# Electropermanent Magnet not working?

I have made an electropermanent magnet based on summary provided on this blog.

Here are my details -

• I have used AlNiCo and Neodymium Magnet together in the core with 10mm diameter and 20mm length.
• I have used two Iron rings on the end of the magnet with 45mm diameter and 2mm thickness.
• I have used insulated Copper wire with 2mm diameter (diameter of the Copper wire inside is 1mm).
• About coil turns are taken in total.
• I am using a 9 volt battery for power supply.

Here's the image of the experiment -

However, when I pass currents of different polarity through the circuit, no difference in the magnet field is observed. What is the reason for this?

Is this because of low windings, low electricity supply or something else?

If you want more details about electromagnet, then please check this paper.

Thanks a lot :)

• A typical 9V PP3 battery cannot supply much current. Also you don't say how many turns, how you measure the field, how the magnets are arranged etc. Dec 21, 2015 at 12:56

An Nd magnet typically has a surface field strength of about 1 Tesla. To overpower that with an electromagnet is really difficult. Using this, you would probably need around 1000 turns and 10 Amps

• He isn't trying to change the Nd magnet. He's trying to change only the Alnico. The Nd magnet is there solely to bias the field. Dec 21, 2015 at 13:51
• Well, the bias is still going to be huge compared to what he appears to be generating with his coil and battery
– user56903
Dec 21, 2015 at 14:06
• My "gut feeling" as an engineer is that he needs around 1000 Ampere turns
– user56903
Dec 21, 2015 at 14:08
• Thanks a lot for your answer :) Not many resources are available online. I just have one more question regarding the same topic. In my actual experiment, I will need magnets of diameter less than 1mm and length of 1.5mm. How many turns and amps do I need then? Dec 22, 2015 at 12:50
• The answer is "I don't know". The problem is that there are all kinds of other problems going to intrude on ideal conditions, not least the fact that the diameter of any wire you use is going to need to be very small in comparison to the solenoid you wind. That ups its resistance and you might end up with heat problems. Then we have "end effects" to take into account. I can only suggest you experiment.
– user56903
Dec 22, 2015 at 13:02

Agree with RedGrittyBrick. 9V battery won't provide the necessary current (even with many windings). To change the field in the Alnico, you need quite a few windings of thinner wire and a strong pulse that should (for efficiency sake) be very short--which means put a big capacitor across the battery (get the polarity right, if the cap is polarized!) and then touch the wire briefly for the pulse. I suggest start with a really big cap of several thousand uF and if that works, try smaller ones until it doesn't work.

• By the way, touching a wire is actually a slow process electrically, and so you need a big cap to continually provide power while the wire eventually makes good contact. A well-designed circuit will probably use a fast transistor (like an SCR), so you can get by with a much smaller cap and proportionally less total energy. Dec 21, 2015 at 14:09
• Thanks a lot for your answer :) Not many resources are available online. I just have one more question regarding the same topic. In my actual experiment, I will need magnets of diameter less than 1mm and length of 1.5mm. How many turns and amps do I need then? Dec 22, 2015 at 12:51
• There are two things to consider for efficiency's sake: You want to make the pulse brief (there is a lower limit but its sub microseconds) and you want to match the impedance of the coil and the driving circuit. So first decide how fast drive it (i.e. whether you are touching wires (~10s of ms) or firing an SCR (microseconds)), and then set the coil inductance so its reactance is close to the impedance of the driving ckt. For the smallish device you have there, I would expect the reactance to be about 100 ohms. So number of windings depends on your pulse time. Dec 22, 2015 at 13:06
• BTW, it might be worth wrapping only the Alnico. Dec 22, 2015 at 13:09
• Thanks again for the answer. By the way, are you sure only wrapping Alnico will work? I read the PhD thesis of Ara - cba.mit.edu/docs/theses/10.06.knaian.pdf and it didn't mention anything about ignoring Neodymium magnet. Dec 22, 2015 at 13:12