1. Core Causes of Overheating and Overload

1.1 Load-side Causes (Most Common)

• Actual load exceeds rated load: For example, blockages in the pipelines of water pumps and fans increase resistance, excessive cutting volume of machine tools, and jamming of conveyor belts. These cause the motor’s output torque to continuously exceed the rated torque, and the current far exceeds the rated current (the overload current is usually 1.2-2 times the rated current), leading to a sharp increase in copper loss (I²R) and subsequent heating.
• Frequent start-up/forward-reverse rotation of the load: The starting current of the motor is 5-8 times the rated current during start-up. Frequent start-stop will cause the heat generated by short-term large currents to accumulate, especially for small and medium-sized asynchronous motors, where the start-up loss accounts for a higher proportion.
• Excessive load fluctuation: For equipment such as crushers and vibrating screens, the load fluctuates greatly. The motor needs to adjust the torque frequently, and current fluctuations lead to heat accumulation.

1.2 Motor Self-causes

• Winding faults: Inter-turn short circuits, phase-to-phase short circuits, or ground short circuits in the stator windings will reduce the effective number of turns of the windings and cause abnormal current increase, resulting in severe local overheating (e.g., the temperature at the inter-turn short circuit can exceed 200℃ instantly). Open circuits in the rotor windings (for wound rotors) or poor contact of slip rings will cause uneven rotor current and additional loss heating.
• Iron core faults: Damage to the insulation between the silicon steel sheets of the stator core (such as aging and wear) will increase “eddy current loss” and “hysteresis loss”, causing the iron core to heat up and transfer heat to the windings. Loosening of the iron core laminations increases magnetic resistance, which also intensifies heating.
• Mechanical faults: Wear, oil shortage, or jamming of bearings increase the rotational resistance of the rotor, and mechanical losses are converted into heat. Uneven air gap between the stator and rotor (such as bearing inner/outer ring runout) leads to uneven magnetic field distribution, excessive local magnetic flux density, and increased additional losses.

1.3 Power Supply-side Causes

• Abnormal power supply voltage: Excessively high voltage (more than 10% above the rated voltage) will saturate the magnetic flux density of the stator core and sharply increase iron loss. Excessively low voltage (more than 10% below the rated voltage) will reduce the motor’s output torque. If the load remains unchanged, the motor needs to increase the current to maintain the torque, resulting in increased copper loss.
• Abnormal power supply frequency: The industrial frequency in China is 50Hz. If the frequency decreases (e.g., below 48Hz), the speed of the stator rotating magnetic field decreases, the rotor slip rate increases, and the rotor copper loss increases. An increase in frequency will increase the motor’s iron loss.
• Three-phase power supply imbalance: If the three-phase voltage difference exceeds 5%, the three-phase current of the stator will be unbalanced. Negative-sequence current generates a reverse rotating magnetic field, increasing additional losses and heating, and especially causing the rotor to overheat.

1.4 Operating Environment Causes

• Poor heat dissipation conditions: Damage to the motor’s cooling fan, blockage of the fan cover, or installation of the motor in an environment with high temperature (exceeding 40℃), excessive dust, and poor ventilation prevent heat from being effectively dissipated, leading to temperature accumulation.
• Mismatched protection class: For example, using a motor with IP23 protection class (protects against solid foreign objects but not water) in a humid environment allows moisture to enter, reducing the insulation of the windings and increasing leakage current, which causes heating.

2. Preventive Measures for Overheating and Overload

1. Reasonably match the load and the motor
   • When selecting a motor, ensure that its rated power is 10%-20% higher than the actual load power (i.e., the “load rate” is controlled at 80%-90%) to avoid “a small horse pulling a big cart”. For equipment requiring frequent start-up and forward-reverse rotation, select “frequent start-up type motors” (such as YZR series wound asynchronous motors).
   • When installing the load, ensure that the coaxiality of the equipment’s mechanical transmission system (such as couplings and pulleys) meets the requirements to avoid additional load due to misalignment.
2. Regularly maintain the motor
   • Winding inspection: Use an insulation resistance meter (megohmmeter) to test the insulation resistance of the stator windings to the ground every month, which should not be lower than 0.5MΩ (for low-voltage motors). If it is too low, the windings need to be dried or replaced. Regularly check the appearance of the windings for discoloration and burning odor.
   • Iron core and mechanical inspection: Check whether the iron core laminations are loose every quarter, whether the bearings have abnormal noise and oil leakage, and replenish or replace the grease (such as No. 2 lithium-based grease) regularly according to the instructions. Check the air gap between the stator and rotor, and adjust the bearings or rotor if it is uneven.
   • Cooling system inspection: Clean the dust on the motor heat sink and fan cover every week to ensure that the fan blades are intact and the air duct is unobstructed.
3. Ensure stable power supply
   • Install voltage and frequency monitoring devices to ensure that the power supply voltage fluctuates within ±5% of the rated value and the frequency fluctuates within ±1Hz. For three-phase equipment, install a three-phase unbalance protector to automatically shut down the machine when the three-phase current unbalance exceeds 10%.
   • For scenarios with unstable voltage (such as factory workshops), install a voltage stabilizer or variable frequency power supply to avoid motor overload caused by abnormal voltage.
4. Optimize the operating environment
   • Install the motor in an environment with good ventilation, temperature below 40℃, and no dust or corrosive gas. If the environment is harsh, select a motor with a high protection class (such as IP54, IP65) and install a cooling fan or cooler (such as forced air cooling, water cooling).
   • Avoid exposing the motor to direct sunlight or placing it near heat sources (such as boilers, heaters). If necessary, install a sunshade or heat insulation board.

3. Emergency Handling Methods for Overheating and Overload

1. Stop the machine immediately: Disconnect the motor power supply to avoid further expansion of the fault (such as winding burnout). If the thermal relay acts, wait for it to cool down (about 5-10 minutes) before resetting.
2. Troubleshoot the cause:
   • Touch the motor housing and bearing end cover by hand to determine the heating part (e.g., heat on the winding side may be a problem with the load or power supply, while heat on the bearing side may be a mechanical fault);
   • Check whether the load is jammed and the transmission system is normal, use a multimeter to detect whether the power supply voltage and three-phase current are balanced, and use a megohmmeter to detect the insulation resistance of the windings;
   • If a bearing fault is suspected, remove the end cover to check the bearing wear, or use a stethoscope to listen for abnormal noise during operation.
3. Targeted handling:
   • If it is load overload: Reduce the load or replace the motor with a higher power;
   • If it is abnormal power supply: Contact an electrician to adjust the voltage and repair the three-phase unbalance;
   • If it is a winding fault: Dry the damp windings or replace the windings with short circuits/open circuits;
   • If it is a mechanical fault: Replace the worn bearings and adjust the alignment of the stator-rotor air gap or transmission system;
   • If it is poor heat dissipation: Clean the cooling system and install cooling devices.
4. Test run verification: After handling, run the motor without load for 5-10 minutes to check whether the current and temperature are normal; then run it with the rated load for 30 minutes. Confirm that there is no overheating before resuming normal operation.

4. Summary