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Hi trying to understand how to build an electromagnet
It must be 38-40 kg strong, and powered by 1,5-3v rechargeable batteries (operating for at least few years) and using as few amp as possible, it will be switched on and off 1-2 times per sec

I know that it's possible cause it exists already;

Mine just needs to be a little stronger and work 24/7
I would be thankful if you could propose any other source of constant energy (to replace the batteries) or a solution to make them last longer

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Can't see your Grainger link, but I doubt Grainger fools with 3 volt electromagnets even in locks. I have a six volt lock that works for as long as the batteries self discharge. The force of an electromagnet is

https://www.easycalculation.com/engineering/electr...

BTW a 10 Newtons force is 2.248 Pounds of force

I include for you a method to accomplish a greater field flux density for a lower energy by incorporating a supplementary NIB magnet buried in your electromagnet, see the picture.

How long does it take to discharge your batteries?

I'm not sure if i could use a NIB magnet as it will make the electromagnet stick to the iron even when it's not supposed to and the power is off.

Then you did not notice, when you pass a smaller negative DC current in the other direction it will ZERO out the all the flux UN-STICK the magnet.

.

Regrettably you do not appreciate the significance of this reveal :-( to you.

THEORY => A final limiting feature of powerful electro-magnet design with lots of turns is resistive heating.... Heating so much that liquid cooling is ultimately required..... Well this design concept allows more flux at less heat !!!

I use the batteries once a day.

Batteries self discharge on a shelf after several years, that is how long.

I do appreciate your answer, but for my application I will probably have to use it without a permanent magnet. I do not think there will be a heating problem, in this case the door holder that is almost of the same strength as my electromagnet would burn the door, which is not the case.

I'm really interested in the specs of your device, you've told me that it uses 6v, is it on 24/7 and in your opinion what is it's strength in kg

This is an old device that pulls a small pin releasing a clever mechanism that uses the individual pushing against the door as the primary unlocking force.

I suspect the force to overcome a small pin spring to be in the single digit ounce range for a very short one time unlock.

To make an electro magnet stronger you use a larger core and more turns of the wire around it. In turn the magnet requires more power to run. To make it run longer you need to use larger capacity batteries or connect several batteries in parallel. Unfortunatly you will not get the holding power your looking for out of 1.5V to 3V. Your link is to a door holder that uses 24VAC at raround 1.5mA but can only hold 3lbs. A small fractonn of what your looking for (about 88lbs). Pulsing the power allows it to run longer but doesn't reduce the voltage and current required to power the magnet.