Axial flux motors, with their high power density, compact structure, and excellent torque characteristics, are increasingly used in new energy vehicles, industrial servos, wind power, and other fields. However, as operating hours accumulate and working conditions become more complex, the rotor—the core rotating component of the motor—will inevitably experience various faults. Among them, surface damage of the Axial Flux Motor Rotor, permanent magnet (magnetic steel) demagnetization, and dynamic balance failure are the three most common fault types. Faced with these problems, the core concern of maintenance personnel is: Which faults can be repaired? Which require replacement? Can performance and reliability be guaranteed after repair?
1. Rotor Surface Damage: Minor Damage Is Repairable, Severe Damage Requires Replacement
1.1 Fault Causes and Manifestations
Surface damage of an Axial Flux Motor Rotor is typically caused by rubbing (friction between stator and rotor), foreign object intrusion, or rotor sinking due to bearing failure. Identifying the type of damage helps locate the root cause: if the rotor surface has a single rub mark while the entire stator surface is scratched, it is often caused by a bent shaft or rotor imbalance; if the stator surface has only one rub mark while the rotor surface is scratched around its entire circumference, it results from non-concentricity between stator and rotor, commonly due to deformation of the frame and end shield spigots, or severe bearing wear.
1.2 When Can It Be Repaired?
Minor surface damage is generally repairable. According to industry standards, scraping or grinding methods are allowed to eliminate light damage on the inner surface of the stator and outer surface of the rotor, provided that the motor surface temperature after repair complies with relevant standards. Specific criteria are:
The damage depth is within the machinable range (usually less than 0.5 mm) and does not affect the overall structural integrity of the rotor core.
No large-area short circuit or melting of silicon steel sheets has occurred. If localized burning of core teeth occurs, the melted and fused parts can be filed off, and the damaged areas can be repaired with epoxy resin.
After repair, the air gap uniformity can still meet design requirements, and the surface temperature rating is satisfied.
As for repair techniques, light scratches and rust spots can be polished with fine emery cloth dipped in oil, with roundness deviations checked frequently using a micrometer. For mating surface damage such as shaft journal wear, surface engineering technologies like laser cladding, brush electroplating, and thermal spraying can be used. These repair processes operate at low temperatures and will not cause shaft deformation or alter the metallographic structure.
1.3 When Must It Be Replaced?
The damage depth is too large, exceeding the design tolerance range, and continued repair would destroy the core structure.
Large-area short circuits or delamination of silicon steel sheets have occurred, leading to significantly increased eddy current losses and core overheating.
The rotor core has suffered irrecoverable structural deformation, and air gap uniformity still cannot be guaranteed even after repair.
The damage has extended to weak points in the rotor base structure, and the repair cost is close to or exceeds the replacement cost.
2. Magnet Demagnetization: Mild to Moderate Is Repairable by Re-magnetization, Severe Requires Replacement
2.1 Causes and Mechanisms of Demagnetization
The essence of permanent magnet demagnetization is an irreversible change in the magnetic domain structure, which, based on the cause, mainly falls into three categories:
Thermal Demagnetization: Occurs when the permanent magnet temperature exceeds the tolerance limit of its material grade. For NdFeB, for example, the Curie temperature is about 310°C, above which total magnetic loss occurs. Experimental data shows that after 1000 hours of continuous operation at 150°C, NdFeB magnets can experience a flux loss of 3% to 5%.
Reverse Field Demagnetization: Reverse magnetic fields generated by abnormal conditions such as overload or short circuits cause local magnetic domain reversal. In one new energy vehicle motor, under 200% overload conditions, the magnetic flux density dropped by 7% to 12%.
Chemical Corrosion Demagnetization: NdFeB materials oxidize in hot and humid environments, causing a gradual decay in magnetic properties. Salt spray tests indicate that unprotected magnets can experience up to 15% flux loss after 500 hours.
How to determine on-site whether the magnets are demagnetized? The most intuitive method: after demagnetization, the motor’s no-load speed increases markedly, the load current rises, and the braking torque decreases. More precise detection requires using a Tesla meter (Gaussmeter) to measure the surface magnetic field strength, or by detecting the back EMF and comparing it with the original parameters.
2.2 When Can It Be Repaired?
The repairability of demagnetization depends on the degree of demagnetization, and it is recommended to assess based on the following classification:
Demagnetization Degree | Flux Drop Percentage | Repairability | Recommended Solution |
Mild Demagnetization | <10% | Highly reversible | Re-magnetization + operating condition optimization |
Moderate Demagnetization | 10%–20% | Partially reversible | Partial magnet replacement + full re-magnetization |
Severe Demagnetization | >20% | Essentially irreversible | Rotor assembly replacement or entire motor replacement |
Mild demagnetization is usually caused by short-term overheating or slight overcurrent and has strong reversibility. The treatment plan includes first optimizing heat dissipation, limiting overload, and stabilizing the power supply, then using a high-voltage pulse magnetizer to directionally magnetize the rotor permanent magnets. After magnetization, verify with a Gaussmeter that the magnetic field has recovered to its original value. According to industry practice, professional magnetization equipment can recover over 95% of original performance.
Moderate demagnetization requires disassembling the motor, testing permanent magnets one by one, picking out severely demagnetized units, bonding or embedding new magnets of the same grade and size precisely according to the original polarity, and after full magnetization, conducting no-load current, torque, and efficiency tests.
2.3 When Must It Be Replaced?
The following situations call for decisive replacement rather than further repair attempts:
The remanence of permanent magnets is below 80% of the design value and cannot be restored to rated performance after magnetization.
Magnets show structural damage (cracks, fractures, severe corrosion) such that mechanical strength and service life cannot be guaranteed even after magnetization.
Irreversible demagnetization has occurred, meaning the permanent magnet material itself has aged or suffered chemical corrosion to the point that remanence cannot be restored through magnetization.
Demagnetization has led to such severe drops in motor efficiency and abnormal temperature rise that repair costs exceed the cost of replacing the entire motor.
3. Dynamic Balance Failure: The Vast Majority Are Repairable, Very Few Require Replacement
3.1 Failure Causes and Diagnosis
Rotor imbalance is the most common fault source in rotating machinery—statistics show that 70% of vibration faults in rotating machinery stem from rotor system imbalance. The root cause is the misalignment of the rotor’s center of mass with its geometric axis, creating mass eccentricity that generates centrifugal inertial force during rotation, manifesting as increased radial vibration and accelerated bearing wear.
However, before performing dynamic balance correction, one important thing must be done first—analyze the root cause of the abnormal vibration, because it may not be a dynamic balance problem. If the equipment has severe looseness, resonance, cracked shafts, bearing damage, misalignment, or foundation settlement, dynamic balance correction will not achieve the expected results.
The typical vibration signature of imbalance is that the vibration period is synchronous with the operating speed (dominated by 1× rotational frequency), the radial vibration amplitude is the highest, and the amplitude and phase exhibit stability and repeatability.
3.2 When Can It Be Repaired?
The vast majority of dynamic balance failure problems can be recovered through on-site or factory-based correction, unless the rotor itself has suffered structural damage.
On-site dynamic balancing is a mature technology widely used in industry today. This method performs vibration measurement and balance correction under the rotor’s actual operating speed and installation conditions, without the need to dismantle the rotor and send it back to the factory. It can save about 3–5 days of time and transportation costs, while avoiding the risk of secondary damage during disassembly and reassembly. Correction methods primarily include weight addition (attaching balance weights, screws, riveting, welding) and weight removal (drilling, grinding, milling), with the specific choice depending on the rotor structure and process requirements.
Correction accuracy follows the ISO 1940-1 / GB/T 9239.1 standards, and residual unbalance can be controlled at extremely low levels. In precision manufacturing scenarios, dynamic balance accuracy can reach G1 grade (the highest accuracy grade in ISO 1940-1), effectively eliminating vibration hazards.
The rotor disc frame of an Axial Flux Motor Rotor is mostly made of non-magnetic composite materials and is relatively light in mass. However, once the balance state changes during operation due to the following reasons, correction becomes even more critical:
Corrosion, wear, or scaling of rotating components during operation.
Foreign object adhesion causing mass eccentricity.
Slowly varying imbalance caused by thermal or mechanical deformation.
In the vast majority of the above cases, normal function can be restored through professional dynamic balance correction.
3.3 When Must It Be Replaced?
In the following situations, dynamic balance correction is ineffective, and the rotor needs to be replaced:
The rotor shaft shows cracks or fractures. It should be noted that if the crack extent does not exceed 10% of the shaft journal circumference, repair welding followed by machining flat can allow continued use; however, if it exceeds this range, the shaft should be replaced. If the crack has propagated to the shaft core, the entire rotor must be replaced.
The rotor core has undergone irreversible structural deformation or damage, and balance accuracy still cannot be guaranteed after correction.
Rotating components have detached (e.g., balance weights falling off, blade fracture) and the damage is irreparable.
Vibration still exceeds limits after multiple dynamic balance corrections, indicating serious existing problems with the rotor base structure.
It is worth mentioning that, due to their modular structural design, Axial Flux Motors have a certain advantage during maintenance—only the faulty module needs to be replaced, reducing overhaul difficulty and maintenance costs.
4. Summary: A Table to Understand Repair vs. Replacement
Fault Type | Repairable | Must Be Replaced |
Rotor Surface Damage | Minor scratches and scores (depth <0.5 mm); no large-area short circuit of silicon steel sheets; air gap uniformity meets design requirements after repair. | Large-area deep damage; severe short circuit or delamination of silicon steel sheets; irrecoverable core structure deformation. |
Magnet Demagnetization | Mild (flux drop <20%): re-magnetization or partial magnet replacement followed by full magnetization. | Severe (flux drop >20%); structural magnet damage; irreversible demagnetization where magnetization is ineffective. |
Dynamic Balance Failure | In most cases, repairable by on-site dynamic balancing (weight addition/removal methods). | Shaft fracture (crack exceeds 10% of circumference); core structure damage; detachment of rotating components that are irreparable. |
5. Maintenance Recommendations and Preventive Measures
1. Regular Inspection Is the Prerequisite: Establish a routine inspection mechanism. Use a Gaussmeter for periodic spot checks of magnetic field attenuation, and a vibration analyzer for regular dynamic balance testing, to eliminate faults in their early stages.
2. Diagnose Before Acting: Before any repair operation, the fault cause must first be clearly identified. Especially for dynamic balance issues, non-balance factors such as bearing damage, misalignment, and looseness must be ruled out first; otherwise, balance correction will be futile.
3. Re-magnetization Requires Professional Operation: Magnetization operations involve high-voltage pulse equipment and must be carried out by qualified personnel in an insulated and shielded environment. After magnetization, verify performance with a Gaussmeter, and conduct no-load and load commissioning after reinstallation.
4. Material Upgrades to Prevent Recurrence: For high-temperature or high-vibration operating conditions, prioritize selecting high-grade permanent magnets (e.g., H, SH series) and apply surface protective treatments such as PVD aluminum coating or epoxy composite coatings to the magnets to extend service life.
5. Maintenance Economic Evaluation: A cost comparison needs to be made between rotor assembly replacement and complete motor replacement—when the stator windings are still in good condition, replacing with a genuine rotor of the same model is sufficient, with costs and turnaround time better than a full motor replacement, and performance restored to like-new. However, when repair costs approach or exceed 60%–70% of a new motor’s cost, prioritizing complete motor replacement is recommended.
