Hysteresis loss and eddy current loss are two important sources of losses present in motor cores. This article focuses on the hysteresis loss in motor cores.
Definition of Motor Cores
The core plays a crucial role in the entire motor. Its function is to increase the magnetic flux through the inductor coils to achieve maximum conversion of electromagnetic power. The motor’s core is generally composed of a stator and rotor, with the stator core being the main component of the motor’s magnetic circuit.
The stator core is typically made of laminated silicon steel and also supports the winding part. During the operation of the generator, the core is subjected to the combined effects of mechanical forces, thermal stresses, and electromagnetic forces.
Functions of the Core
| Enhancing Magnetic Flux: The motor core is usually made from magnetic materials like silicon steel, which have high magnetic permeability. It provides a low reluctance path for the magnetic field, allowing magnetic flux to pass more easily, thus enhancing the magnetic flux through the inductor coils, achieving maximum conversion of electromagnetic power and improving the motor’s efficiency and performance.
| Forming the Magnetic Circuit: The stator and rotor cores together constitute the magnetic circuit of the motor. The stator core is the principal part of this circuit, providing the foundational structure for the formation and distribution of the magnetic field, allowing the magnetic field to circulate within the motor along a specific path, ensuring normal operation.
| Supporting the Windings: The core provides a platform for the motor’s windings to be installed and supported. The windings are key components for generating electromagnetic force. The core holds the windings in the correct position to ensure stability during motor operation and also aids in heat dissipation, prolonging the lifespan of the windings.
Types of Core Loss
With the development of electric vehicles, higher performance requirements for motors have emerged, making high energy efficiency and power density increasingly important. The design of the motor core is one of the key influencing factors.
Two significant sources of loss within the core are hysteresis loss and eddy current loss. Both types of loss increase with the frequency of the motor; hysteresis loss is proportional to frequency, while eddy current loss is proportional to the square of frequency.
Definition of Hysteresis Loss
Hysteresis loss refers to the energy consumed due to the hysteresis phenomenon during the repeated magnetization of ferromagnetic materials. The hysteresis phenomenon involves a lag in magnetization intensity behind magnetic field intensity when the magnetic state of the ferromagnetic material changes, resulting in the formation of a hysteresis loop between magnetic flux density (B) and magnetic field strength (H).
During each magnetization cycle, the hysteresis loss per unit volume of the core is proportional to the area of the hysteresis loop. This energy is converted into heat, leading to a device temperature rise and reduced efficiency. Hysteresis loss is a significant component of iron loss in electrical equipment, and in alternating current motors and other devices, minimizing the impact of hysteresis loss is desirable.
Factors Affecting Hysteresis Loss
| Type of Material: Different types of magnetic materials have distinctly different hysteresis loops, directly determining the magnitude of hysteresis loss. Generally, soft magnetic materials have high permeability and low coercivity, resulting in smaller hysteresis loop areas and, consequently, lower loss. For example, silicon steel is commonly used in transformer cores due to its low hysteresis loss, which can effectively improve transformer efficiency.
| Magnetic Field Frequency: The frequency of the magnetic field also has a notable impact on hysteresis loss. At low frequencies, magnetic domains have sufficient time to magnetize and demagnetize, resulting in relatively low hysteresis loss. Conversely, at high frequencies, the magnetization and demagnetization processes of magnetic domains are constrained, leading to increased hysteresis loss.
| Temperature: Temperature significantly affects hysteresis loss in magnetic materials. As the temperature increases, the magnetization process of the materials becomes relatively easier, and the loss decreases. However, when the temperature exceeds the Curie temperature of the material, the magnetic material loses its magnetism, sharply increasing hysteresis loss. For instance, certain magnetic materials exhibit low hysteresis loss at room temperature, but as the temperature approaches their Curie temperature, their hysteresis loss can rise dramatically, leading to a severe decline in device performance.
Measures to Optimize Hysteresis Loss
| Selecting Low Hysteresis Materials: Using materials with low hysteresis characteristics, such as amorphous and nanocrystalline soft magnetic alloys, which exhibit lower hysteresis loss due to their microstructural characteristics.
| Optimizing Composition Design: Adjusting the chemical composition of materials—such as adding trace elements (e.g., Si, Al, Cr)—to improve magnetic performance and reduce hysteresis loss.
| Applying Transverse Magnetic Field Techniques: Applying a transverse magnetic field, perpendicular to the alternating magnetic field, on soft magnetic composite materials, can significantly reduce hysteresis loss.
| Reducing Grain Size: Refining grain size can decrease the size of magnetic domains, lowering hysteresis loss. This can be achieved through specific heat treatment processes or rapid solidification technologies.
| Improving Processing Techniques: Employing appropriate processing and heat treatment methods, such as oriented crystallization and hot pressing, can optimize the internal structure of materials, reducing defects and thus lowering hysteresis.
Significance of Reducing Hysteresis Loss in Motor Cores
| Reducing Heating: Minimizing the loss can decrease heating, maintaining the core temperature at lower levels, which helps prolong the lifespan of the core and, by extension, the entire motor.
| Improving Motor Efficiency: Lowering the loss reduces energy waste, allowing more electrical energy to be converted into mechanical energy, thereby enhancing motor efficiency.
| Reducing Maintenance Costs: High temperatures can accelerate the aging of other materials inside the motor, such as insulation materials. Reducing the loss leads to lower temperatures, which can slow the aging process of these materials, increasing the reliability and stability of the motor and reducing maintenance costs caused by material aging.
| Meeting High-Speed Operation Requirements: In some high-speed motor applications, such as high-speed machining equipment, the motor needs to operate at high speeds, which increases the magnetization frequency of the core and subsequently increases hysteresis loss. Lowering the loss allows the motor to maintain high efficiency and performance during high-speed operation, fulfilling the demands of these high-speed applications.
Motor Provider
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