Advice on short duration, high current electromagnet circuit

Jessie · 2009-09-24T11:15:27+00:00

I have participated in an informal competition held by the electronicssociety at my uni to design a working high strength electromagnetusing commodity off the shelf (COTS) components. The electromagnet isto be powered by any commonly available means with a maximum budget of=A3100. There are basically no rules to the competition except thebudget and COTS requirement.As far as I know, everyone else is working on the principle of asteady state electromagnet circuit. Typically, with the appropriate AC-> DC voltage rectification circuit and the maximum current determinedby the ampacity of the windings on the electromagnets. Alternatively,other people are working on electromagnets powered off batteries whichobviously have limitations of internal resistance.I think that since the measurements of the electromagnet's strengthwill be done over perhaps 30 seconds in the competition, the ampacityof the windings is not particularly relevant, i.e. it is possible tooverload the current and rely on the thermal mass of the assembly tosoak up the excess heat generated. Therefore, I would like to go for acircuit that can deliver a sustained, high current pulse to anexternal load.I would like to try to build a circuit that charges up a capacitorbank in parallel and discharges it across the electromagnet in series.A Marx cascade seems to be of the sort I need but Marx cascades workwith high voltage capacitors (to trigger breakdown of the sparkjunctions) and I think for a ~30 second sustained current, I will needcapacitors with a high capacitance. Capacitors with a high capacitancetend to have a low voltage (I could find a 30F aluminium electrolyticcapacitor on Farnell with a voltage of 2.3V)As term has just started, please assume I just have knowledge ofelectronics from A-level physics.Does anyone have any comments on my approach?Will the maximum magnetic field strength I can achieve be entirelylimited by the saturation of the core? I know the saturation ofdifferent ferromagnetic...

Sep 24, 2009 22 Replies

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JosephKK 16 years ago

This drives for maximal use of scrap, cheap but labor intensive. If you use a core, perhaps the combined cores from 10 to 30 discarded microwave ovens could help.

How is the strength of the magnet scored? It is really driven by ampere turns. How long does the field have to last?

Based on what i have seen so far, my approach would be on the order of

10 turns of 1/4 inch copper tubing supplied by 1 or 2 lead-acid (auto) batteries.

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Frnak McKenney 16 years ago

I realize I'm coming into this thread a bit late, but did you give any thought to bypassing the power supply completely and building an AC electromagnet?

I dimly recall plans for building one of these from year... er, decades back in a hobbyist magazine (Popular Electronics?); one of its interesting features was that it would pick up non-ferrous metals.

Anyone know how AC and DC electromagnets compare as far as efficiency when both are powered from an AC source?

Frank McKenney

A striking fact of the last two years of financial trouble is how accountability has differed in the public and private spheres. On Wall Street and across the country, decades-old firms have failed, fortunes have vanished, and some former captains of finance face jail or fines. In Washington, meanwhile, most regulators and Members of Congress remain on the job, often with enhanced power. -- "Bernanke\'s Second Chance" / Wall Street Journal 08/26/09

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JosephKK 16 years ago

What

Once the iron saturates all reasonable measurements are iffy at best. The force measured will be limited by saturation flux in the iron.

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