Die-Cast Aluminum Rotors: Analysis Of Causes For Excessive Temperature

Excessively high motor temperature is a common abnormal condition under actual operating conditions. It may stem from continuous overload or jamming of the transmission system, or winding inter-turn faults. Most root causes can be identified through abnormal noise readings or abnormal temperature readings at individual measuring points from online temperature monitoring devices. However, diagnosing issues related to die-cast aluminum rotors proves far more challenging.

Problems in the transmission system produce obvious observable symptoms, and the stationary stator can be monitored in real time. By contrast, the rotor rotates during operation and is difficult to inspect. Abnormalities are usually suspected only after unexplained tripping, reduced output power, insulation aging and other failures occur. Subjected to prolonged operation at excessive temperatures due to inadequate monitoring, the rotor suffers accelerated fatigue aging of aluminum material and expansion of defective areas. This results in deteriorated overload capacity, higher operating energy consumption, and even severe accidents such as broken rotor bars and winding burnout, posing hidden risks to safe and stable motor operation.

Abnormal temperature rise of die-cast aluminum rotors is rarely caused by overload alone. Instead, it arises from the superposition of multiple factors including electromagnetic losses, mechanical failures, structural defects and ineffective heat dissipation.

Heat generation originating from electromagnetic effects in the rotor falls into two categories: sharply increased internal heat generation and heat buildup induced by structural defects. Internal heat mainly comes from resistance loss, namely Joule heat generated when rotor current flows through rotor resistance. During frequent start-stop cycles, prolonged overload or locked-rotor operation, rotor current can reach 5 to 7 times the rated operating current, leading to a sharp surge of Joule heat in aluminum bars. Inherent defects such as blowholes, shrinkage cavities and rotor eccentricity formed during manufacturing cause uneven current distribution in the bars, triggering localized current concentration and heat accumulation, and forming fixed high-temperature zones.

Heat generated by abnormal mechanical factors overlaps and compounds electromagnetic heating. Rotor eccentricity leading to stator-rotor rubbing, bearing seizure and poor coaxiality of the rotating shaft generates massive frictional heat. The accompanying unbalanced electromagnetic effect creates additional heat, jointly pushing rotor temperature rise beyond its normal tolerance range.

Failure of the external heat dissipation system is a major contributor to excessive temperature rise and often overlooked in routine maintenance. After long-term operation, ventilation ducts accumulate dust, oil sludge and fibrous debris, directly blocking air passages and preventing internal heat from being discharged via convection. Damaged, deformed or loose cooling fan blades reduce air volume and drastically cut heat dissipation efficiency. In addition, confined installation environments with poor natural ventilation, high ambient temperature in summer, or long-term low-speed variable-frequency operation that weakens the motor's self-cooling capability trap rotor heat continuously. This creates a vicious cycle that accelerates rotor aging and energy losses.

Preventing failures triggered by overheated rotors requires measures covering both product manufacturing and operational maintenance.

Regularly disassemble and clean motor ventilation ducts to remove accumulated dust and oil contamination; inspect and replace damaged fan blades to ensure unobstructed airflow. Standardize operating conditions, avoid continuous overload, frequent start-stop and locked-rotor operation, stabilize power supply quality and mitigate interference from grid harmonics.

Optimize die-casting aluminum rotors process to minimize internal defects and eccentricity, eliminating potential risks of localized heat generation at the source. Improve motor ventilation duct design by expanding ventilation area and optimizing airflow routes to enhance passive heat dissipation. For equipment serving high-temperature, confined and heavy-load working conditions, install auxiliary cooling fans and shrouds to adopt forced air cooling. This effectively controls rotor temperature rise, slows fatigue aging of aluminum alloy, and greatly extends motor service life.