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Need help calculating resistor needs in an electromagnet circuit

I am trying to put together a project for high school. The idea is to build a working model of a shock absorber using MR (magnetorheological) fluid.

The initial set up: I got a demo kit from LORD company. It consists of two plastic syringes filled with about 5cc of MR fluid. The syringes are connected together with a short coupler so you can push fluid from one syringe to the other. The kit comes with a strong permanent magnet (I don't know how strong, no specs given). If you place the magnet on the coupler, the MR fluid locks up and you can't push the fluid from one syringe to the other. Remove the magnet and everything moves.

Basically, the magnet solidifies the MR fluid in the coupler and that plugs up the system so you can't push the fluid from one syringe to another.

The coupler: The coupler is about 10mm long. It has a rubber jacket covering a metal cylinder that is pinched in the center to form a "waist" so the MR fluid is flowing through a very small bore tube about 2mm in diameter. The ends of the coupler flare out wide enough to accommodate standard syringes. I'm guessing the total volume of MR fluid in this area is maybe .1 cc or less.

My experiment: I am trying to replace the permanent magnet with an electromagnet.

I replaced the coupler with a piece of plastic tube, with about 4 mm inside diameter and 6mm outside diameter. The tube length between syringes is 3 cm. 9 cm of tubing holds 1 cc of water, so about .3 cc of MR fluid is in this tube coupler. I wrapped 22 gauge wire around this area 50 times and connected the ends to the DC output on a Railpower 1370 transformer. It has 120V AC input,

15V DC output and 18VA total output.

I have a Radio Shack multimeter (cat no. 22-109/17-Range analog multimeter) so I can measure the voltage directly from the transformer as I dial up the power.

Results: When I get the voltage up to about 10 or 12 volts, pushing the syringes becomes a lot harder. As I push the syringe plunger I feel a vibration the the circuit breaker kicks in and the field is lost.

Problem: I know I need to put a resistor on the circuit, but I don't know how to calculate the right load.

Based on my calculations this is what I think I'm doing:

10v/18VA ~ .55A 12v/18VA ~ .66A

I am calculating the magnetic moment of a current-carrying loop with diameter of

6mm (radius 3mm) and 50 turns as follows:

pi*(rE-3)^2 * A

3.14*(3E-3)^2*.66 ~ 1.87E-5 A*m^2 or ~ 1.87E-5 J/T per turn for 50 turns that is about 1E-3 J/T

Questions: How strong a resistor should I put in series so I don't trip the circuit breaker?

If I increase the tube length so I can fit 100 turns to give me a greater magnetic field (about 2E-3 J/T) will I lose the benefit because I have also doubled the volume of MR fluid in the tube? (from ~.3 cc to ~.6 cc)

If I scale up the transformer, what is the formula I need to use to calculate the appropriate resistance needed?

Thanks for any help you can provide.

Reply to
trayos
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18VA/15V = 1.2A

That would be your maximum continuous current draw from your power supply. To be safe, run it at a bit less, say 1A.

That means you want a total resistance of:

15V/1A = 15 Ohms

Now, 22 gauge wire has a resistivity of 16.14 Ohms/1000ft. I doubt that you're using anything close to 1000 feet in your coil, so you're going to need a resisitance of nearly

15 Ohms, and at 1A current it'll need to be a hefty 15W resistor.

You might consider making a multilayer coil in order to increase the magnetic field and put more resistance in the wire of the coil rather than in the external resistor.

You might also consider pre-biasing the field with a permanent magnet so that the coil doesn't have to do so much of the work.

Reply to
Greg Neill

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The proper way to do this is to not add any external resistance but to make a coil with the proper resistance and number of turns to not load the transformer. For example instead of an amp or more, shoot for 100 mA of coil current instead. Right now the magnetizing force is about 100 Amp-turns based on your numbers, more or less. At 100 mA it would take a 1000 turns of wire to get the same force but the load on the transformer would be much less.

To get 100 mA with 12 volts, it would take 120 ohms of coil (12/.1 = 120) and would consume 1.2 Watts which is no issue, nothing gets hot or loads the transformer. Use number 32 AWG wire to get 1/10 the wire area and wind 10 times the length and number of turns (approx 1000turns). It will take about

800 feet to get to 120 ohms, more or less. Exact calculations can be made but that's for you to do. Good luck.
Reply to
Bob Eld

Others have suggested more layers on the coil. That should do it. But why not use an electromagnet wound on a core, instead of wrapping the wire around the plastic tube that contains the liquid? You could wind it on a steel nail or screw and put the head against the coupling, and it should be more effective.

Ed

Reply to
ehsjr

Lots of discarded consumer devices have coils that you can salvage. Hair clippers, for example.

Reply to
Michael Robinson

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