Need help calculating resistor needs in an electromagnet cir

T

trayos

Guest
I am trying to put together a project for high school. The idea is t
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 syringe
filled with about 5cc of MR fluid. The syringes are connected togethe
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, n
specs given). If you place the magnet on the coupler, the MR fluid lock
up and you can't push the fluid from one syringe to the other. Remove th
magnet and everything moves.


Basically, the magnet solidifies the MR fluid in the coupler and that plug
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 meta
cylinder that is pinched in the center to form a "waist" so the MR fluid i
flowing through a very small bore tube about 2mm in diameter. The ends o
the coupler flare out wide enough to accommodate standard syringes. I'
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 insid
diameter and 6mm outside diameter. The tube length between syringes is
cm. 9 cm of tubing holds 1 cc of water, so about .3 cc of MR fluid is i
this tube coupler. I wrapped 22 gauge wire around this area 50 times an
connected the ends to the DC output on a Railpower 1370 transformer. I
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 th
power.

Results:
When I get the voltage up to about 10 or 12 volts, pushing the syringe
becomes a lot harder. As I push the syringe plunger I feel a vibration th
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 t
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 wit
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 circui
breaker?

If I increase the tube length so I can fit 100 turns to give me a greate
magnetic field (about 2E-3 J/T) will I lose the benefit because I have als
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 t
calculate the appropriate resistance needed?

Thanks for any help you can provide.
 
trayos wrote:

[snip]

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.
Click to expand...
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.
 
"trayos" <trayos321@gmail.com> wrote in message
news:m9CdneU-KIykhafW4p2dnAA@giganews.com...
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.
Click to expand...

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.
 
trayos wrote:
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.
Click to expand...
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
 
"ehsjr" <ehsjr@nospamverizon.net> wrote in message
news:hhemu8$qlh$1@news.eternal-september.org...
trayos wrote:
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.



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