Temperature rise is a crucial indicator of the reliability of motor products. If the motor temperature rise test value is too low, it means that the effective material consumption is too high and the manufacturing cost is increased; if the test value is too high, the motor is prone to overheating under normal operating conditions, the winding insulation ages prematurely, and if the aging is severe enough, it will directly burn out the motor.
In fact, one of the important goals of motor design is to control the temperature rise within a certain range while minimizing material consumption. Here, MS . will discuss specific measures to reduce temperature rise during motor manufacturing, based on personal practical experience. Assuming that the design parameters have been optimized to their best state, the decisive factor affecting the motor's temperature rise depends on the manufacturing process.
Factors affecting temperature rise during the manufacturing process
Winding impregnation
Impregnation not only strengthens and cures the motor's insulation, but also, through advanced vacuum pressure impregnation equipment, fills the air gaps between winding turns and between windings and ground. Because the thermal conductivity of the insulating impregnating varnish is much greater than that of air, all the heat generated by losses in the motor stator windings is mainly transferred to the core through the winding insulation layer, i.e., the impregnating varnish, and the core then directly conducts the heat to the motor housing for dissipation.
If there are problems with the motor pre-drying temperature, varnish viscosity, and impregnation time, it can lead to severe varnish dripping during the drying process, incomplete varnish filling of the gaps between the windings and the core slots, resulting in voids or even the windings within the core slots not being fully impregnated, causing a significant rise in winding temperature. Therefore, quality control in the winding impregnation process has a crucial impact on reducing the temperature rise of the motor.
Stator core and frame mating
The mating surfaces of the stator core and the frame are crucial for conducting heat from the stator windings and dissipating heat from the core. A sufficiently tight fit between the stator core and the frame significantly reduces the resistance to heat transfer and lowers the core temperature. Various defects often occur during manufacturing: deviations in the inner diameter of the frame, rough machining, uneven stator core stacking, and excessively thick residual varnish layers on the outer surface of the core during impregnation, etc.
In addition to severely affecting the coaxiality of the stator and rotor of the motor, these defects also cause another fatal flaw: the increased gap between the stator core and the frame leads to increased thermal resistance, making it difficult for the motor to dissipate heat and causing the winding temperature to rise.
Surface treatment of machine base
When the heat generated inside the motor is conducted to the surface of the frame, it is mainly carried away by the air blown across the motor surface by the fan. Therefore, in order to effectively control the temperature rise of the motor, the heat dissipation level of the frame surface should be increased as much as possible, so that the iron shell with high thermal conductivity is in as close contact as possible with the cooling medium air, or in other words, the insulation layer should be as thin as possible.
This situation requires special attention because motor manufacturers place great importance on appearance quality. To make it look aesthetically pleasing and high-end, they apply thicker layers of putty and paint. While this does compensate for defects on the casing surface, it affects the actual heat dissipation performance of the base surface.
Motor and fan compliance
As the main air pressure component of a self-ventilated motor, the size and shape of the fan are key factors determining the temperature rise level. If the blades of a highly variable mixed-flow fan are accidentally bumped or subjected to heavy pressure during stacking or transportation, resulting in bending or deformation, the fan's rotation angle and impact angle will be altered, inevitably leading to an increase in the temperature rise of the motor windings.
From a design perspective, fans for motors with the same center height but different pole numbers have certain differences. If a small fan is used by mistake, its ventilation and heat dissipation effect will be greatly reduced; conversely, if a large fan is used by mistake, it will cause the motor efficiency index to deviate.
Fan blades of motors that rotate bidirectionally or have an uncertain direction of rotation generally have no directionality, but for motors with a fixed direction of rotation, it is essential to ensure that the direction of the fan blades meets the requirements.
Pulping quality control
Core loss is one of the five major losses in a motor, and the quality of the laminations must be strictly controlled to ensure the lamination ratio. This is mainly controlled from the following three aspects:
The material of the laminations. This requirement involves two aspects: firstly, selecting the silicon steel sheet supplier to ensure the material performance is met; and secondly, preventing the use of materials with grades that do not meet design requirements. For example, using W800 silicon steel sheets when W600 should be used.
Blade control during lamination. Most core manufacturers can control lamination burrs to within 0.05mm , which ensures the lamination effect of the core; otherwise, the effective iron weight will be insufficient, the eddy current in the core will increase, and thus lead to increased iron loss and temperature rise.
Core stacking. After stacking, every effort should be made to avoid filing, grinding, machining, and damaging the insulating oxide film. This problem exists in many motor manufacturers, and each motor manufacturer should make substantial improvements to address this issue, because the grinding and repair during the core manufacturing process directly affects the performance of the motor. Even with the best materials, without good manufacturing processes, the quality control and improvement of the motor will not be very effective.
Purity and parameter control of cast aluminum rotor
For cast aluminum rotors, the quality control of aluminum procurement is crucial, and the equipment used for melting aluminum during the casting process is also directly related. Better companies use ceramic tongs, effectively ensuring the purity of the aluminum during casting; however, some companies use cast iron tongs. With this method, some rotors in each batch will inevitably have poor aluminum purity when using the last of the molten aluminum from the tongs. This directly manifests as high motor impedance during inspection and testing, caused by excessive iron content leading to increased slot leakage flux.
The main process parameters for rotor aluminum casting include core preheating temperature, molten aluminum temperature, pouring speed, centrifugal casting speed, and pressure. These parameters vary among different motor manufacturers and are relatively critical and confidential control parameters. Ms. Can once visited a low-pressure aluminum casting company. The company had excellent equipment and produced very high-quality rotors, but it maintained a high level of confidentiality regarding the process parameters, making it impossible for visitors to see the specific details.
Rotor casting defect control
A broken bar inspection device is used to check the rotor for defects such as thin bars, broken bars, shrinkage cavities, shrinkage porosity, and air holes. For rotors with severe defects, the rotor will exhibit obvious bluing during motor operation, and in more serious cases, the aluminum in the rotor will melt and flow out.
Motor manufacturing is a meticulous production process. The same design scheme produced by different motor manufacturers will produce drastically different results. Different motor manufacturers also have their own unique advantages in different processes. Combining the advantages of many to create high-quality motors is a demand for motor product quality under the new market conditions.
