The bearing system is the operating system of the permanent magnet motor. When a failure occurs in the bearing system, the bearing will suffer common failures such as premature damage and fall apart due to temperature rise.Bearings are important parts in permanent magnet motors. They are associated with other parts to ensure the relative position requirements of the permanent magnet motor rotor in the axial and radial directions.

    When the bearing system fails, the precursor phenomenon is usually noise or temperature rise. Common mechanical failures usually manifest as noise first, and then gradually increase in temperature, and then develop into permanent magnet motor bearing damage. The specific phenomenon is increased noise, and even more serious problems such as permanent magnet motor bearing falling apart, shaft sticking, winding burnout, etc. The main reasons for the temperature rise and damage of permanent magnet motor bearings are as follows.

1.Assembly and usage factors.

    For example, during the assembly process, the bearing itself may be contaminated by a bad environment, impurities may be mixed in the lubricating oil (or grease), the bearing may be bumped during installation, and abnormal forces may be applied during the installation of the bearing. These may all cause problems with the bearing in the short term.

    During storage or use, if the permanent magnet motor is placed in a humid or harsher environment, the permanent magnet motor bearing is likely to rust, causing serious damage to the bearing system. In this environment, it is best to use well-sealed bearings to avoid unnecessary losses.

2.The shaft diameter of the permanent magnet motor bearing is not matched properly.

    The bearing has initial clearance and running clearance. After the bearing is installed, when the permanent magnet motor is running, the clearance of the motor bearing is the running clearance. The bearing can operate normally only when the running clearance is within the normal range. In reality, the matching between the inner ring of the bearing and the shaft, and the matching between the outer ring of the bearing and the end cover (or bearing sleeve) bearing chamber directly affect the running clearance of the permanent magnet motor bearing.

3.The stator and rotor are not concentric, causing the bearing to be stressed.

    When the stator and rotor of a permanent magnet motor are coaxial, the axial diameter clearance of the bearing is generally in a relatively uniform state when the motor is running. If the stator and rotor are not concentric, the center lines between the two are not in a coincident state, but only in an intersecting state. Taking a horizontal permanent magnet motor as an example, the rotor will not be parallel to the base surface, causing the bearings at both ends to be subjected to external forces of the axial diameter, which will cause the bearings to operate abnormally when the permanent magnet motor is running.

4.Good lubrication is the primary condition for the normal operation of permanent magnet motor bearings.

1)The matching relationship between the lubricating grease effect and the operating conditions of the permanent magnet motor.

    When selecting lubricating grease for permanent magnet motors, it is necessary to select according to the standard working environment of the permanent magnet motor in the motor technical conditions. For permanent magnet motors operating in special environments, the working environment is relatively harsh, such as high temperature environment, low temperature environment, etc.

    For extremely cold weather, lubricants must be able to resist low temperatures. For example, after the permanent magnet motor was taken out of the warehouse in winter, the hand-operated permanent magnet motor could not rotate, and there was obvious noise when it was powered on. After review, it was found that the lubricant selected for the permanent magnet motor did not meet the requirements.

    For permanent magnet motors operating in high-temperature environments, such as air compressor permanent magnet motors, especially in the southern region with higher temperatures, the operating temperature of most air compressor permanent magnet motors is above 40 degrees. Taking into account the temperature rise of the permanent magnet motor, the temperature of the permanent magnet motor bearing will be very high. Ordinary lubricating grease will degrade and fail due to excessive temperature, causing the loss of bearing lubricating oil. The permanent magnet motor bearing is in a non-lubricated state, which will cause the permanent magnet motor bearing to heat up and be damaged in a very short period of time. In more serious cases, the winding will burn out due to large current and high temperature.

2) Permanent magnet motor bearing temperature rise caused by excessive lubricating grease.

    From the perspective of heat conduction, permanent magnet motor bearings will also generate heat during operation, and the heat will be released through related parts. When there is excessive lubricating grease, it will accumulate in the inner cavity of the rolling bearing system, which will affect the release of heat energy. Especially for permanent magnet motor bearings with relatively large inner cavities, the heat will be more serious.

3) Reasonable design of bearing system parts.

    Many permanent magnet motor manufacturers have made improved designs for motor bearing system parts, including improvements to the motor bearing inner cover, rolling bearing outer cover and oil baffle plate to ensure proper grease circulation during the operation of the rolling bearing, which not only guarantees the necessary lubrication of the rolling bearing, but also avoids the heat resistance problem caused by excessive grease filling.

4)Regular renewal of lubricating grease.

  When the permanent magnet motor is running, the lubricating grease should be updated according to the frequency of use, and the original grease should be cleaned and replaced with grease of the same type.

5.The air gap between the stator and rotor of the permanent magnet motor is uneven.

    The influence of the air gap between the stator and rotor of the permanent magnet motor on efficiency, vibration noise, and temperature rise. When the air gap between the stator and rotor of the permanent magnet motor is uneven, the most direct feature after the motor is powered on is the low-frequency electromagnetic sound of the motor. The damage to the motor bearing comes from the radial magnetic pull, which causes the bearing to be in an eccentric state when the permanent magnet motor is running, causing the permanent magnet motor bearing to heat up and be damaged.

6.The axial direction of the stator and rotor cores are not aligned.

    During the manufacturing process, due to errors in the positioning size of the stator or rotor core and the deflection of the rotor core caused by thermal processing during the rotor manufacturing process, axial force is generated during the operation of the permanent magnet motor. The rolling bearing of the permanent magnet motor operates abnormally due to the axial force.

7.Shaft current.

    It is very harmful to variable frequency permanent magnet motors, low voltage high power permanent magnet motors and high voltage permanent magnet motors. The reason for the formation of shaft current is the effect of shaft voltage. To eliminate the harm of shaft current, it is necessary to effectively reduce the shaft voltage from the design and manufacturing process, or disconnect the current loop. If no measures are taken, the shaft current will cause devastating damage to the rolling bearing.

    When it is not serious, the rolling bearing system is characterized by noise, and then the noise increases; when the shaft current is serious, the noise of the rolling bearing system changes relatively quickly, and there will be obvious washboard-like marks on the bearing rings during disassembly inspection; a big problem accompanied by the shaft current is the degradation and failure of the grease, which will cause the rolling bearing system to heat up and burn in a relatively short period of time.

8.Rotor slot inclination.

    Most permanent magnet motor rotors have straight slots, but in order to meet a performance indicator of a permanent magnet motor, it may be necessary to make the rotor into an oblique slot. When the rotor slot inclination is large, the axial magnetic pull component of the permanent magnet motor stator and rotor will increase, causing the rolling bearing to be subjected to abnormal axial force and heat up.

9.Poor heat dissipation conditions.

    For most small permanent magnet motors, the end cover may not have heat dissipation ribs, but for large-sized permanent magnet motors, the heat dissipation ribs on the end cover are particularly important for controlling the temperature of the rolling bearing. For some small permanent magnet motors with increased capacity, the heat dissipation of the end cover is improved to further improve the temperature of the rolling bearing system.

10.Rolling bearing system control of vertical permanent magnet motor.

    If the size deviation or the direction of the assembly itself is incorrect, the permanent magnet motor bearing will not be able to operate under normal working conditions, which will inevitably cause the rolling bearing noise and temperature rise.

11.Rolling bearings heat up under high-speed load conditions.

    For high-speed permanent magnet motors with heavy loads, relatively high-precision rolling bearings must be selected to avoid failures due to insufficient precision of the rolling bearings.

    If the rolling element size of the rolling bearing is not uniform, the rolling bearing will vibrate and wear due to the inconsistent force on each rolling element when the permanent magnet motor is running under load, causing metal chips to fall off, affecting the operation of the rolling bearing and aggravating the damage to the rolling bearing.

    For high-speed permanent magnet motors, the structure of the permanent magnet motor itself has a relatively small shaft diameter, and the probability of shaft deflection during operation is relatively high. Therefore, for high-speed permanent magnet motors, necessary adjustments are usually made to the shaft material.

12.The hot-loading process of large permanent magnet motor bearings is not suitable.

    For small permanent magnet motors, rolling bearings are mostly cold pressed, while for medium and large permanent magnet motors and high-voltage permanent magnet motors, bearing heating is mostly used. There are two heating methods, one is oil heating and the other is induction heating. If the temperature control is poor, excessively high temperature will cause rolling bearing performance failure. After the permanent magnet motor has been running for a certain period of time, noise and temperature rise problems will occur.

13.The rolling bearing chamber and bearing sleeve of the end cover are deformed and cracked.

    The problems mostly occur on the forged parts of medium and large permanent magnet motors. Since the end cover is a typical plate-shaped part, it may undergo large deformation during the forging and production processes. Some permanent magnet motors have cracks in the rolling bearing chamber during storage, causing noise during the operation of the permanent magnet motor and even serious bore cleaning quality problems.

    There are still some uncertain factors in the rolling bearing system. The most effective improvement method is to reasonably match the rolling bearing parameters with the permanent magnet motor parameters. The matching design rules based on the permanent magnet motor load and operating characteristics have also been relatively complete. These relatively fine improvements can effectively and significantly reduce the problems of the permanent magnet motor bearing system.

14.Anhui Mingteng’s technical advantages

    Mingteng（https://www.mingtengmotor.com/） uses modern permanent magnet motor design theory, professional design software and self-developed permanent magnet motor special design program to simulate and calculate the electromagnetic field, fluid field, temperature field, stress field, etc. of the permanent magnet motor, optimize the magnetic circuit structure, improve the energy efficiency of the permanent magnet motor, and solve the difficulties in on-site bearing replacement of large permanent magnet motors and the problem of permanent magnet demagnetization, fundamentally ensuring the reliable use of permanent magnet motors.

  Shaft forgings are usually made of 35CrMo, 42CrMo, 45CrMo alloy steel shaft forgings. Each batch of shafts is subjected to tensile tests, impact tests, hardness tests, etc. according to the requirements of the “Technical Conditions for Forged Shafts”. Bearings can be imported from SKF or NSK as needed.

    In order to prevent the shaft current from corroding the bearing, Mingteng adopts an insulation design for the tail end bearing assembly, which can achieve the effect of insulating bearings, and the cost is much lower than that of insulating bearings. It ensures the normal service life of the permanent magnet motor bearings.

    All permanent magnet synchronous direct drive permanent magnet motor rotors of Mingteng have a special support structure, and the on-site replacement of bearings is the same as that of asynchronous permanent magnet motors. Later bearing replacement and maintenance can save logistics costs, save maintenance time, and better guarantee the user’s production reliability.

Copyright:This article is a reprint of the WeChat public number “Analysis on Practical Technology of Electric Motors”, the original link:

https://mp.weixin.qq.com/s/77Yk7lfjRWmiiMZwBBTNAQ

This article does not represent  our company’s views. If you have different opinions or views, please correct us!