Whether it's a high-voltage or low-voltage motor, customers frequently report issues such as shaft misalignment or increased bearing temperature due to axial force. The ideal operating condition for a motor is when it experiences no axial force, ensuring an ideal connection between the motor and the load; however, in reality, all motors, to varying degrees, experience some form of axial force.
For a three-phase AC motor, when the stator winding is connected to the power supply, a rotating magnetic field is generated, which induces a current in the rotor due to electromagnetic induction. The rotor is magnetized and rotates due to electromagnetic attraction.
After being excited, a synchronous motor rotates at synchronous speed; an asynchronous motor experiences slippage during rotation. The attraction of the rotating magnetic field of a motor is due to the shortest magnetic field line characteristic. The electromagnetic force keeps the motor running on the electromagnetic center line, meaning that the electromagnetic forces of the stator and rotor act radially, balancing them and preventing the generation of axial magnetic force. However, in actual operation, axial force can be generated due to the following factors.
Causes of axial force
● The axial force generated by the airflow of cooling air during no-load operation of the motor rotor fan.
● Due to factors such as manufacturing process and motor assembly errors, the mechanical centerline of the stator and rotor may deviate from the electromagnetic centerline of the motor, resulting in axial force.
● In order to reduce the influence of tooth harmonics, the stator or rotor is made into a skewed slot structure. Therefore, when the load is running, the electromagnetic force forces the rotor to deflect to one end and generates an axial force.
● The electromagnetic centerline of an electric motor serves as the reference line for the connection between the motor shaft and the coupling of the driven machinery. However, since electric motors are not designed and manufactured with an electromagnetic centerline marking, the installation drawings provided in the accompanying documents, used for motor installation and coupling connection, are based on the designed mechanical centerline. This inevitably leads to misalignment during actual operation due to the mechanical centerline not coinciding with the electromagnetic centerline, resulting in errors in motor installation and coupling connection, and consequently, axial forces. These axial forces exert a pushing or pulling force on the driven machinery, potentially damaging the bearings or the entire machine of both the motor and the driven machinery.
For increased safety brushless synchronous motors, there are even more factors involved. Therefore, from a manufacturing perspective, improving lamination quality, reducing lamination burrs, strictly adhering to the process discipline of lamination number and lamination pressure, ensuring the length of stator and rotor cores, and eliminating human error in installation and assembly are all necessary to ensure that the axial force is reduced.
In addition, designers must be proficient in manufacturing processes and product assembly processes, and comprehensively consider the magnitude and direction of axial forces generated by inherent factors such as the motor's rotation direction, fan arrangement, and skew slot direction in the structural design, ensuring that these forces cancel each other out or are reduced to a minimum. Furthermore, product assembly technicians are required to participate in the initial debugging test after product assembly.
In the debugging experiment, it is even more important to require the inspection technicians to accurately require the assembly process to adjust the displacement value and displacement direction of the rotor position based on the actual offset data of the motor centerline and electromagnetic center measured in the no-load and load tests during the prototype assembly experiment.
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