Tips for controlling motor temperature rise and temperature
For a motor, although it is not a homogeneous material, the basic characteristics of the heating process are generally applicable to the motor. In order to make the temperature rise of the motor not exceed a certain value, it is necessary to reduce the loss generated in the motor on the one hand, and increase the heat dissipation capability of the motor on the other hand.
With the increasing capacity of the single unit of the motor, improving the cooling system and improving the heat dissipation capacity to limit the temperature rise of the motor have always been one of the main problems in the development and progress of the motor.
When the motor is operating normally under a certain capacity, its temperature rise is also constant. Therefore, only the temperature rise of the motor is specified to make the capacity of the motor have a definite meaning. The purpose of the temperature rise calculation is generally to calculate whether the temperature rise of several heat-generating components in the motor during the rated operation exceeds the allowable limit value, and taking into account the necessary margin, Ms. Sharing.
Principle of determining the temperature rise limit
When the motor is operated for a long period of time under rated conditions and its temperature is stable, the allowable limit value for temperature rise of each component of the motor is called the temperature rise limit. The temperature rise limit of the motor is specified in the national standard, and different heat resistance levels correspond to different temperature rise values.
In the case of windings, the temperature rise limit is basically determined by the maximum temperature allowed by the insulation structure and the temperature of the cooling medium, but also the temperature measurement method, the heat transfer and heating conditions of the winding, and the heat flow intensity allowed in the winding. Relevant, respectively, are as follows:
● The materials used in the insulation structure of the motor winding will gradually deteriorate in mechanical, electrical and physical properties under the action of temperature. When the temperature rises to a certain extent, the characteristics of the insulating material will change substantially. Finally, even the ability to lose insulation. In electrical engineering, the insulation structure or insulation system in motors and electrical appliances is often divided into several heat-resistant grades according to the limit temperature. Insulation structures or systems operate for long periods of time at the appropriate grades of temperature and generally do not produce qualitative changes in properties.
● The insulation structure can obtain a relatively economical service life at the specified limit temperature. Theoretical derivation and practice have proved that the service life of the insulation structure is exponential with temperature, so it is very sensitive to temperature. If the operating temperature exceeds the limit temperature of 8-14 ° C, its service life will be reduced by an average of half. For some special-purpose motors, if the service life is not very long, in order to reduce the size of the motor, the allowable limit temperature of the motor can be improved based on experience or test data.
●The temperature of the cooling medium varies depending on the cooling system and cooling medium used. However, for various cooling systems currently in use, the temperature of the cooling medium is basically dependent on the atmospheric temperature, and numerically and atmospherically. The temperature is roughly the same. However, the atmospheric temperature varies with different times and locations within a year. According to statistics, the average annual temperature in China is below 22 °C, the average maximum temperature does not exceed 35 °C, and the absolute maximum temperature is generally between 35 and 40 °C. A few areas are between 40 and 45 °C. At present, countries around the world generally use the absolute maximum temperature of the atmosphere as the temperature of the cooling medium. Therefore, the national standard of China specifies the temperature of +40 °C as the cooling medium.
● The method of measuring temperature is different, which will cause the difference between the measured temperature and the hottest temperature of the tested component, and the hottest temperature of the tested component is the key to judge whether the motor can operate safely for a long time.
● Under normal circumstances, the temperature rise limit is specified for areas where the altitude does not exceed 1000 meters and the maximum ambient temperature is 40 °C. In areas with higher altitudes, the air is thinner and the heat dissipation conditions are worse. One motor operates under such conditions. The national standard of the temperature specifies that the altitude of the motor is higher than the test site.
Control of temperature rise limit under special circumstances
In some special cases, the temperature rise limit of the motor windings often does not depend entirely on the maximum allowable temperature of the insulation structure used, but also other factors:
● Further increase in the temperature of the motor windings generally means an increase in motor losses and a decrease in efficiency, which is not necessarily economically advantageous.
● Increase in winding temperature (for example, above 150 °C) may cause difficulties in the operation of the bearing lubrication system.
● For motors with commutators, the increase in winding temperature (eg above 200 °C) can cause difficulty in commutation
• An increase in winding temperature will cause an increase in thermal stress in some related component materials.
● Others, such as the dielectric properties of the insulation, the mechanical strength of the conductor metal material, etc., will have an adverse effect.
Therefore, in some current motor windings, although the F-class or H-class insulation structure is used, the temperature rise limit is often still in accordance with the B-level specified value, which not only takes into account some of the above factors, but also increases the reliability of the motor. It is more advantageous and can extend the life of the motor.
