guitar sustainer, I noticed some minor sparking when the guitar string (the string is grounded) touched the sustainer (mild steel) core....I measured about 3V DC present on the mild steel core, so obviously when that 3V gets grounded via the guitar string (cos the string vibrates & sometime glances the top of the sustainer core), that's a direct short of 3V to ground ...which is why I'm seeing sparkage.

I measured the DC resistance from the coil start & end wires to the core...it's in the order of megaohms....so where is the 3V I measured coming from? Is it being induced from the AC signal into the mild steel core?

 

[peskywinnets]

Scratch that, transpires there was a short from the coil to the steel inner core, therefore that's why there is present!

 

[IRstuff]

You can't "induce" a DC voltage from an AC voltage without a rectification process.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529