The difference and connection between temperature rise test and ambient temperature test

In summer, engineers always worry about the motor overheating. But everyone knows that to measure the degree of heating of the motor is to use "temperature rise" instead of "temperature". The temperature test involved in the motor test is mainly the temperature rise test and the ambient temperature test. This article mainly introduces the difference and connection between the two.
    1. Motor temperature rise test
    The motor starts to run at normal temperature (the temperature of each part is the same as the ambient temperature), and the temperature continues to rise. When it is higher than the ambient temperature, it continues to absorb heat and slowly rises up. On the other hand, it started to radiate heat to the surroundings. When the motor is in the heat balance state and the temperature no longer rises, the difference between the temperature of the motor and the ambient temperature is called the motor temperature rise. That is: temperature rise = motor temperature-ambient temperature, using K as the unit.
    The allowable temperature of the motor is the temperature that the winding can withstand. When used for a long time at this temperature, the physical, mechanical, chemical and electrical properties of the insulating material will not undergo significant malignant changes. If the temperature exceeds this temperature, the properties of the insulating material will change qualitatively or cause rapid aging. Therefore, the allowable operating temperature of the insulating material is determined according to its economic service life. The allowable temperature of the motor is determined, and the limit of temperature rise at this time depends on the temperature of the cooling medium. Generally, the cooling medium in motors is air, and its temperature varies with regions and seasons. In order to manufacture motors that can be applied throughout the country throughout the year, and to clarify unified inspection standards.


    Figure 1 Comparison table of motor insulation class


    Although many advanced technologies have been used to measure the temperature of motor windings and other parts, they can still be summarized into three basic methods: resistance method, thermometer method, and embedded thermometer method.
    Resistance method: Conductor resistance increases with temperature. Resistance and temperature rise have the following relationship. The temperature rise measured by resistance method is the average temperature rise of the winding, which is about 5 degrees Celsius lower than the hot spot of the winding. The measurement of resistance can be measured by voltammetry or bridge method. After the power is cut off, the measured temperature rise is lower than the actual temperature at the moment of the power cut.
    Thermometer method: Use a thermometer to directly measure the temperature rise of the motor. When the motor reaches the rated operating state, its temperature gradually rises to a certain stable value and no longer rises. At this time, a thermometer can be used to measure the temperature of the motor. The temperature measured by this method is the local temperature of the measuring point.
    The embedded thermometer is to embed the thermocouple or thermal resistance thermometer in the motor manufacturing process, which can not be reached after the motor is manufactured. This method is mainly used to measure the temperature of the AC stator winding, iron core and structural parts. Using this method requires at least six thermometers to be embedded between the winding layers of the motor, and they are distributed along the circumference. Under the premise of ensuring safety, they should be placed as much as possible in the hot part of the winding, and avoid the thermometer and Cooling air contact, for the use of air-cooled motors, the reader can determine whether the temperature rise of the winding meets the requirements with a thermometer.


    Figure 2 Motor embedded thermometer


    2. Motor ambient temperature test
    For some motors that will actually run in a high temperature environment, the actual ambient temperature should be simulated during the type test, and then the load test should be performed; for this type of test, a thermostat is generally used to cover the motor under test and heat it , Simulate the actual ambient temperature for experiments.
    The motor ambient temperature test generally faces two directions:
    1. Do extreme environmental durability tests for special high and low temperature motors. For example, high-temperature motors are made of high-temperature-resistant insulating materials, which can ensure the safe operation of the motor in a high-temperature environment without aging or burning. The high-temperature motor has a wide range of applications and has a promising market prospect. The motor performance is designed according to the characteristics of the fan and pump load, which can save energy.
    2. Measure the operating state of the motor at a constant temperature to eliminate the influence of temperature on the operating parameters of the motor.


    Figure 3 Schematic diagram of the connection of the motor environment experiment box


    Three, the influence of ambient temperature changes on the temperature rise of the motor
    The temperature rise measured in the cold winter of the same motor is low, and the temperature rise measured in the midsummer time. For a normal running motor, theoretically, its temperature rise has nothing to do with the ambient temperature under the rated load, but it is actually affected by the ambient temperature. And other factors. When the ambient temperature drops, the temperature rise of the normal motor will be slightly reduced. This is because the winding resistance r decreases and the copper loss decreases. When the temperature drops by 1℃, r drops by about 0.4%.