Axial force arises from the electric motor.
For the machinery and equipment driven by the electric motor, the axial force or axial thrust is generated by the error of the installation and coupling alignment benchmark and the structural factors of the ac motor.
For the medium and large size of the motor with sliding bearing structure, the axial force is generated because the motor is not running on the electromagnetic centerline.
The rotor will inevitably cause axial runout, and the axial pull or axial thrust generated will cause impact to the driven mechanical equipment.
If the deviation between the electromagnetic centerline and the mechanical centerline is too large, the bearings of the electric motor and the mechanical equipment driven by it will cause serious damage and lead to major accidents of the whole machine.
Similarly, for the motor with rolling bearing, although the rotor axial position is fixed, the axial force acting on the rolling bearing will definitely reduce the life of the rolling bearing, and in serious cases, it will lead to the accident of rolling bearing burning.
Therefore, it is very important to consider the relationship between the offset of electromagnetic centerline and axial force from both operation and design aspects.
Calculation method of electric motor axial force
According to Maxwell’s law: the electromagnetic force acting on a magnetized ferromagnetic object is proportional to the total area of the magnetic lines of force crossing the magnetic poles and the quadratic of the magnetic induction intensity in the air gap.
If the magnetic induction strength B is uniformly distributed along the surface of the pole, the electromagnetic force is calculated according to equation (1).
F=SBδ2/2µ0…………………………………………. (1)
formula (1)
F – electromagnetic force in J/cm;
Bδ – air gap magnetic induction strength, unit Wb/cm2;
S – the total area of the magnetic pole surface, in cm2;
µ0 – air gap magnetic permeability.
formula (1) is the electromagnetic force of the rotating magnetic field in the operating condition where the motor is running on the electromagnetic centerline and the axial force is zero.
The axial force is generated due to the determined installation and coupling alignment reference error, as well as the structural factors of the motor to make the motor run in the deviation from the electromagnetic centerline state of the axial pull or axial thrust.
The calculation of axial force should take into account the saturation effect of the magnetic circuit and the marginal effect of the air gap.
In the actual engineering calculations, formula (1) is not practical, usually the calculation of axial force by 1mm per kilowatt offset electromagnetic centerline calibrated by the baseline, the axial force of 0.000478N/kW a brief estimate is sufficient.
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