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I saw a magnetic lock on the front door of my apartment building and was wondering the holding force of it. I read that they easily have a pulling force of thousands of newtons. The one in my building only run at 12VDC and 500mA. I was surprised by this low current value.

How can I calculate the holding force of an electromagnet at a given distance ? How many turns the coil inside typically has ?

Electromagnet

The winding and the core are shown on this picture:

Electromagnet

Taken from this question

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    $\begingroup$ This is not straightforward. See Force between two bar magnets section in this Wiki article: en.wikipedia.org/wiki/Force_between_magnets. The force depends on many different factors, most importantly on magnetic properties (flux density etc.) and geometry (area, distance etc.). $\endgroup$ Commented Nov 30, 2021 at 13:50
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    $\begingroup$ Is the keeper on the left of my last picture a permanent magnet? I thought the whole core was just ferrite, and the coil produced a flux in the magnetic circuit. $\endgroup$
    – Ultra67
    Commented Nov 30, 2021 at 14:03
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    $\begingroup$ Even if it's not a permanent magnet, it's still not a straightforward problem. For this device, the literal answer to "how do I calculate the holding force?" is "computer modeling & finite element analysis." $\endgroup$ Commented Nov 30, 2021 at 14:07
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    $\begingroup$ You are right, it is not a magnet, it is probably some ferrite. But the point is still the same, to the best of my knowledge calculating the force is not that simple. In some special cases it is possible to estimate the force via power/energy, but this does not apply here. Your best option is to measure it :) $\endgroup$ Commented Nov 30, 2021 at 14:07
  • $\begingroup$ Thank you for your answers. What are the special cases when an estimation is possible? $\endgroup$
    – Ultra67
    Commented Nov 30, 2021 at 14:51

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This is not straightforward. For example, see Force between two magnets section in the following Wiki article:

https://en.wikipedia.org/wiki/Force_between_magnets

Although this does not directly apply to your question, you get the idea that the force depends on many different factors, most importantly on magnetic properties (magnetic flux) and geometry (area, distance etc.).

In your case, the other side is not a magnet but some kind of a ferrite core. The point is still the same - calculating the force is not straightforward and would require some heavy procedures such as FEM.

In some special cases it would be possible to estimate the force. For example, in synchronous electric machines (stator and rotor), the mechanical power equals electrical power (current times voltage) minus losses, and the mechanical power equals torque times shaft speed, and the torque equals force times rotor length. Therefore, to estimate force you need to measure current, voltage and speed, and you need to know length.

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