Why Does Hysteresis Loss Occur?
When we experimentally test a piece of NdFeB material and repeatedly change its magnetization direction to the completely opposite state using an external magnetic field, after the external magnetic field disappears, the material does not fully return to a non-magnetic state but retains a certain magnetism. This phenomenon originates from tiny magnetic regions within the material called magnetic domains. These domains resist changes in the direction of the magnetic field. All this resistance primarily stems from the internal resistance of the domains flipping. The work done to overcome this is consumed within the material and converted into heat, leading to an increase in the material’s temperature—this is the ultimate manifestation of hysteresis loss, and this heating effect is known as hysteresis loss.
What Problems Can Hysteresis Loss Cause?
1.Temperature Rise
In electrical equipment, heat dissipation is an eternal challenge. Continuous hysteresis loss causes the iron core temperature to rise, accelerating the aging of insulation materials and shortening the equipment’s lifespan.
2.Reduced Operating Efficiency
In operating equipment, not 100% of the energy is used for useful work; instead, a portion is wasted as useless heat. In a city’s power system, these losses accumulate to form an astronomically large energy waste. Improving efficiency is a key topic in global energy conservation and emission reduction efforts.
Where Can We See Hysteresis Loss?
1.AC Motor Applications
Transformers and most electric motors use alternating current. Alternating current generates a magnetic field that periodically changes in both direction and magnitude, causing the magnetic materials in the motor’s stator/rotor to undergo high-frequency magnetization-demagnetization cycles. This results in extremely significant hysteresis loss. Designers must use special materials and cooling systems to manage it and prevent the machine from overheating.
2.DC Motor Applications
In many DC motors, the armature iron core rotates such that, although the external excitation magnetic field is produced by direct current and remains constant in direction, for any point on the armature iron core, the effective magnetic field direction it experiences is constantly changing. In DC motors, hysteresis loss mainly occurs in the armature iron core because it rotates in a constant magnetic field, leading to periodic changes in the effective magnetic field direction experienced internally. In contrast, the stator yoke experiences a constant magnetic field, so its hysteresis loss is typically very small.
How to Reduce Hysteresis Loss?
1.Use Soft Magnetic Materials
The advantages of soft magnetic materials are low coercivity and low remanence, allowing for a smaller hysteresis loop area. This is the most fundamental measure.
2.Use Materials with Small Hysteresis Loop Areas
The area of the hysteresis loop is equivalent to the energy loss. Adding silicon to silicon steel optimizes the magnetic properties of iron, making it easier to magnetize and demagnetize.
3.Optimize Material Properties Through Processes
Through processes such as refining and heat treatment, internal stresses and crystal defects in the material are reduced, thereby decreasing coercivity and hysteresis loss.
Calculation of Hysteresis Loss
To understand why an appliance with an iron core heats up, we can trace how energy is “wasted” step by step.
For example:
l = length of the iron rod
A = cross-sectional area of the rod
N = number of coil turns
i = current at any moment
H = magnetizing force = (N × i) / l
B = magnetic flux density
Volume of the iron rod V = A × l
Suppose we have an iron rod of length l and cross-sectional area A, with a volume of V = A × l.
When the current i in the coil undergoes a tiny change di, according to the law of electromagnetic induction, this induces an electromotive force e in the coil, attempting to resist the change in current. To continue changing the current, the power source must do work against this electromotive force, e. In a very short time, dt, the work done by the power source is: dW = e × i × dt.
Using the physical formula e = N × A × (dB/dt) for derivation, we can fully convert this micro-work into physical quantities describing the internal magnetic state of the material: dW = V × H × dB. This result is profoundly significant: it tells us that every tiny change in the material’s magnetic state requires energy input.
When the current completes a full cycle, the magnetization state of the material also cycles along the hysteresis loop once. By summing all the incremental work dW along the way, we can obtain the total energy loss for one cycle: Energy loss per cycle = Material volume V × Hysteresis loop area.
If such magnetization cycles occur f times per second, the power loss is: Hysteresis loss power Pₕ = V × loop area × f.
This continuous power eventually converts into Joule heat, which is one of the fundamental reasons why transformers or motor casings heat up.
I'm dedicated to popular science writing about magnets. My articles mainly focus on their principles, applications, and industry anecdotes. Our goal is to provide readers with valuable information, helping everyone better understand the charm and significance of magnets. At the same time, we're eager to hear your opinions on magnet-related needs. Feel free to follow and engage with us as we explore the endless possibilities of magnets together!