What impact does the insulation level of a motor have on the motor
The insulation level of a motor is determined by the heat resistance level of the insulation material used. Insulation materials can be classified into seven levels based on their allowable limit temperatures under normal conditions: Y, A, E, B, F, C, and H. Their limit temperatures are Y ≤ 90 °, A ≤ 105 °, E ≤ 120 °, and B ≤ 130 °, respectively F ≤ 155 °, H ≤ 180 °, C>180 °. Currently, the commonly used insulation levels for Hengli motors are F or H.
So how should users choose the appropriate insulation level in actual working conditions?
With the occurrence of motor faults, overheating of the motor usually occurs, which is very detrimental to the insulation of the motor. The most sensitive part of an electric motor to heat response is the stator winding insulation. Each insulation material can only withstand a certain temperature. Exceeding its own allowable temperature will accelerate insulation aging, shorten the service life of the motor, and may also cause various accidents due to insulation damage. Therefore, the inspection, detection, and protection of motor overheating faults are very important for reducing accident rates, minimizing accident losses, and improving the economic benefits of enterprises.
1. The relationship between motor temperature rise and insulation level. Due to the differences in the location and ambient temperature of the motor, "temperature rise" is often used to indicate the actual heat resistance of the insulation material of the constant force motor. The so-called temperature rise refers to the temperature at which electrical equipment (including electric motors) exceeds the ambient temperature. The rated temperature rise of an electric motor refers to the maximum allowable temperature rise of the motor winding at the designed ambient temperature (+40 ℃), which depends on the insulation level of the winding. The insulation level of an electric motor refers to the level at which the insulation material used for the motor winding can withstand the temperature limit. The relationship between the maximum allowable temperature (rated temperature rise) of the motor and the insulation level is shown in Table 1. The rated temperature rise of the motor Δ t0=maximum allowable temperature of the motor t-ambient temperature t0, where t0=+40 ℃, and t refers to the maximum allowable temperature of the motor winding. For the running motor, due to the inability to directly measure the actual temperature of the winding, only the external temperature of the casing (i.e. the temperature inside the lifting ring hole) can be indirectly measured, which is about 10 ℃ lower than the hottest point of the motor winding. The maximum allowable temperature t of the motor is determined by adding a temperature difference of about 10 ℃ to the measured temperature on the outer surface of the casing.
2. The maximum allowable temperature rise of each part of the motor is shown in the table
3. Method for measuring the temperature of electric motors (1) Hand feel method: It is a method of estimating the temperature of the electric motor by touching the designated part of the motor with the hand and estimating the strength of the hand feel heat. See Table 3. (2) Thermometer method: directly measuring the temperature rise of an electric motor using a thermometer. When the motor reaches its rated operating state, its temperature gradually rises to a stable value and no longer rises. At this time, a thermometer (preferably an alcohol thermometer) can be used to measure the temperature of the motor. The method is to wrap the ball of the alcohol thermometer with tin foil and insert it into the motor lifting ring hole, so that the thermometer ball is tightly attached to the surrounding area of the hole, and then seal the hole tightly with cotton. At this point, the temperature measured by the thermometer is about 10 ℃ lower than the hottest point of the motor winding. Therefore, adding 10 ℃ to the measured temperature and subtracting the ambient temperature (+40 ℃) is the actual temperature rise of the motor. Table 1 Relationship between Temperature Rise and Insulation Level of Electric Motor Insulation Level Y A E B F H C Maximum allowable temperature ℃ 90 105 120 130 155 180>180 Rated temperature rise ℃ 50 65 80 90 115 140>140 Table 2 Maximum allowable temperature rise of various parts of the electric motor (ambient temperature:+40 ℃) The temperature rise limits of different insulation levels for different part names are ℃ A E B F H thermometer method, resistance thermometer method, resistance thermometer method, resistance thermometer method, resistance thermometer method, stator winding 55 60 65 75 80 85 100 125 wound rotor winding 55 60 65 75 70 80 85 100 105 125 stator core 60 75 80 100 125 slip ring 60 70 80 90 100 sliding bearings 40 40 40 40 rolling bearings 55 55 55 55 55 55. Note: The data in the table is the temperature rise limit considering the actual situation. Table 3 shows the relationship between the surface temperature of the motor casing and the hand feel. The surface temperature of the casing is ℃. The hand feel indicates that it is slightly colder than body temperature, and the feeling is slightly cooler. The degree of warmth felt is 45 degrees when touched by hand, I feel warm at 50 degrees Celsius and slightly hot when touched for a long time. My palm turns red at 55 degrees Celsius and can only be touched with my hand for 5-6 seconds. It's even hotter at 60 degrees Celsius and can only be touched with my hand for 3-4 seconds. The constant force motor is very hot and can only be touched with my hand for 2-3 seconds. After leaving my hand, my palm still feels very hot at 70 degrees Celsius and can only be touched with one finger for about 3 seconds. It's extremely hot and can withstand 1-2 seconds. It's 80 degrees Celsius and even if I suspect that the motor has been burned out, I can't touch it with one finger. Ethylene resin shrinks to 85-90 degrees Celsius and is extremely hot. If I suspect that the motor has been burned out, I can touch it with my finger as if it's on fire. (3) Resistance method: using the principle that conductor resistance increases with temperature for measurement. As long as the cold and hot resistance of a certain phase winding of the motor are measured separately, the average temperature rise of the motor can be calculated as Δ t0=t2-t0=[(R2-R1) (235+t1)/R1]+t1-t0. In the equation, t1 represents the cold temperature of a certain phase winding of the motor, ℃ t2 represents the hot temperature of a certain phase winding of the motor, and ℃ t0 represents the ambient temperature+ 40 ℃ R1- Cold resistance of a certain phase winding of the motor, constant force motor Ω R2- Hot resistance of a certain phase winding of the motor (must be measured within half a minute after the motor is powered off), Ω
4. Causes of motor overheating faults: (1) Overloaded operation of the motor. For example, overload faults such as friction between the stator and rotor (commonly known as chamber sweeping), assembly failure, friction or jamming of the driven mechanical parts. (2) The electric motor is running in phase loss. (3) The imbalance degree of three-phase voltage and three-phase current exceeds the allowable range specified. (4) The power supply voltage is too high or too low, exceeding the allowable variation range of the rated voltage of the motor (i.e. ± 10%). (5) The wiring of the motor winding is incorrect. For example, the connection at one end of the stator winding is reversed. (6) There is a fault in the motor winding. For example, short circuits between turns or layers of windings, and grounding of windings. (7) The insulation between the silicon steel sheets of the stator core is damaged, resulting in a short circuit of the stator core, causing an increase in eddy current in the stator core and overheating of the motor. (8) Frequent startup. (9) The air duct of the electric motor is blocked and the ventilation is poor. (10) The ambient temperature around the motor is too high (exceeding the design requirement of+40 ℃), resulting in poor heat dissipation and poor cooling effect.
