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Exploring Rotor Balancing in Micro Motors

May 23, 2024
coupled-unbalance-diagram

An electric motor features two main subassemblies: the stator and the rotor. These components are made using various mechanical techniques like machining, forging, drawing, stamping, sintering, molding, and casting, with each of these methods imparting specific tolerances based on factors like size, shape, and process limits. Assembly of these components adds even more tolerances, leading to unevenness in the motor's structure and mass distribution. This asymmetry causes an eccentric rotation of the rotor, resulting in a rotor unbalance; the unbalance also creates extra centrifugal forces, leading to vibrations and stress on the rotor and its supports. The blog will dive into the topic of rotor balancing, including the main types of unbalances and why properly balancing the rotor is crucial, particularly in high-speed motors, to ensure safe and efficient operation.

Types of Rotor Unbalances

There are three types of rotor unbalances:

Static Unbalance. Static unbalance is a condition in which the principal inertia axis of a rotor is offset from, and parallel to, the shaft rotational axis. This is mostly in rotating parts where the mass is concentrated near their center plane, like in disc magnet rotors. This unbalance can be fixed by either finding the center location and removing mass or by placing an equal mass at 180° opposite to, and at the same radius of, the unbalanced mass.

coupled-unbalance-diagram

Coupled Unbalance. Coupled unbalance is a condition in which the principal inertia axis of the rotor intersects the shaft rotational axis at the center of gravity, where two equal masses are positioned on a rotor at opposite ends and 180° from one other. This unbalance can be corrected by placing another coupled mass opposite the existing coupled mass.

static-unbalance-diagram

Dynamic Unbalance. Dynamic unbalance is a condition in which the principal inertia axis is neither parallel to nor intersects with the shaft rotational axis. It is the vectorial summation of static unbalance and coupled unbalance and fully represents the total unbalance of the rotor and the most familiar unbalance in motors. The correction of dynamic unbalance requires at least two correction planes perpendicular to the shaft center.

dynamic-unbalance-diagram

Rotor Balancing Methods

Rotor balancing is the process of trying to improve the mass distribution of a rotor so that it rotates without uncompensated centrifugal forces. This is done by adding balancing masses to the rotor at prescribed locations, with most of the outer rotor motors being balanced by this process. It should be noted that the added balancing mass must be firmly installed on the rotor so that it won’t fall off when the rotor rotates at high speed. A related balancing method is the removal of fixed quantities of material from the rotor. This is used in most of the BLDC inner rotor motors, where balancing rings are attached at rotor ends and are drilled to remove material for balancing.

The rotor can be balanced using a single plane, two planes, or a multi-plane balancing method. The selection of the balancing method depends on the type of unbalance and the type of rotor. Single-plane balancing is the most basic procedure for balancing a rotor and is mostly performed on thin rotors with static unbalance. Rotors with higher length-to-diameter ratios require dynamic two or multiplane balancing.

The radial unbalance is the product of mass and radius. To ensure intended motor performance, the magnitude of this unbalance should be lower than a permissible value. The commonly used balancing standard in the motion solutions industry is ISO Standard 1940. This standard defines common levels of acceptable unbalance for various types of machines and applications. Generally, for miniature motors G2.5 balance quality grade is used.

Benefits of Rotor Balancing

There are many benefits to having a balanced rotor. These include:

  • Lower vibration in the motor
  • Stable motor output
  • Improve bearing life
  • Reduce mechanical stresses
  • Reduce overall noise
  • Better reliability of motor

Conclusion

Rotor balancing is a critical process in ensuring the smooth and efficient operation of electric motors. By addressing the various types of rotor unbalances (static, coupled, and dynamic) through meticulous balancing techniques, we can significantly reduce vibrations, mechanical stresses, and noise while enhancing motor stability, bearing life, and reliability. Proper rotor balancing not only optimizes the functionality of high-speed motors, but also safeguards against potential failures. Have a question regarding rotor balancing? Our engineers are ready to assist - reach out to us here to start collaborating!