Fault Analysis and Modification of High Voltage Motor Bearings

In the production process of power plants and large enterprises, electric motors are the main source of power. In the event of failure, the losses caused by the entire production are considerable, especially large and medium-sized high-voltage motors, which can seriously affect the entire production process. . According to our statistics on motor faults, the motor damage caused by bearing damage or bearing damage accounted for more than 70% of all motor faults. For this reason, we analyzed and modified the motor with more motor bearing faults. The utility model solves the problems of heat generation, oil leakage, low life and sudden damage during operation of the motor bearing, greatly reducing the maintenance cost and providing a reliable guarantee for safe operation.

The main problems of frequent bearing damage:

At present, most of China's large-scale high-voltage motors are separated sliding bearings. The failure of bearings is generally less, and the problem of oil leakage of the bearing bush has little effect on the motor. The structure of some large high-voltage motors and medium-sized motors is “end-cap sliding bearings” and “end-capped rolling bearings”. There are many problems in the bearings of these motors:

1. End cap type sliding bearing: This kind of motor generally has the phenomenon that the axial movement of the rotor is large, the bearing bush is heated and oil leakage occurs. Corrosion of the motor windings due to oil leakage causes excessive motor ash inside the motor to overheat the motor. In addition, the sliding bearing is much more complicated than the inspection of the rolling bearing.

2, the general structure of the motor: China's box-type motor has a part of bearing failures, after analysis, the main problem is: the outer side of the bearing is equipped with a small oil-shield from the bearing clearance. This is not conducive to the heat dissipation of the bearing and the circulation of the grease, and increases the difficulty and cost of the maintenance work, and after the loosening of the hole and the shaft in the oil retaining plate, abnormal sound is generated or released from the shaft, causing serious failure.

3. Double-bearing motor: Part of the box type high-voltage motor, the load side adopts the structure of double bearing. The structure of this type of motor brings great difficulty to the maintenance work. When the motor is inspected and repaired, the bearing cannot be cleaned and inspected and must be replaced, resulting in an increase in maintenance costs. In addition, the motor of this structure has a relatively high temperature during operation and a low service life.

4. Types of bearings: Most of the bearings on the negative side of the motor are “cylindrical roller bearings” and the air side is “central thrust ball bearings”. During the operation of the motor, the change of the rotor length is adjusted by the negative side. At this time, if the coupling between the motor and the machine is a "elastic coupling", there is no significant influence on the motor and the machine, and if it is a "rigid coupling", the motor or mechanical vibration occurs, and even the bearing is caused. damage.

5, bearing bearing and allowable speed: China's motor bearings, the general negative side is mostly selected "medium roller bearings." The bearing capacity of the bearing greatly exceeds the calculated value, but the allowable rotational speed of the bearing is little or not enough with the actual rotational speed of the motor, so this is also an important reason for the frequent heating and damage of the bearing.

6. Design of bearing accessories: At present, most of the bearing accessories of China's motors are: bearing sleeves and inner and outer oil cover structures. The advantage of this type of construction is that it is simple and has the disadvantage of poor sealing. Dust enters the inside of the bearing, accelerating the wear and damage of the bearing, and most of the bearing structure causes the bearing to be hot due to the small oil chamber and the presence of the oil retaining plate.

Design advantages of some imported motors:

We analyzed the imported high-voltage motors of the same type and found that most of the bearings have a continuous service life of more than 5 years. After several inspections, we found out the advantages of its design:

1. The negative side uses a larger "deep groove ball bearing" and the empty side uses a smaller "cylindrical roller bearing". The analysis believes that this can ensure that the axial displacement of the rotor on the air side when the cold and heat length changes, to ensure the positioning of the motor and mechanical connection.
2. Bearing load and allowable speed: After calculation, under the condition that the motor running power is fully guaranteed, the allowable speed of the selected bearing greatly exceeds the actual speed of the motor, so the service life of the bearing can be greatly increased.

3. Bearing attachment structure: The bearing sleeve adopts “spheroidal graphite cast iron” material to ensure the stability of the bearing operation and prolong the service life. An aluminum alloy "trapezoidal groove seal" is installed on the outside of the inner and outer bearing caps to ensure the inside of the bearing is clean.

4. The positioning structure of the bearing is: a structure with a slotted positioning nut and a 19-tooth lock pad. The device not only plays a good positioning role for the bearing, but also can further reduce the temperature of the bearing, and the space design of the bearing oil chamber is large, so that the temperature of the bearing during operation can be reduced, and the service life of the bearing is greatly increased.

5, good bearing plus, oil drain device: the fueling device is basically the same as our structure, the oil drain device is well designed. The thicker oil drain pipe is installed in the lower part of the larger oil chamber, and a small bolt can be easily taken out to clean the waste oil.

Motor bearing modification method:

Before the transformation of the motor with more bearing failures, we first calculated the selection of the bearing and found that there was a problem in the selection, that is, redesign. The transformation principle is based on the actual structural size of the motor, and the structure of the imported motor is used as much as possible. The specific design and renovation projects are basically as follows:

1. The negative side double bearing structure is changed to a single bearing structure as much as possible;

2. According to the connection mode with the machine being driven, try to design the negative side as “deep groove ball bearing” and position it on the negative side;

3. After calculation, in the case of ensuring the power of the motor, try to select the bearing that allows the speed to greatly exceed the motor speed;

4. According to the actual axial dimension of the motor to be modified, try to increase the oil chamber on the outside of the bearing;

5. According to the actual axial dimension of the motor to be modified, try to install the aluminum alloy trapezoidal groove seal on the outer side of the inner and outer bearing caps;

6. The fixing method of the bearing adopts a filament, a 4-slot nut and a 19-tooth lock pad structure;

7. The bearing sleeve is made of “spheroidal graphite cast iron” material, and the inner and outer bearing caps are made of gray cast iron material;

8. The bearing and oil discharge device of the bearing is completely made according to the structural design of the imported motor.

The example of motor bearing modification: Over the years, we have completely modified all the high-voltage motors of Huangtai Power Plant's bearings, which greatly ensured the safe operation of the unit. In recent years, the motors of the “Lingcheng Power Plant”, “Jining Power Plant”, “Hequ Power Plant” and “Wei County Power Plant” have been reconstructed, and good results have been achieved. However, because of the different equipment, the specific modification scheme of each motor is different. The brief introduction is as follows:

1. The primary Fan Motor of the 300,000 kW unit of Laicheng Power Plant. The motor is a YKK560-4 type, 1250KW high-voltage motor produced by Xiangtan Motor Factory. The motor has many bearing damage faults, especially the outer bearing and the airside bearing on the negative side, and the shortest damage period is several days. The structure of the bearing is that the load side is a double bearing, the outer side is a #N332 cylindrical bearing, the inner side is a #6232 ball type bearing, and the idle side is a #N332 column type bearing. According to our analysis, the motor is mainly a bearing selection problem, and the allowable rotation speed of the bearing is a fundamental cause of damage to the no-load side bearing and the load side outer bearing. For this, we changed the negative side of the motor to #6236 ball type single bearing structure, and the air side was changed to #NU232 light type roller bearing. After calculating the bearing capacity of the bearing can fully meet the requirements of the motor, the modified bearing allows the speed to increase by 300 rev / min, and the other parts are completely modified according to the structure of the imported motor. At present, all the fan motors of all the units have been reconstructed, and the original transformation has been running for 5 years with good results.

2. The 600,000 kW unit coal mill motor of Hequ Power Plant. The motor is YTM630-6 and 1120KW produced by Shanghai Electric Machinery Factory. The main problem in the operation of the motor is "bearing heat", especially the load side bearing, the temperature can be as high as 80 degrees or more. According to our analysis, the unreasonable structure of the motor bearing accessory is the main reason for the "bearing heat": the oil bearing chamber of the motor is too small and the oil retaining plate installed outside the bearing is the main problem causing the bearing heat in operation. The output of the motor is on the negative side, and the friction coefficient of the roller bearing on the negative side is greater than that of the spherical bearing on the air side, so the bearing temperature on the negative side is high. For this, we have changed the bearings on both sides, the negative side bearing was changed to #6334 type, the air side bearing was changed to #NU234, and the other parts were completely modified according to the structure of the imported motor. At present, the motor after the transformation is operating normally, and the bearing temperature on the negative side is lower than the temperature of the negative side cover by 5 ° C or more, achieving good results.

3. Primary fan motor of Jinan Power Plant's 300,000 kW unit. The same as Shanghai production, YKK560-4 type, power 1250KW, its bearing fault is basically the same as the coal mill motor of Hequ Power Plant, so we have adopted the same transformation plan as the Hequ Power Plant. Because of its different speeds and bearings, the specific projects for its transformation are also different. The negative side bearing was changed from #NU332 to #6332, and the airside bearing was changed from #6330 to #NU230. After the transformation, it achieved good operation.

4. Hebei Weixian Power Plant Feed Pump Motor. The motor is produced by Changsha Electric Machinery Factory, YKK4504-2 type, power 800KW. The bearing structure is: the negative side is double bearing structure, the outer bearing is #NU221 type roller bearing, the inner bearing is #6221 type ball bearing, the air side bearing is #NU221 Roller bearing. The bearing temperature of the motor is up to 90 °C, and the bearing is damaged after two months of operation. According to our analysis, the first one is that the allowable speed of the #NU221 bearing is close to the actual speed of the motor. The second is that the structure of the double bearing is tight and the heat dissipation is poor, which is the main cause of bearing heat and damage. After calculating the bearing capacity of the bearing, the negative side bearing was changed to #6221 type single bearing structure, and the air side bearing was changed to #NU220 type, which increased the allowable rotation speed of the bearings on both sides. The other parts were completely modified in accordance with the structure of the imported motor. After the motor is modified according to the above scheme, the operating temperature of the bearing is reduced to below 40 °C, which completely solves the problem of bearing heat generation and frequent damage.

Summary development:

Through the statistics of the motor operation after the transformation, the continuous operation cycle has basically reached a maintenance cycle of the unit, which greatly reduces the temporary shutdown of the unit caused by the damage of the motor, and solves the problem of frequent damage of the large and medium-sized motor bearings. According to the information that is known, in many power plants and large-scale manufacturing enterprises across the country, high-voltage motors almost all have such problems. Therefore, we will further explore and work hard in the future to apply advanced and mature motor bearing modification technology to practical work more widely, and make greater contributions to the development of electric power and other businesses.