The material used to electrically insulate the device is called insulating material.
The combination of one or several insulating materials is called an insulating structure.
Represents the maximum allowable operating temperature of the insulation structure, and at such temperatures it can maintain its performance for a predetermined period of use, within the allowable range and its grade is the heat resistance grade. The heat resistance grade is divided into seven grades: Y-class 90 ℃, A-class 105 ℃, E-class 120 ℃, B-class 130 ℃, F-class 155 ℃, H-class 180 ℃ and H-class.
Beyond this limit, the life of the insulating material is drastically shortened and may even burn out. This temperature limit is called the allowable temperature of the insulating material.
The difference between the temperature at a point and the reference (or reference) temperature is called the temperature rise. It can also be called the difference between the temperature at a certain point and the reference temperature. The temperature difference between a part of the motor and the surrounding medium is called the temperature rise of the part of the motor.
When the motor reaches a thermally stable state under long-term operation under the rated load, the allowable limit of the temperature rise of each component of the motor is called the temperature rise limit.
The allowable temperature of the insulation material is the allowable temperature of the motor; the life of the insulation material is generally the life of the motor.
When the motor is running under load, starting from exerting its role as much as possible, the larger the load, that is, the larger the output power, the better (if mechanical strength is not considered). But the greater the output power and the greater the loss power, the higher the temperature. We know that the weakest thing in the motor is the insulation material, such as enameled wire. There is a limit to the insulation material's temperature resistance. Within this limit, the physical, chemical, mechanical, electrical, and other aspects of the insulation material are stable in performance, and the working life is generally about 20 years. According to the allowable temperature, five types of insulation materials commonly used in motors are A, E, B, F, and H.
The temperature of the motor is measured by "temperature rise" instead of "temperature". When the "temperature rise" suddenly increases or exceeds the maximum operating temperature, it indicates that the motor has failed. Some basic concepts are discussed below.
1 Insulation class of insulation material
Insulation materials are divided into 7 grades according to their heat resistance: Y, A, E, B, F, H, C, and their extreme operating temperatures are 90, 105, 120, 130, 155, 180 ° C, and above 180 ° C.
The so-called extreme working temperature of the insulating material refers to the temperature of the hottest point in the winding insulation during the expected life of the motor during operation. According to experience, the life of Grade A materials at 105 ° C and Grade B materials at 130 ° C can reach 10 years, but in actual conditions, the ambient temperature and temperature rise will not reach the design value for a long time, so the general life is 15 ~ 20 year. If the operating temperature exceeds the limit operating temperature of the material for a long time, the aging of the insulation will increase, and the life will be greatly shortened. So when the motor is running, temperature is one of the main factors of life.
2 Temperature rise
The temperature rise is the temperature difference between the motor and the environment, which is caused by the heat of the motor. The iron core of the motor in operation will generate iron loss in the alternating magnetic field, copper loss will occur after the winding is energized, and other stray losses. These will increase the motor temperature. On the other hand, the motor will also dissipate heat. When the heat generation and heat dissipation are equal, an equilibrium state is reached, and the temperature no longer rises and stabilizes at a level. When the heat is increased or the heat is reduced, the balance will be broken, the temperature will continue to rise, and the temperature difference will be enlarged, the heat will be increased, and a new equilibrium will be reached at another higher temperature. However, the temperature difference at this time, that is, the temperature rise has increased than before, so the 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, The motor is faulty, or the air duct is blocked or the load is too heavy.
3 Relationship between temperature rise and temperature
For a normally running motor, the theoretical temperature rise under the rated 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 normal motors will decrease slightly. This is because the winding resistance r decreases and the copper loss decreases. For every 1 ° C drop in temperature, r decreases by about 0.4%.
(2) For self-cooled motors, each 10 ° C increase in ambient temperature will increase the temperature rise by 1.5 to 3 ° C. 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) For every 10% higher humidity, the temperature rise can be reduced by 0.07 ~ 0.38 ° C due to the improved thermal conductivity, with an average of 0.19 ° C.
(4) The altitude is 1 000 m as the standard. For every 100 m, the temperature rise increases by 1% of the temperature rise limit.
4 Extreme working temperature and maximum allowable working temperature
Generally speaking, the limit working temperature of A-class is 105 ℃, and the maximum allowable working temperature of A-class is 90 ℃. So how is the extreme operating temperature different from the maximum allowable operating temperature? In fact, this is related to the measurement method. Different measurement methods reflect different values and different meanings.
(1) The thermometer method's 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, which is the "hottest point". This method is the simplest and is most widely used in the field of small and medium motors.
(2) The resistance method's measurement results reflect the average temperature of the entire winding copper wire. This number is 5 to 15 ° C lower than the actual maximum temperature according to different insulation levels. This method measures the cold and hot resistance of the conductor and calculates the average temperature rise according to the relevant formula.
(3) In the embedded thermometer test, copper or platinum resistance thermometers or thermocouples are embedded in windings, iron cores, or other parts that need to measure the highest expected temperature. The measurement results reflect the temperature at the contact point of the temperature measuring element. Large motors often use this method to monitor the operating temperature of the motor.
There is a certain difference between the temperature measured by various measurement methods and the actual maximum temperature. Therefore, the "maximum allowable operating temperature" must be subtracted from the "limit operating temperature" of the insulating material.
5 Temperature limit of each part of the motor
(1) The temperature rise of the core in contact with the winding (thermometer method) should not exceed the temperature rise limit of the winding insulation in contact (resistance method), that is, Class A is 60 ℃, Class E is 75 ℃, and Class B is 80 ℃ Class F is 100 ° C and Class H is 125 ° C.
(2) The temperature of rolling bearings shall not exceed 95 ° C, and the temperature of sliding bearings shall not exceed 80 ° C. Too high temperature will change the oil quality and damage the oil film.
(3) In practice, the temperature of the cabinet is always based on the hotness.
(4) The stray loss on the surface of the squirrel-cage rotor is very large, and the temperature is high, which is generally limited to not jeopardize the adjacent insulation. Can be pre-painted to estimate.
6 Troubleshooting of Motor Thermal Failure
When the temperature of the motor exceeds the maximum operating temperature or the temperature rise exceeds the stipulation or the temperature rise does not exceed the stipulation, but the temperature rise suddenly increases at a low load, it indicates that the motor is faulty. The judgment and elimination methods are:
(1) The temperature rise under the rated load does not exceed the temperature rise limit. Only because the ambient temperature exceeds 40 ° C, the motor temperature exceeds the maximum allowable operating temperature. This phenomenon indicates that the motor itself is normal. The solution is to reduce the ambient temperature manually. If this is not possible, the load must be reduced.
(2) The temperature rise under the rated load exceeds the nameplate. No matter what the situation is, the motor is faulty and must be stopped and inspected. Pay special attention to the sudden rise in temperature. The external reasons are: the grid voltage is too low or the line voltage drop is too large (more than 10%), the load is too heavy (more than 10%), and the motor and machinery are improperly matched; the internal reasons are: single-phase operation, inter-turn short circuit, inter-phase short circuit , Stator grounding, damaged or unsecured fan, blocked air duct, damaged bearings, friction between stator and rotor, heating of motor and cable joints (especially copper-aluminum or aluminum-aluminum connection), corrosion or moisture of motor. In addition, in theory, the motor can be forward and reverse, but some motor fans have directionality. If the fan is reversed, the temperature rise will exceed a lot. In short, you must troubleshoot for specific situations.
Motor insulation level
The classification of the machine insulation grade is based on the allowable limit temperature of the insulation material used by the motor. There are several grades such as Y, A, E, B, F, H, C, etc. The allowable limit temperature of each level is as follows.
The so-called allowable limit temperature refers to the maximum allowable operating temperature of the motor insulation material, which reflects the heat resistance of the insulation material.
Insulation materials are divided into Y-level, A-level, E-level, B-level, F-level, H-level and C-level according to heat resistance.
Allowable temperature (° C) 90, 105, 120, 130, 155, 180, 180 ° C or more.
The temperature rise limit (resistance method) of the stator winding when the motor adopts Class B insulation should not exceed 80K;
When the motor adopts F-level insulation, the temperature limit of the stator winding should not exceed 105K;
The temperature rise limit (thermometer method) of the collector ring of the YR motor should not exceed 80K;
The allowable temperature of the motor bearing (thermometer method or embedded thermometer method) should not exceed 95 ° C for rolling bearings;
For sliding bearings (when the oil temperature is not higher than 65 ° C), it should not exceed 80 ° C or according to mutual agreement.
Motor temperature rise description: The temperature rise of a part of the motor is the difference between the temperature of the cooling medium in that part and the unit is K. The temperature rise of the motor includes the temperature rise of the stator and rotor windings, the temperature rise of the stator and rotor cores, the temperature rise of the slip ring, and the allowable temperature of the bearing (as explained earlier). The temperature rise limit of B-level motor windings is 80K; F-level motors are also 80K according to B-level assessment; 105K according to F-level assessment. According to the corresponding standards, the working temperature that B-level insulation materials can withstand for a long time is 130 ° C, and F-level can be Withstand 155 ℃ for a long time, calculated according to the actual maximum ambient temperature of the motor of 40 ℃, the allowable working temperature of the motor is:
B grade ≤120 ℃ (ring temperature 40 ℃ + temperature rise 80) <130 ℃
F level ≤145 ° C (ring temperature 40 ° C + temperature rise 105) <155 ° C