## How is the axial force of a three-phase AC motor formed?

Whether it is a high-voltage or low-voltage motor, from time to time there will be customers who report the problem of the shaft, or because of the axial force, the bearing temperature rises. The best working condition of the motor is that it is not axially stressed, and that the motor is ideally associated with the load; in fact, more or less, there is an axial force in different motors.
For a three-phase AC motor, when the stator winding is turned on, a rotating magnetic field is generated, so that an induced current is generated by electromagnetic induction in the rotor, and the rotor is magnetized to generate electromagnetic attraction and rotate.
The synchronous motor rotates at the synchronous speed after the excitation, and the rotation of the asynchronous motor produces a slip. The gravitational force of the rotating magnetic field of the motor is caused by the shortest characteristic of the magnetic field line. The electromagnetic force causes the motor to run on the electromagnetic center line, that is, the electromagnetic force of the stator and rotor of the motor acts in the radial direction to balance each other, so that no axial magnetic force is generated. However, in actual operation, axial force is generated due to the following factors.

Cause of axial force

The axial force generated by the flow of the cooling air of the motor rotor fan during no-load operation.

● Stator and rotor The axial center force is generated by offsetting the mechanical centerline from the electromagnetic centerline of the motor due to factors such as manufacturing process and motor assembly error.
● In order to reduce the influence of the tooth harmonics, the stator or the rotor is made into a chute structure, so that the electromagnetic force forces the rotor to shift toward one end to generate an axial force during load operation.
● The electromagnetic centerline of the motor is the reference line for the motor shaft to interface with the driven mechanical coupling. However, since the motor is designed and manufactured without the electromagnetic centerline mark, the installation size of the motor and the random pattern provided by the coupling when the coupling is docked is based on the designed mechanical center. In this way, in actual operation, the motor will inevitably appear to be offset due to the misalignment between the mechanical centerline and the electromagnetic centerline, that is, the error of the motor installation and the coupling of the coupling, and the axial force generated. The direction of its axial force is an axial force that pushes or pulls on the driven machine, thus causing damage to the bearings or the complete machine of the motor and the driven machinery.

For the increased safety type brushless excitation synchronous motor, the factors are more, so from the perspective of process manufacturing and processing, the quality of the punching sheet is improved; the punching burr is reduced; the process of strictly observing the number of laminated sheets and the lamination pressure is strictly observed. Discipline, to ensure the length of the stator and rotor core; eliminate the artificial installation and assembly errors, and require the technical workers of the enterprise to have high technical quality and fine assembly skills, is the process guarantee to reduce the axial force.

In addition, the designer must be proficient in the manufacturing process, product assembly process, and consider the size and direction of the axial force generated by the inherent factors such as the steering of the motor, the fan arrangement, and the direction of the chute in the structural design. Can cancel or reduce to each other to a minimum. At the same time, the product assembly technicians are required to participate in the initial commissioning experiment after the product assembly.

In the commissioning experiment, it is more important to require the inspection technician to accurately assemble the technical center file and the electromagnetic center actual offset data according to the actual test data of the motor center line and the electromagnetic center in the prototype test. The process further adjusts the displacement value and the displacement direction of the rotor position.