What impact does rotor unbalance have on the quality of an electric motor?

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Vibration and noise are the most common problems in motor testing and operation. There are many causes of vibration and noise, and it is not easy to fully distinguish between them. In many cases, we attribute vibrations and the noise they cause to the rotor dynamic balancing process; however, very often no definite cause can be found in the rotor itself. Ultimately, there is a certain discrepancy between theoretical analysis and actual conditions; only by ensuring that every aspect involved meets the required standards can high-quality motors be produced. The mechanical vibration and noise of motors include vibration and noise generated by imbalance on the motor rotor, noise from bearing vibrations, axial vibration and noise of the end cover induced by bearing excitation, as well as frictional vibration and noise between the brushes and the commutator (or slip ring), and so on. Today, the equipment expert will share with you the issues of vibration and noise caused by rotor mechanical imbalance. Dynamic imbalance and static imbalance of the rotor: Generally, the imbalance of a motor rotor can be classified into static imbalance, dynamic imbalance, and mixed imbalance. The centrifugal force resulting from static imbalance induces vibrations of equal amplitude and identical phase at the two supports. The couple of centrifugal force resulting from dynamic imbalance induces vibrations of equal magnitude but opposite phase at the two supports. In practice, the most common issue is mixed imbalance, which results from the combined effect of residual static unbalance centrifugal forces and dynamic unbalance centrifugal couples acting on the two supports, thereby generating vibrations of different amplitudes and phases. The mechanical imbalance of the rotor can be eliminated through balancing. Unbalanced vibration of the rotor: Residual imbalance from manufacturing; excessive accumulation of dust due to long-term operation. During operation, thermal stress causes the shaft to bend. Thermal displacement of rotor components causes unbalanced loads. The centrifugal force of the rotor components causes deformation or eccentricity. The shaft bends due to external forces (belts, gears, poor direct coupling, etc.). Improper installation of the bearings (axis precision or locking) causes the shaft to bend or the bearings to deform internally. Suppress rotor imbalance: Maintain it within the allowable imbalance level. Improvement of excessive tight fit between the shaft and core. Design improvements for the anisotropy of thermal expansion. Improvement in strength design or assembly. Adjustments to shaft strength design, changes in the types of shaft couplings, and corrections for alignment with the center. Prevention of the bearing end face from shifting relative to the shaft attachment section or the locking nut. Abnormal vibration and noise in bearings: Damage inside the bearing: Abnormal axial vibration in the bearing is caused by the axial spring constant and the rotor mass, which together form a vibration system that generates excitation. Friction sounds: Caused by poor lubrication and bearing clearance in cylindrical roller bearings or large-diameter high-speed ball bearings. Bearing replacement: Appropriate axial spring preloading helps to mitigate variations in bearing clearance. Choose a soft grease or one with excellent low-temperature performance, and keep the residual gap small (be mindful of temperature rise). Rotor dynamic balancing methods: After performing dynamic balance measurements on a rotor using a dynamic balancer, the rotor can be balanced as needed through either the adding-weight method or the removing-weight method. The adding-weight method involves attaching corrective weights in the direction opposite to that of the imbalance. Common methods include welding, soldering, riveting, screwing, and adding counterweights. The deduplication method involves removing a certain amount of weight in the unbalanced direction. Common methods include: boring, drilling, chiseling, milling, grinding, etc.