Driving coils for spinning magnetic field

Thread Starter

kiweered

Joined Dec 13, 2021
28
Hey,

I would like to build a magnetic agitator (the one used in chemistry lab) but where the mixing magnet is drive by coils and no with a spinning magnet.
Here is the existing technology i'm trying to copy :
1676040725556.png

I can't figure how to choose the magnetic coil for my project.
I need to have the strongest magnetic field for current around 1-2 A. The diameter of the coil should be around 2cm or less.
I saw that in electromagnet you have iron core to help the magnetic field but I don't know if you can find such inductance.

Can you help me choose the propper part by telling me which parameters I should look ?
And if it exists already made small electromagnet like this I can buy ?

Thanks for your help,
Adrien.
 

Thread Starter

kiweered

Joined Dec 13, 2021
28
It's a personnal project to help a chemist friend who can't afford the industrial material.

I'll drive the coils with stepper motor driver, so I have some current limitations (I haven't chosen which one yet so let say around 1.5A max).
 
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MisterBill2

Joined Jan 23, 2018
19,422
The stepper motor driver will cost more than a DC motor and simple speed control plus a power supply. Not nearly as elegant but simple and reliable.
 

LowQCab

Joined Nov 6, 2012
4,294
It would appear that You are trying to modify an existing device for "more" Power.
This may complicate getting the desired result, rather than making it easier.

Several pictures,
and detailed description of the type of container(s) to be used,
and a general description of the Liquids to be mixed, and the quantities,
would get You some good ideas about how to achieve
the desired result at the least expense.

Generally, there will need to be some means of "clamping" the container to the device.

A slowly rotating platform, ( maybe ~60-RPM ), ( with clamps ),
that is angled at approximately ~30-degrees from vertical,
will do an excellent job of thoroughly mixing Liquids or Powders.
( Powders may require a steeper-angle than ~30-degrees )

A Stepper-Motor-Driver would probably not work very well with Solenoids,
but it could run an actual Stepper-Motor to turn the above described rotating "platform",
and the vibrations coming from the Stepper-Motor might even be a bonus in the mixing process.

What is your general budget ?
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Thread Starter

kiweered

Joined Dec 13, 2021
28
The stepper motor driver will cost more than a DC motor and simple speed control plus a power supply. Not nearly as elegant but simple and reliable.
Yes I also think I can drive 4 coils with a motor driver like L298 full bridge (in its datasheet you find a Two Phase Bipolar Stepper Motor Circuit with a L297).

If variable current is required, what about one of the simple 555 PWM circuits.
I think it's not suitable here, as I'll use a microcontroller to control the speed of the rotation. With a 555 I believ the parametters of the output signal is defined by resistances/capacitors...

A Stepper-Motor-Driver would probably not work very well with Solenoids,
Thanks for your detailled answer, however the purpose of my project is to remove any mechanical moving part in a mixing process (let see this as the project brief). The liquids are not visquous so I don't need a lot of torque/power in the agitation process.
Can you be more precise when you say that a steper driver would not be suitable to drive solenoids/electromagnets ?

I found 3 similar projects that correspond to what I would like to reproduce but they are not very detailed :
https://fablab.ruc.dk/magnetic-stirrer/
https://fablab.ruc.dk/magnetic-stirrer-version-2/

In these you can see they used big soleinoids/inductances to generate the magnetic field but they don't give the references or special indications about them (one is 1410604C https://www.mouser.fr/ProductDetail/Murata-Power-Solutions/1410604C?qs=5CKLVr1iF0kSPSRQWGAVyQ== ).

Do you have any knowledges about which type of inductance would be the best to create a powerful magnetic field ?

Thx
 

LowQCab

Joined Nov 6, 2012
4,294
It seems like there is a translation problem ........
When You said ..........
"" I would like to build a magnetic agitator (the one used in chemistry lab)
but where the mixing magnet is drive by coils and no(t) with a spinning magnet. ""

I interpreted this to mean that You wanted to use some other method,
different than a Spinning-Magnet, to mix chemicals.

If all You are looking for is a "Coil" to make a Magnet spin, that's super easy ..........
go to your local Hobby-Store, and buy the cheapest Quad-Copter-Motor that they have,
a used or broken one is even better, because it will be less expensive.
They may even give You a damaged Motor for free.
Or, You can get brand-new Motors from China for ~$20.oo or less.
.
Quad Drone Motor .png.
Drone Motor 1 .png
.
Disassemble the Motor,
You do not need any of the "moving-parts", Bearings, Shafts, Magnets, etc.
The Core-Armature inside these Motors is designed perfectly to spin a Magnet.
It is precisely what You need.
They come in a wide variety of sizes,
the best fit for what You want to do is the type of Motor that is large in Diameter, but very "Flat".
Some of these Motors are not very large in diameter, and are longer,
this type "may" work OK, but is not preferred.

Any of these types of Motors will handle as much Power as You can find,
but lots of Power will not be necessary, because this will be a very efficient device.

You can use either a Stepper-Motor-Driver, ( with Current-Limiting ),
or, an ESC, ( Electronic-Speed-Controller ), designed specifically for a Quad-Drone-Motor,
they are cheap, and easy to use, instead of a Stepper-Motor-Driver.

Any Epoxy-Coated-Magnet will work as a mixer-element.
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Thread Starter

kiweered

Joined Dec 13, 2021
28
Sorry for the misunderstanding, yes I would like to spin a magnet inside a sealed container to mix liquids inside.
I thought about brushless motor but not in this way.... Are you saying that with the stator of such a device I will generate a spinning magnetic field above it ?

1676277993801.png
 

shortbus

Joined Sep 30, 2009
10,049
Exactly,
It will work very well, and its cheap.
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I think your wrong. It works in the BLDC motor because the magnetic force is pointed inward, toward the rotor. Just like in solenoid, toroidal transformers and Ecore transformer, the flux is concentrated toward the inside of the winding. There are magnetic field lines outside but they exert little or no force. It's the same reason the earth doesn't have east and west poles, only north and south.
 

shortbus

Joined Sep 30, 2009
10,049

MisterBill2

Joined Jan 23, 2018
19,422
Once again, it will function more effectively with some flux extenders affixed to the outer pole pieces to direct te flux upward, certainly there may be a concentration of flux in the center, but that does not matter,. also, magnetic flux , a lot like electric currents, tries to take a path to complete the loop .'
Very few people can actually see magnetic flux, because it is normally invisible.
 

LowQCab

Joined Nov 6, 2012
4,294
Just because it's not ~99.9% efficient,
and doesn't follow any of the "best-practices" for designing a proper Brushless-Motor,
doesn't necessarily mean that it won't do the job in a thoroughly impressive manner.
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MrAl

Joined Jun 17, 2014
11,715
Sorry for the misunderstanding, yes I would like to spin a magnet inside a sealed container to mix liquids inside.
I thought about brushless motor but not in this way.... Are you saying that with the stator of such a device I will generate a spinning magnetic field above it ?

View attachment 287477
Hi,

You may get some spin from that but there are a few things about electromagnets that you should be aware of.

The first, and probably most important for that kind of construction, is the distance between the electromagnet pole and the piece to be attracted.
The force decreases as the square of the distance, so you want the distance to be very small. In the construction above if it were to be used with a bar magnet as shown on top, the distance between the pole faces and the magnet could be as much as 2cm which is quite large. If it was just 1cm for example, that would mean the force would be 4 times as great. if it was just 1/2 cm, the force would be 16 times as great. So you can see how important the distance is.

Now if you could use a bar magnet that was as wide as the container and have the coil pole faces pointing inward (like a stepper motor) then you could have the pole faces closer to the ends of the bar magnet. In this case you may even be able to use some strongly magnetically active material like special metal used for transformer cores. A good shape for that would probably be like a plus sign "+" so you have four faces. The propellor could be mounted on top of that.

There are a lot of possibilities here though so it ends up depending mostly on what you can buy and what you can do as far as machining and winding of coils yourself, so it might be worthwhile to mention what you are capable of obtaining and what kind of metal work you can do.

The cores for the windings should be highly active magnetic material such as those used for regular transformer cores or those used in stepper motors and the like. The core material increases the strength 100 fold or more due to the permeability of the material. It could actually be 1000 fold.

The active area is also important. If the pole face is 1 square centimeter and the force is 2 units, with pole face of 2 square cm the force would increase to 4 units.

The current is also important, the force is proportional to the square of the current, so 2 amps will produce 4 times as much force as 1 amp.
The number of turns is similar, with 20 turns the force will be 4 times that of just 10 turns.

So to sum up...
You want a core material that has high permeabilty.
You want the most number of turns possible.
You want the highest current possible.
You want the largest pole face area and target piece (magnet or high mu magnetic metal piece) face to match.
You want the smallest distance between the pole face and the target piece face.

There are a few things we can say about the purely physical aspect of the construction also...
If the fluid viscosity is very low then you dont need as much force.
If the friction between the moving piece and the container is low you need less force also.
These two taken together means you can use a needle type bearing which has low friction.
Of course you cant have the piece that is immersed in the fluid to react to the fluid so you may have to coat it with a protective layer.

Just to note, you can also use a motor with an offset cam to vibrate a container for mixing. Does not work well with very thick fluids though. Maybe add some marbles to help with the mixing too.

What size is the container you need to use with this: height, widths or diameter, and volume?

Here is a drawing showing the coils on the outside of the container instead of underneath it. The container is a cylinder and shown in red.
 

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MisterBill2

Joined Jan 23, 2018
19,422
Like I had mentioned earlier, extending the pole pieces up, towards the bottom of that sealed container will improve the efficiency. BUT if that container is electrically conductive, that will block much of the effect of the rotating magnetic field.
It suddenly hit me that the TS had not mentioned Glass container, only a sealed container. If it is a conductive material all bets are off.
 
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