Information about magnetostriction
Magnetostriction is a property of magnetic materials that causes them to change their shape or dimensions during the process of magnetization. The variation of materials' magnetization due to the applied magnetic field changes the magnetostrictive strain until reaching its saturation value, λ. The effect was first identified in 1842 by James Joule when observing a sample of iron.
Magnetostriction is a phenomenon in which magnetic materials undergo slight changes in length under the influence of a magnetic field. This effect can cause vibrations and noise (the so-called "100-hertz hum") in generators and impair efficiency. Various special generator designs and material technologies are used to mitigate magnetostriction in generators.
1. Material Selection:
*Soft magnetic materials: Choosing the right core material is crucial. Soft magnetic materials with low magnetostriction are used. These include:
*Silicon-iron alloys (electrical steel strip):
Special non-grain-oriented (NGO) or grain-oriented (GO) electrical steel sheets with a high silicon content are used. Silicon increases the material's resistivity, which reduces eddy current losses, while also positively influencing the magnetostrictive properties. Grain-oriented electrical steel strip is manufactured so that the easy magnetization direction lies in the rolling direction, which minimizes magnetostriction in this direction.
*Nickel-iron alloys (Permalloy):
These alloys exhibit very low or even negative magnetostriction and are therefore well suited for applications where low noise and vibration are critical.
*Amorphous and nanocrystalline alloys:
Due to their disordered atomic structure, these materials often have very low magnetostriction and low core losses.
2. Design measures:
*Laminate stacking:
The stator and rotor core of a generator consists of many thin, mutually insulated laminations (laminations). This prevents large eddy currents and thus reduces losses and the heat and noise associated with magnetostriction.
*Stress relief annealing:
After punching and machining the laminations, internal stresses can develop in the material, which negatively affect the magnetostrictive properties. Subsequent stress relief annealing can relieve these stresses and improve the magnetic properties.
*Impregnation and bonding:
The laminations are often joined under pressure and impregnated or bonded with resins. This increases the mechanical stability of the core and dampens vibrations that could be caused by magnetostriction.
*Geometry optimization:
Slot geometry: The shape and arrangement of the slots in the stator and rotor can be optimized to minimize local magnetic flux density fluctuations. These fluctuations are a major cause of magnetostriction.
*Air gap field harmonics:
Tooth-wound coil windings in permanent magnet synchronous machines (PMSMs) can exhibit strong stator air gap field harmonics, which amplify rotor eddy currents and thus also magnetostriction. The severity of these harmonics can be reduced by optimizing the winding and rotor geometry.
Robust mechanical design:
A rigid and low-resonance design of the entire generator is important to effectively dampen the vibrations generated by magnetostriction and minimize their transmission to the housing.
3. Active Damping Methods (less common in generators):
Although passive measures are generally preferred in generators, there are also approaches to active damping, albeit more complex.
*Active Noise and Vibration Control:
In very specific applications, sensors can detect vibrations, and actuators generate counter-vibrations to neutralize the vibrations caused by magnetostriction.
Research and Development:
Research continues to focus on developing new alloys with even lower magnetostriction and on optimizing design and manufacturing processes to further minimize magnetostrictive effects and improve the efficiency and quietness of generators.
Multidisciplinary design automation that considers electromagnetic, thermal, structural, rotordynamic, and NVH (noise, vibration, and harshness) aspects is playing an increasingly important role in this process.
