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Noah Hoppis

Sun Mar 24 2013, 08:09PM

Registered Member #10072 Joined: Thu Feb 14 2013, 05:12AM
Location: seattle wa
Posts: 21

I was wondering about constructing a large electromagnet for my cyclotron vs just buying one. Would plain steal or iron work? what about laminated? (if yes what kind of surface contact is required to transfer the magnetic flux?)

Ash Small

Sun Mar 24 2013, 09:15PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I know nothing about cyclotron magnets, but I assume it's just a huge, presumably DC, solenoid type thingy.

I imagine steel may have some benefits over iron, I read somewhere I think, that It's about 1.2 times as efficient than iron in this type of application, but you'd need to check this.

I imagine you just follow the usual advice for coils in general, but just think a bit bigger.

I imagine some of the coil gun builders can supply the maths for a 0.9T magnetic field.

doctor electrons

Sun Mar 24 2013, 09:35PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Laminated steel cores are constructed as such to eliminate eddy currents in the core and stabilize the
larger field around the entire core. That's why most transformers, if not all, are constructed that way.
The larger electromagnets you see in scrap yards are a different story. They use a large solid steel or iron core.
They also have a backup battery system in case of power failure while making a pick. Generally that backup battery
only provides enough time to warn anyone in the area to get away! The latter is about as simple an electromagnet
as it gets. In both cases the coils are wound around the cores. Surface contact is not an issue because the core is magnetized by the flux generated by the energized coil. If the core is inside the flux it will magnetize.

klugesmith

Mon Mar 25 2013, 05:04AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Hi Noah.
Since your cyclotron magnet is DC, with a large gap between the poles:
- Laminations are unnecessary.
- Air gaps between core sections are no problem, if much thinner than the main gap.
They only matter in transformers and lifting magnets which ideally have -no- gap.
- at 0.9 teslas, materials more expensive than ordinary steel won't improve performance significantly.

Time to learn the difference between your B and your H. I'd recommend sticking with SI units, and reading a tutorial. Just found this one, which looks quite respectable.

If you read the first five chapters, or equivalent in another reference, you'll be able to calculate the ampere-turns you need, and you'll understand what laminations are for.

Here's an alternate starting point.

Good luck!
-Rich F.

Noah Hoppis

Mon Mar 25 2013, 11:23PM

Registered Member #10072 Joined: Thu Feb 14 2013, 05:12AM
Location: seattle wa
Posts: 21

I knew what lamination were for in transformers; to avoid eddy currents at 60 Hz AC and to stop unnecessary heating. The Questions were more posed towards whether the lamination would cause unnecessary loss, because 4"x1"x18" bar is much easier to get and bolt together than a several hundred pound magnet core!

Ash Small

Mon Mar 25 2013, 11:48PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Noah Hoppis wrote ...

I knew what lamination were for in transformers; to avoid eddy currents at 60 Hz AC and to stop unnecessary heating. The Questions were more posed towards whether the lamination would cause unnecessary loss, because 4"x1"x18" bar is much easier to get and bolt together than a several hundred pound magnet core!

I think as long as it's steel, it's not going to matter much.

Steve Conner

Tue Mar 26 2013, 11:01AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A cyclotron magnet needs a very uniform field. Any wiggles and lumps will break the beam up. Traditionally the field was tweaked by hand after assembly, by inserting steel shims between the magnet pole and the chamber lid.

I'm sure you could make a perfectly good magnet out of smaller steel bars bolted together, especially if you used some simulation beforehand to work out what the field would end up looking like. Maybe there would be some weird arrangement of bars that would give a more uniform field than the obvious arrangement.

klugesmith

Fri Apr 05 2013, 10:39PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Stumbled on this uniform-field magnet on ebay.

Its size can be guessed.

Because of the small amount of wiring, and no plumbing, I bet it uses permanent magnets.
But... there may be some electromagnet coils also, to fine-tune field strength, for example as the permanent magnet temperatures change.

Along those lines, an amateur cyclotron magnet might be designed with main coils powered by rectified but unregulated mains voltage (aptly named). The natural inductance would limit current ripple to a few percent, if my calc's are right. Then smaller "vernier" coils using much less electrical power could be driven dynamically to cancel the effects of ripple current and mains voltage fluctuation.
Might not be worth the trouble, compared with just making a multi-kW regulated DC supply for the main coils.

Ash Small

Sat Apr 06 2013, 01:15AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

klugesmith wrote ...

Might not be worth the trouble, compared with just making a multi-kW regulated DC supply for the main coils.

How many kW will be required for 0.9T?

I was thinking a lead/acid battery bank would be the easiest way to provide a 'smoothed' supply, but how much power are we talking here?

klugesmith

Sat Apr 06 2013, 03:00PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Depends on your pole diameter and gap length, and the resistivity of copper.
Got some dimensions in mind?
Last month Noah and I analyzed a design for D,G = 8",2". Then I did one for 3", 1" .
The unreviewed results (on the order of 3 or 4 kW and 1kW) aren't here now, but I'll post 'em later. Someone can compare them with historical & university cyclotron numbers.

Spreadsheet calculations demonstrated 2 principles.
1) You can design for any voltage by choice of wire gauge, with no change in power, coil size, or copper weight and cost.
2) You can reduce power by using more copper, but that needs longer or (less efficiently) wider coils, thus a bigger yoke.

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