Although an electric motor is not a uniform material, the basic characteristics of its heating process generally apply. To prevent the motor's temperature rise from exceeding a certain level, it is necessary to both reduce losses within the motor and increase its heat dissipation capacity.
With the increasing capacity of individual motors, improving the cooling system and enhancing heat dissipation to limit the temperature rise of the motor has always been one of the main issues in the development and progress of motors.
When a motor operates normally at a certain capacity, its temperature rise is also constant. Therefore, only by specifying the temperature rise of the motor can the motor's capacity have a precise meaning. The purpose of temperature rise calculation is generally to verify whether the temperature rise of several heat-generating components in the motor exceeds the allowable limit value during rated operation, and to take into account the necessary margin. Ms. Can will share some simple insights on the topic of temperature rise with you today.
Principles for determining temperature rise limits
When a motor operates under rated conditions for an extended period and its temperature stabilizes, the permissible limit of temperature rise for each component of the motor is called the temperature rise limit. The temperature rise limit of a motor is specified in national standards, with different heat resistance grades corresponding to different temperature rise values.
For windings, the temperature rise limit depends primarily on the maximum temperature allowed by the insulation structure and the temperature of the cooling medium, but it is also related to factors such as the temperature measurement method, the heat transfer and heating conditions of the windings, and the allowable heat flux intensity, which are explained below:
● The materials used in the insulation structure of motor windings gradually deteriorate in terms of their mechanical, electrical, and physical properties under the influence of temperature. When the temperature rises to a certain level, the properties of the insulation material undergo a fundamental change, eventually even losing its insulating ability. In electrical engineering, the insulation structures or systems in motors and electrical appliances are often classified into several heat resistance classes according to their extreme temperatures. Under normal conditions, long-term operation of the insulation structure or system at the corresponding temperature class will not result in any qualitative changes in its performance.
● Insulation structures can achieve a relatively economical service life under specified extreme temperatures. Theoretical derivation and practical experience have proven that the service life of insulation structures has an exponential relationship with temperature, making it highly sensitive to temperature. If the operating temperature exceeds the extreme temperature by 8-14°C, its service life will be shortened by an average of half. For certain special-purpose motors, where a long service life is not required, the allowable extreme temperature can be increased based on experience or experimental data to reduce motor size.
●While the temperature of the cooling medium varies depending on the cooling system and the medium itself, for most currently used cooling systems, the cooling medium temperature is essentially determined by the atmospheric temperature and is roughly the same numerically. However, atmospheric temperature varies throughout the year and by location. Statistics show that the annual average temperature across my country is below 22℃, the average maximum temperature does not exceed 35℃, and the absolute maximum temperature is generally between 35℃ and 40℃, with only a very few areas between 40℃ and 45℃. Currently, most countries worldwide use the absolute maximum temperature of the atmosphere in most regions as the temperature for the cooling medium; therefore, my country's national standard specifies +40℃ as the temperature for the cooling medium.
● Different methods of temperature measurement will result in different differences between the measured temperature and the temperature of the hottest spot in the measured component. The temperature of the hottest spot in the measured component is the key to determining whether the motor can operate safely for a long time.
●Generally, temperature rise limits are specified for areas with an altitude not exceeding 1000 meters and a maximum ambient temperature of 40℃. In areas with higher altitudes, the air is thinner and heat dissipation is poorer. The national standard stipulates that the temperature rise limit for a motor operating under these conditions should be extrapolated when the altitude of the motor's location is higher than that of the test location.
Control of temperature rise limits under special circumstances
In certain 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; other factors must also be considered:
● Further increasing the temperature of the motor windings generally means increased motor losses and decreased efficiency, which is not necessarily economically worthwhile.
● An increase in winding temperature (e.g., above 150°C) may cause difficulties in the operation of the bearing lubrication system.
● For motors with commutators, increased winding temperature (e.g., above 200°C) can cause difficulties in commutation.
●Increased winding temperature will cause an increase in thermal stress in the materials of certain related components.
●Other effects, such as on the dielectric properties of insulation and the mechanical strength of conductor metal materials, will also have an adverse impact.
Therefore, although some motor windings currently use F-class or H-class insulation structures, their temperature rise limits are often still specified according to the B-class value. This not only takes into account some of the factors mentioned above, but is also more beneficial for increasing the reliability of the motor during use and can extend the service life of the motor.
Disclaimer: This article is a reprint. If there are any copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
