Detailed explanation of the working principle of electromagnet: from current magnetic effect to magnetic circuit design

The working principle of an electromagnet can be summarized as a chain: current passing through a coil generates a magnetic field (current magnetic effect) → the coil concentrates the magnetic field into a tubular shape → the soft iron core is magnetized, amplifying the magnetic field hundreds or thousands of times → the magnetic circuit and air gap determine the actual output suction force. After power is cut off, the magnetic domains of the iron core return to disorder, and the magnetism disappears - this is the physical essence of the "controllability" of an electromagnet.
Key points of this article (30-second quick overview)
Principle in three steps: Electrifying a wire to generate magnetism → Magnetizing a solenoid coil to gather magnetism → Magnetizing a soft iron core (with a magnetic permeability hundreds to thousands of times that of air).
Four elements of magnetic force: ampere-turns NI, core material, air gap δ, and magnetic pole area A; when unsaturated, the attraction force ∝ (NI)², and under ideal magnetic circuit conditions, it is ∝ 1/δ².
Ohm's Law in Magnetic Circuit: Φ = NI / Rm. Despite the air gap being only a few tenths of a millimeter, the magnetic reluctance often accounts for the majority of the total magnetic circuit.
Magnetic saturation is the ceiling: the working magnetic density is generally taken as 70% to 80% of the saturation value. Beyond this, adding turns only results in heat generation without increasing force.
Two-piece AC electromagnet: Silicon steel laminations suppress eddy current heating, and a short-circuit ring eliminates zero-crossing vibration noise.
I. What is an electromagnet: It consists of three components: the coil (excitation winding): wound with insulated enameled wire, which generates a magnetic field when energized and is the source of magnetic force; the iron core (yoke and pole shoes): a soft magnetic material magnetic conductor, which amplifies the coil's magnetic field and guides it to the working air gap for output; the power supply and control system: determines the power-on, power-off, and current magnitude, making the magnetic force controllable like a "switch".
In 1820, Oersted discovered the magnetic effect of electric current, and in 1825, Sturgeon made the first electromagnet; thereafter, with Henry's improvement of the winding process, the electromagnet became the power core of relays, solenoid valves, motors, and lifting equipment.
II. The working principle in three steps: Step ①: Current magnetic effect - electricity generates magnetism. When a wire is electrified, a circular magnetic field appears around it, and the direction is determined by the right-hand rule. This is the starting point for all electromagnetic devices.
Step ②: Solenoid Magnetization. After the wire is wound into a solenoid, the magnetic fields of each turn are superimposed in the same direction, forming a nearly parallel and dense tubular magnetic field inside the solenoid; the more turns, the stronger the magnetic field.
Step ③: Increase the magnetization of the soft iron core. After inserting the soft iron core, the disordered "magnetic domains" within the core align neatly under the influence of an external magnetic field, effectively amplifying the magnetic field by a factor of μr. The relative magnetic permeability of soft iron can reach hundreds to thousands, so under the same number of turns, the magnetic field with a core is hundreds to thousands of times stronger than that of an air-core coil.
After power is cut off: the external magnetic field disappears, the magnetic domains of soft magnetic materials return to disorder, and only a very weak remanence remains - hence the switching characteristic of "magnetic when powered on, non-magnetic when powered off".
Figure 1 (see HTML version): Three-step schematic diagram of the working principle of an electromagnet - ① circular magnetic field of an energized straight wire; ② tubular magnetic field of a solenoid; ③ neatly arranged magnetic domains within the iron core, resulting in amplified magnetic field.
III. Four Factors Determining Magnetic Force: When considering a single air gap and uniform magnetic density, the engineering approximation formula for the attractive force of an electromagnet is as follows:
F ≈ B²·A / (2μ0), where μ0 = 4π×10⁻⁷ T·m/A.
Example: When the air gap magnetic density B=1T and the magnetic pole area A=10cm² (0.001m²), F≈398N, which is approximately equivalent to a suction force of 40kg.
factor
physical quantity
The influence on magnetic force
Design Highlights
Excitation intensity
Ampere-turns NI
When the magnetic circuit is not saturated, the suction force ∝ (NI)²
Increase the number of turns or amplify the current, but subject to temperature rise and power supply limitations
core material
Permeability μ, saturation magnetic flux density Bs
μ determines the amplification factor, while Bs determines the upper limit of magnetic force
Soft iron, silicon steel, Bs is approximately 1.5~2.2T
Working air gap
Air gap length δ
Under an ideal magnetic circuit, the suction force ∝ 1/δ²
Minimize the air gap as much as possible under the given structural constraints
Magnetic pole area
End area A
Suction ∝ A
Increasing the area of the pole shoes can directly enhance the suction force
IV. Introduction to Magnetic Circuit Design: Treating the Magnetic Circuit as an Electrical Circuit for Calculation 1. Ohm's Law for Magnetic Circuits: Magnetic flux Φ = NI ÷ Rm, and magnetic reluctance Rm = l / (μ·A). Analogy: Magnetomotive force NI is equivalent to supply voltage, magnetic reluctance Rm is equivalent to resistance, and magnetic flux Φ is equivalent to current. The magnetic reluctance of the iron core segment is small, while that of the air gap segment is large, with the total magnetic reluctance being dominated by the air gap.
2. Air Gap: The relative magnetic permeability of air, which constitutes the majority of the magnetic reluctance, is 1. When the iron core material has a permeability of thousands, i.e., μr=1000, the magnetic reluctance of a 1mm air gap is approximately equal to that of a 1m-long iron core path. Minimizing the air gap as much as possible under structural constraints is the first lever to enhance the attraction force; it is precisely this principle that the air gap tends to approach zero and the attraction force increases sharply several times after the attraction-type electromagnet is attracted.
3. Magnetic saturation: After the knee point of the magnetic ceiling magnetization curve (B-H curve), the growth becomes significantly slower: as the current continues to increase, the magnetic density almost no longer rises, and all energy is converted into heat. In engineering, the working magnetic density is generally taken as 70% to 80% of the saturation value, and the number of turns and current are deduced accordingly.
4. Two exclusive designs of laminated cores for AC electromagnets: Alternating magnetic flux induces eddy current heating in the conductive core, and the surface insulation and laminated use of silicon steel sheets can cut off the eddy current path; Short-circuit ring (magnetic shunt): At 50Hz, the alternating magnetic flux crosses zero about 100 times per second, causing the attraction force to momentarily become zero, which can lead to vibration and sound emission from the armature. After the end face of the magnetic pole is sheathed with a copper short-circuit ring, a phase-lagging magnetic flux is induced inside the ring, and the resultant attraction force no longer crosses zero.
5. The magnetic flux in the leakage part bypasses the working air gap and leaks, reducing the effective suction force. Countermeasures: Reasonably design the yoke cross-section, set up pole shoes, and shorten the bypass around the air gap.
Comparison of commonly used iron core materials
material
Relative magnetic permeability μr (magnitude)
Saturation magnetic flux density Bs (T)
Characteristics and Typical Applications
Pure iron/low carbon steel (annealed)
Thousands
≈2.15
DC electromagnet core, yoke; annealing is required to reduce remanence
silicon steel sheet
Thousands
1.9~2.0
Lamination is used to suppress eddy currents, making it the preferred choice for AC electromagnets and transformers
Permalloy
Up to the order of magnitude of 100,000
≈0.7
Weak magnetic field precision devices, magnetic shielding
ferrite (soft magnetic)
Hundreds to thousands
0.3~0.5
In high-frequency applications, it has high resistivity and low eddy current
Air (air gap)
one
—
The ring with the largest magnetic reluctance in the magnetic circuit
Figure 2 (see HTML version): Analogy diagram of Ohm's law in magnetic circuits - U-shaped electromagnet flux circuit + equivalent circuit (NI=power supply, Rm=resistance, Φ=current).
V. How to apply the principle to typical applications
Focus of principle
Electromagnetic crane
DC excitation achieves a large ampere-turns count; the magnetic pole area is large; after attraction, the air gap approaches zero, leading to a sharp increase in attraction force
Relay / Contactor
Snap-fit type small air gap magnetic circuit, with fast attraction and release of the armature; AC products use shorting rings to eliminate noise
Solenoid valve
The round tube solenoid directly drives the moving iron core (armature) to move the valve core, with a small stroke, fast response, and spring reset
MRI superconducting magnet
The resistance of a superconducting coil is zero, and NI can be made extremely large without generating heat, producing a stable magnetic field of 1.5~3T or higher