Insulation Aging
The three-phase motor insulation system consists mainly of a variety of materials, including paper insulation, varnish insulation, and layers of polymer insulation. The selection of insulation materials directly influences the long-term operating performance of motors. At high temperatures, the molecular structure of the insulation material could be changed by the action of thermal energy and will degrade the mechanical strength and electrical performance of the materials.
The insulation aging in motors is mainly due to thermal stress, oxidation, and moisture in practical applications. Generally speaking, insulation materials at higher operating temperatures have a much faster aging rate. As mentioned in studies, with every 10°C increase, the insulation material would experience half of its lifetime. As an example, an F-class insulation material that has a lifetime of 20 years at 120°C may have a reduced lifetime at higher temperatures. Therefore, it is very important to take into consideration the working environment during design and application so as to prevent excessive aging and failure of insulation materials.
The failure of insulation may have serious effects, including short circuits, equipment downtime, and safety hazards. According to specific conditions of industrial processes, for example, in chemical and metallurgical plants, where there may be a combination of high temperature and corrosive gases, the degradation of insulation materials can be accelerated significantly. In this respect, regular performance tests of insulation and maintenance are very important in assuring reliable motor operation in service.
Increased Resistance
With an increase in temperature, the resistance of the winding increases in three-phase motors, which then reflects on the efficiency and performance of the motors. The dependence of the motor’s resistance on the temperature is usually specified by a temperature coefficient. For copper materials, the resistance rises at around 0.4% per degree of temperature rise. Increased resistance during operation, therefore, leads to increased power losses, which further results in heating.
Increased resistance will influence the flow efficiency of current, and it will also lead to apparatus heating and potential overheating issues. When operating a motor at high temperature, the insulation materials of windings and other electric parts will suffer damage caused by prolonged overheating. Actually, in practical operation, there have been cases where motors have had overheating-induced failures due to the increased resistance that pushed the temperature beyond the tolerance limit of the insulation material.
To address the problems that increased resistance brings about effectively, many manufacturers use high-conductivity materials and good heat dissipation designs. For example, using materials with lower temperature coefficients can reduce the rate of resistance increase with temperature. Besides, checking the operating conditions of the motor regularly and ensuring it is within safe parameters is an important approach to avoid failures effectively.
Reduced Efficiency
In design, the efficiency of three-phase motors is regulated with high significance to ensure their energy consumption rate under different operation conditions. Under high temperatures, influences arise due to factors such as increased copper loss, changes in iron loss, and potential increases in friction loss.
Under high-temperature conditions, the copper losses, that is, the winding losses, increase because of the increased resistance of windings, leading to more power losses. Iron losses, which include hysteresis losses and eddy current losses, may also be affected by temperature. Further, the degradation of lubricants in the motor at high temperature can affect friction losses, hence lowering the overall efficiency of the motor.
Thus, a motor rated at 100 kW has an efficiency as high as 95% at an ambient temperature of about 40°C, while at an ambient temperature of 60°C, its efficiency may drop below 90%. This difference in actual energy consumption can impact production costs and resource use. For instance, if at high temperatures the actual input power of a motor is higher, the increased energy consumption compared to the input power at normal temperatures will directly influence operational efficiency.
It could increase the efficiency of the motor by applying several approaches in high-temperature working conditions: first, the motor design must be optimized by using adequate construction and materials. High-efficiency inverters and control systems can control the operating conditions of the motors to minimize energy waste. Regarding care and maintenance for the motor, that is also very important for keeping the motors in effective running condition.
Cooling Challenges
Cooling systems are very important for running motors, mainly if working in a high-temperature environment. Various cooling systems of motors include natural cooling and forced cooling. Natural cooling depends on the process of natural air convection within and around the motor, while forced cooling relies on mechanical apparatus like fans or pumps.
Under high-temperature working conditions, several factors may affect the performance of a cooling system. Firstly, with the rise of the ambient temperature, there is a corresponding rise in the temperature of cooling media such as air or water, which weakens their cooling effectiveness. Secondly, the efficiency of the cooling system may not be sufficient if the design fails to take high-temperature factors into consideration. For instance, poor airflow inside an enclosure will cause a heat build-up, leading to increased overheating of the motor.
During cooling, which is a challenge, it’s important that the design of the cooling system be optimized. It can be improved by adopting a ventilation design that increases the heat-dissipation area and uses cooling media that work more effectively. Besides, performing regular checks on the status of operation and maintenance is very important in terms of ensuring effectiveness. For example, cleaning the heat exchanger regularly and checking the flow of cooling fluid can help prevent malfunction of the cooling system.
Mechanical Stress
The impact that a high-temperature environment exerts on mechanical parts in a three-phase motor is easily overlooked but is significant. Different materials have different coefficients of thermal expansion, and the size of components changes accordingly under a high-temperature environment. These changes will cause mechanical stress, which impairs stability and reliability during the operation of motors.
With high temperatures, the fit between rotor and stator may be affected, leading to unnecessary vibration and noise. This involves more wear on the equipment and a tendency toward earlier failure. Bearings, as important moving parts of motors, may face accelerated degradation of lubricants in high temperatures, which leads to weakened effectiveness of lubrication and increased friction, accelerating the wearing of bearings.
