I am attempting to create an electromagnetic propulsion system (and no it is not a perpetual motion machine I am not that thick) and for that I need an electromagnet capable of producing a very strong electromagnetic field, currently I am getting a moderately strong field however it is insufficient and I want to know if there are any ways I can increase the strength, I am using a MOT with the secondary removed.

I know that there are three main factors which influence the strength of an electromagnet, 1) Amperage 2) Coil Density 3) Permeability of the core

The core seem to be very permeable and its unlikely I can get anything better than that taking into consideration funds. I also can't really change the coil density

So I am mainly considering amperage and I used http://www.daycounter.com/Calculators/Magnets/Solenoid-Force-Calculator.phtml to do some calculations and it seems like this will definitely work but the trouble is I am currently (pun intended) unsure of how to increase the amperage as while I have connected a 400A power source to it, it obviously isn't absorbing even close to that and I estimate it is getting under 8A so my main question is is there a to make it absorb more amperage? I am not overly concerned about Overamping (not the drug one, the electrical one)as it is unlikely to instantaneously destroy it and I have a spare.

  • \$\begingroup\$ More voltage or less windings of thicker wire are your two options. The max field strength you'll be able to get with an electrical steel core will be around 1.8-2 Tesla. \$\endgroup\$
    – Jon
    Commented Sep 3, 2015 at 8:31

2 Answers 2


If you look at the force generated by a solenoid you will not normally find a term in there for the permeability of the solenoid core: -

\$F = \dfrac{(N*I)^2 μ_0 A }{ (2 g^2)}\$

  • N is number of turns
  • I is amps
  • \$\mu_0\$ is permeability of free space
  • A is cross sectional area of solenoid
  • g is gap between metal wishing to be attracted and one end of the solenoid

This is because the flux produced to attract another piece of metal is mainly flowing through a large air gap. This air gap totally dominates and should not be confused with dimension g in the formula above - g above is the gap between one end of the solenoid and the metal to be attracted BUT flux then has to flow out of that metal and back to the other end of the solenoid (a much bigger gap).

However, should you have a solenoid that is attracting the metal with both pole pieces i.e. one constructed from a simple transformer core then this is improved. However, the path that the mag field takes is still thru a significant air gap and the magnetic reluctance of this air gap will still likely dominate the low reluctance of the core except for very small gaps.

Reluctance adds in series like resistors in a conventional circuit and clearly 1k in series with 1 ohm is still largely 1k.

If you are using a single ended solenoid (I have no idea what a MOT is) then abandon the MOT and make your own from much fatter gauge wire so you can push the current through it. Make the cross sectional area (A) as big as possible and stack the windings as close to each other as possible.


A coil can be approximately described as an ideal inductor in parallel with a resistor. So when you plug a current source into it, the voltage first climbs as high as the source allows, then after a time about L/R (that could be seconds, did you wait before measuring?) decreases to R.I if that is under the source max voltage. If its not, then you must either increase the source compliance, or decrease the coil resistance - that means thicker wire, or better conductor (more pure copper), or parallel coils.


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