When the motor is running, temperature is one of the main factors affecting the service life of the winding.
Temperature rise
Temperature rise is the temperature difference between the motor and the environment, which is caused by the heat generated by the motor. The running motor core is in the alternating magnetic field, which will cause iron loss. When the winding is energized, copper loss will occur, and other stray losses will occur. These will increase the temperature of the motor. On the other hand, the motor will dissipate heat When the heat and heat are equal, the equilibrium state is reached, and the temperature does not rise and stabilizes at a level. When the heat is increased or the heat dissipation is reduced, the balance is broken, the temperature continues to rise, and the temperature difference is increased to increase the heat dissipation and reach a new equilibrium at another higher temperature. However, the temperature difference at this time, that is, the temperature rise has increased more than before, so that temperature rise is an important indicator in the design and operation of the motor, which indicates the degree of heat generation of the motor. During operation, if the temperature rise of the motor suddenly increases, Indicates that the motor is faulty, or the air duct is blocked or the load is too heavy.
Relationship between temperature rise and temperature
For a normally operating motor, the temperature rise under theoretical load should be independent of the ambient temperature, but it is actually affected by factors such as ambient temperature.
(1) When the temperature drops, the temperature rise of the normal motor will decrease slightly. This is because the winding resistance R is lowered and the copper loss is reduced. For every 1 °C drop in temperature, R is reduced by about 0.4%.
(2) For self-cooling motors, the temperature rise is increased by 1.5 to 3 °C for every 10 °C increase in ambient temperature. This is because the copper loss of the winding increases as the temperature rises. Therefore, temperature changes have a greater impact on large motors and closed motors.
(3) The air humidity is 10% higher. Due to the improvement of heat conduction, the temperature rise can be reduced by 0.07~0.38°C, with an average of 0.19°C.
(4) The altitude is 1000m, and 100m per liter, the temperature rise increases by 1% of the temperature rise limit.
Extreme operating temperature and maximum allowable operating temperature
Generally speaking, the ultimate operating temperature of Class A is 105 °C, and the maximum allowable operating temperature of Class A is 90 °C. So, what is the difference between the extreme operating temperature and the maximum allowable operating temperature? In fact, this is related to the measurement method. Different measurement methods reflect different values and have different meanings.
(1) Thermometer method
The measurement results reflect the local surface temperature of the winding insulation. This number is on average about 15 °C lower than the actual maximum temperature of the winding insulation, the "hottest spot". This method is the simplest and most widely used in medium and small motors.
(2) Electrical resistance method
The measurement results reflect the average of the copper temperature of the entire winding. This number is reduced by 5 to 15 ° C depending on the actual maximum temperature. The method measures the cold state and the thermal resistance of the conductor, and calculates the average temperature rise according to the relevant formula.
(3) embedded thermometer
Copper or platinum resistance thermometers or thermocouples are embedded in the windings, cores, or other components that require the highest temperature expectations. The measurement results reflect the temperature at which the temperature measuring element contacts. Large motors often use this method to monitor the operating temperature of the motor.
The temperature measured by various measuring methods has a certain difference from the actual maximum temperature. Therefore, it is necessary to subtract the difference from the "limit operating temperature" of the insulating material to be the "highest allowable operating temperature".
