How to Build an Electromagnet
Shawn Martin | July 29, 2017
Image credit: Kanthal Bethel
Electric and magnetic fields propagate at the speed of light, functioning as a duality in that you cannot have one without the other. For every electrical current there exists a magnetic field that propagates perpendicular to the flow of electrons. The electromagnet is built off of this concept and a simple understanding of classical physics can be used to build a functional electromagnet.
Material Requirements
The weak magnet field produced by a current carrying conductor can be amplified by wrapping a powered coil around a ferromagnetic core to magnetize it. To build your own functional electromagnet, there are three basic requirements: a ferromagnetic core, a current carrying conductor and a power source.
The ferromagnetic core should exhibit low coercivity and high relative permeability. Its permeability is a function of its ability to concentrate the magnetic flux while its coercivity speaks to the ease of which the core becomes demagnetized when the power source is removed from the circuit.
Soft iron makes for an ideal ferrous core as its molecules are easily aligned in the presence of an electrical field and it will exhibit minimal magnetic memory. Care should be taken when forming soft iron, however, as in doing so the material will become hardened and lose its desired properties. If your core needs to be bent or formed into a desired shape, it should then be followed by an annealing process to alleviate any residual stress.
The current carrying conductor should exhibit low resistivity and be electrically insulated so that current flows through the coil rather than taking the path of least resistance. Unalloyed pure metal typically exhibits the lowest resistivity and copper magnetic wire — fully annealed, electrolytically refined copper with a thin coating of polymer film insulation — is most commonly used as copper has excellent physical, electrical conductivity and thermal resistance properties. Aluminum and copper-clad aluminum magnet wire is also available, which can be used for weight reduction and corrosion resistance.
Manufacturers like Kanthal Bethel produce solid magnet wire rather than stranded wire as stranded wire typically exhibits higher resistance and would require a thicker insulating film. The gauge of the wire is also a factor of resistance and while a thicker gauge wire will produce less heat, space constraints come into play and the gauge of the wire should be selected based on the size of the ferromagnetic core so that the tight coil of insulated wire will take up as little space as possible while making as many turns as possible.
A constant voltage power source would function best as the output can be tuned to the length of the wire and its known resistance as to not exceed a predetermined current capacity using Ohm’s law. Over-sizing the power source will melt and short the wire, and if there is little to no load on the circuit current will flow freely and quickly deplete the power source.
Engineered for Strength
The strength of the electromagnet is dependent on the core material, its size, the amount of current flowing through the coil, the number of turns within the coil and the length of the magnetic circuit.
A soft iron core formed in a “C” shape where the air gap is minimized will produce a stronger electromagnet. This is because minimizing the air gap drastically reduces the reluctance.
Reluctance is defined as the length divided by the product of the permeability and the area. Since the permeability of soft iron is roughly 1,000 times greater than air, the length of the air gap will have a significant impact on the total reluctance of the magnetic circuit and, consequently, the strength of the magnet. The ideal core will have a considerable cross-sectional area, a minimal length and a minimal air gap.
The current and the number of turns in the electrical coil are the next two important design considerations. A 28 gauge wire can make 10,000 turns about an iron core with a cross-sectional area 3 cm long and 4 cm wide. This will take approximately one kilometer of wire that will have 213 ohms of resistance and will be limited to a capacity of 1.4 amperes. The size of the core and gauge of the wire can be increased to decrease the resistance and increase the current capacity, but a larger wire will require significantly more room.
The electrical resistance and associated heat produced when operating an electromagnet are considerable. It is often countered by cooling with either water or cryogenic liquids when operating at or near the current capacity. In some, it is more suitable to utilize an electromagnet to demagnetize a permanent magnet when a controlled magnetic field is desired.
Resources:
Calculating the Strength of a Magnet
