Temperature rise is an important performance indicator of an electric motor . An electric motor itself is a heat-generating element during operation. So how does a motor dissipate heat to reach equilibrium? Generally, heat is dissipated from the surface of the heat-generating element into the surrounding medium mainly through two methods: radiation and convection via air or other cooling media. In electric motors, the latter usually plays a dominant role.
Radiative heat dissipation
According to the law of radiation, the amount of heat radiated per second from the surface of a heating element per square meter is given by equation (1):
q=5.7×10-8v(T4-T04)(W/m2)………………(1)
In formula (1):
T—Temperature of the surface of the heating element (K);
T0 — the temperature of the surrounding medium (K);
5.7 × 10⁻⁸ — The radiation constant of a pure black object obtained from experiments;
V – a factor whose value varies depending on the surface condition of the heating element: 1 for pure black objects, 0.97 for coarse cast iron, 0.95 for rough wrought iron, 0.29 for polished wrought iron, 0.2 for rough brass, and 0.17 for polished copper.
According to equation (1), the amount of heat dissipated by radiation depends on two factors: firstly, the characteristics of the surface of the heating element, with objects with dull surfaces having greater radiation capacity than objects with glossy surfaces; and secondly, the temperature of the surface of the heating element and the surrounding medium.
Generally, in a calm atmosphere, heat dissipated by radiation accounts for about 40% of the total heat dissipation. When forced convection is used to cool a motor, the heat dissipated by forced convection is much greater than that carried away by radiation, so radiative heat dissipation is often neglected.
Convection cooling
Heat dissipation can be observed from the direct contact between a solid surface and a fluid. When the temperature of the solid surface is not equal to that of the fluid, heat exchange occurs between them, with heat being transferred from the hotter object to the colder object. This heat exchange actually involves both conduction and convection, but is collectively referred to as convective heat transfer. In electric motors, the heat generated in the core, windings, or other heat-generating components is carried away by cooling fluids (air, hydrogen, water, oil, etc.) flowing over one or more surfaces of these components; therefore, convective heat dissipation is widely present in motor cooling systems. The heat dissipation capacity of this method depends primarily on the fluid's motion on the solid surface.
When a fluid undergoes laminar flow, it flows only parallel to the solid surface. If the fluid is divided into many flow layers parallel to the solid surface, and there is no fluid exchange between the layers, then in the direction perpendicular to the solid surface, heat transfer mainly relies on conduction.
Because fluids have low thermal conductivity, heat dissipation from the solid surface is poor during laminar flow. When a fluid undergoes turbulent flow, its components no longer move parallel to the solid surface but instead form irregular vortices in all directions at an average velocity. In this case, heat transfer mainly relies on convection.
Because the thermal resistance is relatively small during convective heat transfer, the heat dissipation capacity of a solid surface is significantly enhanced when the fluid is in turbulent flow. In turbulent flow, a thin laminar layer still exists near the solid surface, but the higher the fluid velocity, the thinner this laminar layer becomes, and the higher the surface heat dissipation capacity. For convective heat dissipation, the surface heat dissipation capacity is also related to the physical properties of the cooling medium (such as thermal conductivity, specific heat, and density), as well as the geometry, size, and position of the solid surface within the fluid.
Regarding motor temperature rise
Temperature rise is the temperature difference between the heat-generating parts of a motor and the surrounding environment, typically referring to the temperature rise of the stator core and windings. During motor operation, the core, situated in an alternating magnetic field, experiences hysteresis and eddy current losses. Current flowing through the windings generates copper losses, and there are also wind friction losses, mechanical losses, and stray losses caused by air gap magnetic field pulsations from stator and rotor slots and inherent non-working harmonics in the windings. All of these ultimately manifest as heat, causing the motor temperature to rise.
In addition, corresponding to the heat-generating factors, the following heat-dissipating factors also exist:
●There is a temperature difference between the motor surface and the surrounding environment, resulting in heat transfer and heat radiation.
● Heat exchange occurs between the internal heating elements of the motor and the internal air, between the internal air of the motor and the motor housing, and between the motor housing and the air blowing across the surface of the motor.
When the two factors, namely the heat-generating factor and the heat-dissipating factor, reach a balance, the temperature stops rising and stabilizes at a certain level. If there is an external disturbance, such as an increased load or a sudden short circuit, the balance will be disrupted, the motor temperature will continue to rise, and the temperature difference will continue to widen until the new heat dissipation equals the new heat generation, reaching a new balance at a higher temperature. However, at this point, the temperature difference, i.e., the temperature rise, is already greater than before.
One of the most important tasks in motor design is to study and understand the qualitative or quantitative relationship between ventilation and heat dissipation conditions or factors and heat generation, ensuring that the actual temperature rise of the motor fluctuates within a reasonable range. In actual operation, if the motor temperature rise suddenly increases, it indicates that the motor has electrical faults such as inter-turn short circuits, or abnormal conditions such as air duct blockage or sudden load increase.
Factors affecting temperature rise
Theoretically, the temperature rise of a normally operating motor under rated load should be independent of the ambient temperature, but in reality it is still affected by factors such as ambient temperature.
●When the air temperature drops, the temperature rise of a normal motor decreases slightly. (Relationship between copper resistance and temperature)
●For self-cooled motors, the temperature rise increases when the ambient temperature increases.
●Increased air humidity improves heat conduction, which can slightly reduce temperature rise.
●Altitude is based on 1000m. At higher altitudes, the temperature rise will be slightly higher due to the relatively poor heat dissipation.
Passing the temperature rise test during type testing does not necessarily guarantee the safety of motor operation. Temperature rise is the difference in temperature, which is directly related to the actual operating temperature of the motor. When the ambient temperature is high, the actual operating temperature of the motor will also rise accordingly, which involves the matching relationship between the heat resistance rating and the actual operating temperature.
The maximum heat resistance temperature of related components is also directly related, such as the operating temperature control requirements of bearings. For example, the temperature of rolling bearings should not exceed 95℃, and the temperature of sliding bearings should not exceed 80℃. Excessive temperature can cause changes in oil quality and damage the oil film. The direct consequence of this problem is that bearing failure leads to overheating of the motor bearing system, which in turn causes a rapid rise in the temperature of the motor windings, resulting in motor stalling and bearing seizure, ultimately leading to motor burnout.
A motor manufacturer produces screw air compressor motors. During type testing, the motor temperature rise is around 70K. However, after the motors are launched into the market, devastating motor failures such as winding overheating and bearing failure occur intermittently in batches. Statistical data shows that the end customers for these faulty motors are all located in southern regions during the hot season.
There will always be discrepancies between expectations and reality, and the numerous seemingly unavoidable motor failures are actually a good thing. First, they provide motor designers with excellent benchmarks, allowing for more targeted product improvements. Second, they serve as a warning to motor manufacturers, especially those in northern regions, that passing factory testing does not guarantee successful product development; a thorough assessment of the matching relationship between test temperature rise and actual operating conditions is essential. Third, while matching test values with design values is necessary, reliable performance and positive customer feedback are the ultimate tests.
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