Discrimination method for motor temperature rise

The testing of motor temperature rise or temperature has always been a major topic, and experts and scholars in the motor industry have tried countless schemes and conducted countless experiments in an attempt to find an accurate test method to grasp motor temperature rise or temperature. However, no matter which method is adopted, the measured motor temperature always deviates from the actual value. Sometimes, the same method is used on different motors, and the experimental results are very confusing

Hengli Motors roughly divides the temperature rise and temperature testing of motors into three methods: resistance method, thermometer method, and thermistor method. The method to be adopted should be determined according to actual needs, and combined with a large amount of data accumulation and infiltration of empirical methods, striving to minimize the deviation between measured values and actual values.

Thermometer method

The measurement results of the thermometer method reflect the local surface temperature of the winding insulation. Many motor manufacturers place thermometers at the lifting ring position during the type testing process of Hengli motors to measure and test the temperature of the stator core representing the motor. This number is on average about 15 ℃ lower than the actual highest temperature of the winding insulation, also known as the "hottest point".

Resistance method

The measurement results of the resistance method reflect the average temperature of the entire winding electromagnetic wire. This number is 5-15 ℃ lower than the actual maximum temperature according to different insulation levels. The principle of this method is to measure the cold and hot resistance of the conductor and calculate the average temperature rise of the winding according to relevant formulas. It is particularly emphasized that this temperature rise is only an average temperature rise and cannot represent the temperature rise status of each point in the winding. However, from a theoretical analysis perspective, the values measured by this method should be relatively accurate.

Thermal Sensor Testing Method

This method involves embedding thermal sensitive elements such as copper or platinum resistance thermometers or thermocouples in windings, iron cores, or other components that require the highest expected temperature measurement. The measurement results reflect the temperature at the contact point of the temperature measuring element. This method is commonly used to monitor the operating temperature of large motors

Practice has proven that for the same motor, there is a deviation between the temperature rise measured by the resistance method and the data measured by the thermistor testing method, and sometimes the deviation is even significant. This situation is actually Ms The question asked by the participant Xiao D is that there is a huge gap between appearance and tactile sensation, and the test results are somewhat unbelievable. To this end, Ms. Chen will first explain the underlying reasons for the occurrence of this problem, and then share some relevant knowledge or experience.

Differences in test results

The test results of thermal sensitive components are related to the placement of the components, reflecting the temperature at the location where the components are placed. The resistance test result is the average value, and the appearance and tactile sensation are the comprehensive results of the heating situation of various parts of the motor. The heating situation and heat dissipation conditions of different parts of the motor are different, and the uncertainty caused by motor design and manufacturing factors results in vastly different temperature differences. However, the customer's questioning of PT100 actually confuses local temperature and average heating situation, indicating the necessity of finding and eliminating the highest temperature point caused by local overheating during the product trial stage.

Temperature limits for various parts of the motor

The temperature rise of the iron core in contact with the winding (thermometer method) should not exceed the temperature rise limit of the insulation of the winding in contact (measured by resistance method)

The temperature of rolling bearings for general motors should not exceed 95 ℃, and the temperature of sliding bearings should not exceed 80 ℃. Hengli motors can cause changes in oil quality and damage to the oil film due to high temperatures, which can also lead to bearing failure. For motors with particularly high temperature rise and special working conditions, the influencing factors should be identified to prevent bearing failure.

In practice, the temperature of the casing is often based on not being hot to the touch. This is an evaluation method that customers can directly perceive, but for places with particularly high environmental temperatures, this requirement is somewhat strict for motor manufacturers; At the same time, the performance of the motor is also an unavoidable test.

The surface of the squirrel cage rotor has significant stray losses and high temperatures, generally limited to not endangering adjacent insulation. It can be estimated by applying irreversible color changing paint in advance. In the process of handling motors returning to the factory, we often find the fact that there are blue spots on the surface of the rotor, which is an actual manifestation of high motor temperature or overheating of the rotor itself.

Analysis and fault identification of motor heating problem

When the temperature of the motor exceeds the maximum operating temperature or the temperature rise exceeds the specified value, or when the temperature rise suddenly increases at low load although it does not exceed the specified value, it indicates that there is a fault in the motor. The diagnosis and troubleshooting methods are:

Under rated load, the temperature rise does not exceed the temperature rise limit, and the motor temperature exceeds the maximum allowable operating temperature only due to the ambient temperature exceeding 40 ℃. This phenomenon indicates that the motor itself is normal. The simplest solution is to manually lower the ambient temperature, otherwise the load must be reduced during operation.

The temperature rise exceeds the nameplate specifications under rated load. Regardless of the situation, it is a fault with the motor and must be stopped for inspection, especially when there is a sudden increase in temperature rise. Possible reasons include: (1) external factors: low grid voltage or excessive line voltage drop (more than 10%), heavy load (more than 10%), improper coordination between motor and machinery; (2) Internal factors: single-phase operation of the motor (commonly known as phase loss), turn to turn short circuit, phase to phase short circuit, stator grounding, fan damage or loose fastening, air duct blockage, bearing damage, stator rotor rubbing, motor and cable joint heating (especially copper aluminum or aluminum aluminum connection), motor corrosion or moisture, etc. (3) Other: In theory, motors can rotate in both forward and reverse directions, but some motors have directional fans (especially 2P motors). If they are reversed, the temperature rise will exceed a lot; This problem often occurs during the motor repair process.

The various problems and targeted analysis and judgment should be a process that motor manufacturers and customers must interact with. When entering into a product contract, both parties should have sufficient communication on the usage requirements to ensure the suitability of the motor and its use.