I am looking on ebay for some electromagnets. I want to make another stir bar but this time with electromagnets. I found many options on ebay but I was confused when I saw a 24v and 12v electromagnet.
Ok so here is my question: How does the 24v electromagnet create the same field strength as the 12v electromagnet?
I thought the magnetic field strength is dependent on current. So with more current the field would get stronger, but on ebay that does not seem to be true since the 24v is 0.13A and the 12v is .25A.
No, they are the same magnetically. Magnetizing force (MMF) is measured in amp-turns.
The 24V one has twice as many turns and 4 times the winding resistance, so the current is
half that of the 12V one. amps x turns is the same. And twice the length of wire of half
the area is 4 times the resistance but the same amount of copper.
If you keep the mass of copper the same and change the wire diameter and length you
get the same magnetically but at a different DC resistance and AC impedance. Its the
amount of copper that matters (and the amount of iron in the core).
Is it also correct that they will dissipate the same amount of heat?
watts = current * volts
If the wattage rating is the same, I would assume similar amounts of heat.
Now, since you're making a stirrer... I'm curious, are you going to spin the magnet beneath the housing, or are you going to use multiple magnets that are pulsed to move the stir-bar?
Yes, if you use intrinsic quantities (current density, power density, magnetic field strength) you'll
easily see its the same system, you could use one turn even (at a stupidly low impedance to the
external circuit!)
I dont want to use a motor this time. I tried that but I could never get it balanced. This time I am going to use 3 magnets in a triangle pattern or 4 in a square pattern to create a spinning magnetic field. I really have not decided on specifics yet because I need to buy and test the magnets.
I also found similar projects but I want to attempt it by myself first. I think it will be a challenge!
I'd use 4, not 3, otherwise the stir-bar will jump around like crazy - if you use quadrature there' only
a turning force, not a translational force on the thing.
(Actually 3 can be made to work, but you'd basically be winding a 3-phase induction motor stator to get
a properly balanced field without eccentric forces - a lot more work than 4 electromagnet poles)
Wow thank you for the help. I did order 4 because I wasn't sure. I'll try to understand what you mean about three magnets and a stator and hopefully my magnets will be here.
I thought pwm would work for three but maybe not. I'll have to test when they get here.