Thermal Expansion Matching Design for Precision Motor Die Castings and Silicon Steel Sheets: How to Prevent Stress Cracking During High-Temperature Operation?
Publish Time: 2025-09-17
During the operation of modern high-performance electric motors, the internal temperature rises significantly with load changes, causing different degrees of expansion in components such as the stator core, windings, and housing. Precision motor die castings are typically made of aluminum alloy, serving as a supporting and enclosing structure, while silicon steel sheets are stacked to form the core, providing magnetic conductivity. These two materials have significantly different physical properties: the thermal expansion coefficient of aluminum alloy is much higher than that of silicon steel. When the motor starts from cold and operates at full load and high temperature, the inconsistent expansion rates can, if not properly designed, generate significant thermal stress at the interface, potentially leading to cracking of the die casting, loosening of the core, or even insulation failure. Therefore, thermal expansion matching design is crucial for ensuring the long-term reliability of the motor.The root cause of thermal stress lies in the different responses of materials to temperature changes. When the motor is energized, Joule heat is generated in the windings, rapidly conducted to the silicon steel core and then to the die-cast housing. The aluminum alloy housing expands rapidly due to its high thermal conductivity and expansion coefficient; while the silicon steel core, although having lower thermal conductivity, has inherent axial compression space due to its laminated structure, limiting radial expansion. If the die casting and the core are tightly fitted or rigidly fixed, the expansion of one will be constrained by the other. This mutual constraint generates shear and tensile stress at the interface, especially in stress concentration areas such as structural discontinuities, wall thickness transitions, or bolt fastening points, easily leading to microcracks that propagate with repeated thermal cycles, ultimately causing structural failure.To avoid these problems, designers must implement a "thermal compatibility" mechanism at the structural level. The primary strategy is to control the clearance tolerance. At room temperature, an appropriate gap is maintained between the inner cavity of the precision motor die castings and the outer diameter of the silicon steel sheet to allow the two to expand freely during the heating process and avoid forced contact. too large a gap can lead to loose assembly, poor heat dissipation, or vibration noise, while too small a gap cannot relieve stress. Through thermo-mechanical coupling simulation, deformation under various operating conditions can be predicted, guiding the setting of tolerance ranges and achieving a dynamic balance between "tight fit at high temperature and clearance at low temperature."Secondly, stress-relieving features are incorporated into the structural design. Elastic grooves, chamfers, or flexible ribs are added to the inner wall of the precision motor die casting, allowing localized deformation to absorb displacement caused by differential expansion. These structures do not compromise overall rigidity, but provide micro-scale buffering, preventing stress concentration. Sharp corners and abrupt thickness changes are avoided; smooth transitions and uniform wall thickness reduce local stress peaks.Material selection and process control also play a role. Some high-end motors use a special aluminum alloy with added silicon and copper to adjust its coefficient of thermal expansion, matching it more closely to that of the silicon steel sheet. During stacking of the silicon steel sheets, appropriate axial pressure is applied to form a stable assembly, reducing internal relative displacement and the reaction force on the casing. The cooling rate during die casting is strictly controlled to minimize residual stress, ensuring the casting is in a low-stress state before operation.In the assembly process, a staged tightening strategy is used. Bolted connections are not fully tightened initially, but retightened after several thermal cycles, ensuring that all components reach their final position at the operating temperature before achieving a rigid connection. Some designs use elastic washers or limit pins instead of rigid fasteners, allowing for slight movement without compromising structural integrity.Ultimately, thermal expansion matching is not merely a comparison of material properties, but a system-level coordination art. It requires designers to view the motor as a thermo-mechanical-electrical multi-field coupled system, seeking a dynamic balance between cold and hot states. When a motor withstands thousands of start-stop cycles without cracks or loosening, it is not just a material triumph, but a testament to design ingenuity—a delicate balance between expansion and constraint, where differences are harmonized, and power is sustained in quiet reliability.
