Daily inspection and maintenance of the motor
The 3 phase electric ac motor consists of a stator frame, windings and insulation materials, a rotor, bearings at both ends and end covers, etc., which is relatively simple. The causes of motor failure include: power failure, incorrect voltage or frequency; winding short circuit, open circuit, grounding; poor bearing operation; dirty inside and outside, poor heat dissipation (too thick paint on the outside is also the reason for poor heat dissipation), and the built-in cooling fan is broken, ventilation is not smooth; and mechanical equipment is poor; long-term high-load operation; high ambient temperature, etc. More than 90% of the damage to the motor is caused by the maintenance personnel's daily inspection and insufficient maintenance. As long as you insist on careful observation, listening, touching, measuring, and doing, most of the faults can be prevented and avoided, reducing the company's spare parts and repair costs.
1. Look: Check the size and changes of the motor's working current during daily inspections, and see if there is any water leakage or dripping around, which will cause the motor insulation to break down and burn out. Also see if there are objects around the motor that affect its ventilation and heat dissipation environment? Check if the fan cover, blades and the outside of the motor are too dirty and need to be cleaned? To ensure the cooling and heat dissipation effect. No matter who finds the problem, it should be dealt with in time.
2. Listen: Listen carefully to see if the running sound of the motor is abnormal. Because the noise in the machine room is loud, you can use auxiliary tools such as screwdrivers or listening sticks to listen close to both ends of the motor. If you listen frequently, you can not only find the bad vibration of the motor and its towing equipment, but also judge the amount of internal bearing oil, so as to timely add bearing oil, or replace new bearings and other corresponding measures to avoid the motor bearings from being blocked, running outside the circle, and burning due to lack of oil and dry grinding.
Most manufacturers will use open bearings considering the difficulty of dismantling and replacing bearings of large motors. When refueling with an oil gun regularly (2000h), it is necessary to pay attention to using the bearing oil marked on the motor brand (-35℃～+140℃), and open the lower oil drain port or remove the blind screw on the other side to squeeze out the old oil (pay attention to the oil replenishment quota) to prevent the oil from being squeezed into the motor due to high pressure during refueling, splashing on the stator and rotor during operation, affecting the heat dissipation function of the motor, etc.
3. Touch: Use the back of your hand to feel the temperature around the motor. When the bearing is in good condition, the temperature at both ends is generally lower than the temperature of the middle winding section. If the temperature at the bearings at both ends is high, the bearings should be checked in combination with the measured bearing sound. If the overall temperature of the motor is high, the load, equipment and ventilation of the motor should be checked in combination with the working current and corresponding treatment should be carried out.
According to the insulation grade of the insulation material used in 4 pole ac motor, the maximum temperature of the winding insulation that can be used for a long time when the motor is running can be determined, or the allowable temperature rise of the motor (the actual temperature of the motor minus the ambient temperature). The insulation grade standards of various countries vary, but they are basically divided into: Y, A, E, B, F, H, C. Among them, the allowable temperature rise of grade Y is the lowest (45℃) and the allowable temperature rise of grade C is the highest (above 135℃). Considering the insulation oil and other materials, the temperature measured by the thermometer attached to the motor is best controlled below 85℃.
4. Measurement: Offline: When the motor stops running, regularly (monthly) measure the insulation resistance of each phase to ground or between phases with an insulation meter, and compare it with the test results of the previous month, so as to find insulation defects in time. If it is found to be bad, use a drying lamp to bake it to improve the insulation to avoid the breakdown of the winding and burning the motor due to too low insulation (recommended value>1 megohm). For motors equipped with drying electric heating, do not turn off the heating switch at will unless there are special circumstances. Pay special attention to the waterproofing, moisture-proofing and drying of the motor in humid weather and winter. Pay special attention to watertightness for motors in open air and humid places. For motors suspected of being seriously damp or splashed with water, they should be carefully checked before use. If the motor is found to be soaked in water, just disassemble the motor, remove the rotor, blow it dry with compressed air, and then use a baking lamp to bake from both ends of the motor stator until the motor insulation rises to normal.
Some motor windings have only three leads, and the insulation between the windings cannot be detected. The insulation resistance of the rewound motor should generally not be less than 5 megohms.
Online: When the motor is running, its three-phase working voltage and current can be measured to see if they are balanced. The voltage should be basically equal, and the error between the current of each phase and the average value should not exceed 10%. If the difference between the current of each phase measured by the clamp meter is too large, there may be a short circuit between turns. Sometimes it is necessary to disconnect the load to measure the no-load current. Generally, 2 poles are about 2800rpm, 1/3 Ie (rated current); 4 poles are about 1600rpm, about 40% Ie; 6 poles are about 900rpm, 55% Ie. It will vary depending on the number of poles and capacity. In addition, the Y and △ connection of the motor should not be wrong. If the Y is connected to the △, the working current will increase; otherwise, it will decrease. At the same time, if the motor winding is connected incorrectly, or the number of winding turns is reduced, the air gap between the stator and rotor of the old motor is too large (normally 0.2-1.0mm), the no-load current will increase. These points can help us judge the condition of the motor in daily operation.
5. Do: Not only should remedial measures be taken in time for the problems found during the inspection, but also the motor should be screwed and wired tightened, disassembled for inspection, cleaned and maintained according to the maintenance cycle (monthly). If all four fixing screws of the motor end cover are loose, it will cause the sweeping operation to burn out; the loose connection bolts of the motor will cause the phase loss to burn out; the fan blades of the motor will fall off and press against the body to cause the motor to burn out; the motor bearings are poorly lubricated and the operating temperature is high, but the lubricating oil is not added or the bearings are replaced in time, causing the motor to burn out; the insulation resistance of the motor decreases due to humid weather, and the insulation is not improved in time due to baking; the motor burns out in the section because it is not checked and tightened in time; or because the problem has been found but no maintenance and remedy is done. Whether it is not to see or not to do, or just to see and not to do, it will eventually cause failures or accidents.
If the bearings need to be replaced when disassembling and inspecting the motor, imported ones should be used as much as possible. There will be many refurbished old bearings in China, and the quality is difficult to guarantee. If it is found that the outer circle of the bearing and the end cover bearing seat are not tightly matched, that is, when the bearing is running on the outer circle, it should be eliminated according to the degree of difference, and the inner circle of the end cover bearing seat should be pocked, padded with copper skin or inlaid with copper sleeves. The center point must be accurately determined, otherwise it will be damaged again soon. Before putting it into operation, it is necessary to reconfirm the radial swing of the protruding end of the packing and the fastening of the end cover, whether the rotor rotates flexibly, whether the winding lead connection is correct, etc.
There are also many motors. When the pump connected to it leaks, they usually go to pressurize (stuffing box) to extinguish the leak. When adding or replacing the packing, experienced maintenance workers will first turn it by hand to see the rotation, tighten the gland screws evenly, and then start and stop for two or three short times to see if the working current is normal; while inexperienced new maintenance workers only know to tighten and extinguish the leak, but do not pay attention to the related motors. As a result, the motor is too large to start the current of the stalled rotor, and the thermal overload protection is not able to work in time, so the winding is broken and burned. In addition, when repairing, disassembling, inspecting and reassembling mechanical equipment driven by motors (reducers, water pumps, oil pumps, etc.), the center line of the motor and the driven machinery should also be carefully checked and corrected to ensure good alignment. The coupling should be light and flexible when turned by hand. Only by doing every step practically, seriously and meticulously can the equipment's intact rate be improved.
Regardless of YZP, YP2 series variable frequency motors or three-phase asynchronous motors, as long as you insist on careful observation, listening, touching, measuring and doing, most faults can be prevented and avoided, reducing the company's spare parts and repair costs.
