Why do motors have axial forces?

From the functional analysis of motor operation, we utilize the tangential force generated by the rotor part of the motor. The various factors leading to the generation of axial forces are mostly unfavorable. However, sometimes we fully utilize the axial force of the motor to balance certain forces, making the motor operation smoother. For example, in vertically mounted motors, the axial electromagnetic force direction can be designed to be opposite to the direction of rotor gravity, partially offsetting the axial force caused by the rotor's own weight and reducing the axial load on the bearings.

When the rotor of the motor adopts skewed slots, the electromagnetic force acting on the conductors inside the slots is perpendicular to the slots, thereby decomposing into axial and tangential forces. Among them, the tangential force is the part of the motor used to generate mechanical energy. Generally, the angle of the skewed slots should be inclined by one stator tooth pitch.

When the stator and rotor of the motor are not aligned, axial forces can also be generated. When the stator and rotor of the motor are aligned, the force exerted by the air gap magnetic field on the rotor is radial and tangential, without axial force. However, when the magnetic force centerlines of the stator and rotor do not align, the air gap magnetic field generates an axial component, causing the rotor to be subjected to axial electromagnetic force. The direction of the axial electromagnetic force always tends to align the stator and rotor. Therefore, there may be axial displacement of the rotor before and after the motor is energized, especially in motor structures using sliding bearings, where the displacement can be visually inspected. This axial displacement may have adverse effects on the motor's bearing system. For motors with segmented cores, such as those with radial ventilation ducts, if the stator and rotor are offset, it will not only affect the axial force but also affect the motor's temperature rise performance.

In addition to affecting the motor's bearing system, the axial force of the motor may also have adverse effects on direct drive motors. In order to objectively analyze and grasp the magnitude of axial forces, hydraulic jacks, force gauges, and lathe live centers can be used to connect to the motor shaft end. When the motor runs unloaded at rated speed, the axial force can be directly read from the force gauge. The measured data play an important role in motor design, application, and related simulation software design.

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