common faults that appear in three-phase motors include overheating, , bearing failure, breakdown of insulation, problems in power supply, misalignment of the motor, and rotor failure. The general measures would pertain to controlling the load, avoiding fluctuations in power, keeping correct lubrication for bearings, checking insulation resistance at regular intervals, checking alignment, and installation coupled with routine maintenance. For an increase in the life of motors and safe operation, regular inspection along with timely maintenance and optimization of the environment is necessary.
Overheating Issues
Overheating may directly relate to performance, service life, and even safety in the operation of three-phase motors. Overheating mainly comes from some overheating factors, such as excessive current load, poor cooling system, high ambient temperature, and heat accumulation inside the windings. In full-load conditions, the losses of the motor mainly come from two sources: copper losses (the current loss of the windings) and iron losses (the eddy current loss and the hysteresis loss of the core). If the load current is excessive, then the copper losses in the windings are increased considerably. Therefore, the temperature of the winding rises sharply. If the cooling system is proper, internal heat can be dissipated in time. However, if there is insufficient cooling, the insulation material will suffer from high temperature, aging, and an accelerated risk of breakdown.
Local overheating could, therefore, be associated with abnormal rotor core loss owing to poor design of the rotor slot that leads to uneven magnetic flux density, hence increased localized core heating. Besides, the complication of cooling channels in motors, especially the high-speed ones, means partial clogging or any failure either in the ventilation fan or in the external cooling system can keep heat tapped inside the motor and its temperature rises even higher. In addition, long running at high temperature will result in thermal fatigue, destroying the insulation of the stator winding and burning the motor.
In actual operation, it is necessary to , unblock the heat dissipation channels, clean up the dust at regular intervals, and periodically check the operation status of the fans. If a motor has to work in a serious environment for a long time, it is possible to install a real-time temperature sensor or thermal switch within its winding and core to automatically monitor temperature by shutting it off if the temperature exceeds a certain limit.
Voltage Imbalance
Voltage imbalance may cause three-phase motors unstable operation and shorten its life. Voltage imbalance normally comes from the problem of power grid, line losses, loose connections, or internal winding inter-turn shorts. Three-phase motors are designed for symmetrical phase voltage and balanced phase current; voltage imbalance will cause uneven phase currents, resulting in additional heating in the stator windings, thus accelerating deterioration of the insulation.
In practical applications, it normally controls voltage imbalance within 2%. If the imbalance exceeds 5%, the exponential increase in motor temperature rise occurs. Voltage imbalance also reduces the load capacity of the motor because unbalanced currents can cause overloading on one phase winding. Voltage imbalance is most damaging to asynchronous motors, creating negative sequence components that increase the , periodic mechanical vibrations, and noise, while seriously impacting operating stability.
Regularly check the balance and stability of the voltage in the power supply to the motor. In cases where there is repeated fluctuation or imbalance in voltage, the installation of a voltage balancer or power filter at the input to the motor may be helpful. Moreover, when connecting a load motor, check the electrical connection of the power line in order to avoid voltage drops and imbalances due to poor contact.
Bearing Failures
Bearings are one of the important supporting parts in motors, affecting directly rotational performance of motors and mechanical transmission efficiency. Bearing failures are most familiar with motors in poor lubrication, over-load, seal failure, or premature wearing. If the bearings of high-speed rotating three-phase motors either have poor lubrication or aged lubricants, friction drought is very easily caused, increasing frictional heat and causing overheating phenomena. Especially in high-temperature conditions, lubricating oil evaporates or oxidizes and then loses its lubrication effect, which increases friction.
When , external contaminants, such as dust or moisture, enter into the bearing chamber, leading to corrosion and wear of the rolling elements, increasing uneven bearing clearance, causing abnormal vibration and noise. If the bearing clearance is over the limit or has uneven wear on the rolling elements, the vibration of the motor in operation is natural, affecting the overall stability. Moreover, excessive bearing load resulting from misalignment of the motor, incorrect installation, and more axial force will aggravate its fatigue failure and thus affect the efficiency of the motor.
Among others, preventive measures include periodic renewal of lubricants, cleanliness of the lubricant, integrity of the seals, and bearing operational clearance. Furthermore, in the presence of any strange noise or vibration during motor operation, operation should be stopped and the motor should be inspected to avoid spreading the failure of the bearings to more important components.
Insulation Breakdown
The insulation performance of a motor is closely related to safety operation. Generally, insulation failure is caused by overload operation of the motor, overheating, humidity, high-frequency pulse voltage, among other factors. Generally, three-phase motor winding insulation is made of polyimide, polyester, and other materials that have strong resistance to heat and moisture. However, in such hostile environments of high temperature, high humidity, and high vibration, insulating materials must be oxidized, degraded, cracked, expanded, or peeled, thereby further causing windings to short circuit, turns to short, or phases to short.
Under overload conditions, the winding copper losses are greatly increased, generating heat that rapidly advances the aging process of the insulation, which in turn reduces insulation strength. Usually, the first effect of insulation deterioration is a reduction in insulation resistance; if below rated values, leakage current between windings might rise, even leading to turn-to-turn winding shorts.
Insulation breakdown is also one of the and usually appears on those which start more frequently. The high current at the time of startup produces a high-frequency pulse voltage, which exerts electrical stress on the insulation layer, causing it to weaken gradually. Besides, when operating under high-humidity conditions, moisture can easily go through the insulation layer to cause surface leakage or insulation breakdown.
The insulation stability in the motor calls for testing and maintenance of the insulation system of the motor regularly, along with insulation resistance and withstand voltage tests. In case the insulation performance decreases, the insulation material is to be replaced or repaired without wastage of time, and operation of the motor for a long period under extreme conditions is to be avoided.
Power Supply Problems
Power quality is the prime factor for stability in motors. Supply of low voltage, fluctuation, and sagging in voltage used to affect the performance of motors. Low voltage raises the starting current of the motor, hence increases the starting time, winding temperature, and reduces aging of insulation. Voltage fluctuations occurring regularly in the power network lead to variations in operational currents that make unstable the motor torque, thereby reducing efficiency during operation.
It will not only change the performance of the startup but also frequent shutdowns. Voltage fluctuation, especially in an industrial setting where equipment loads vary, is unavoidable. Poor power quality can result in transient high or low voltage, adding thermal shock to windings and shortening the life of windings.
In addition, voltage regulators or filters installed at the motor power supply end will also be able to reduce the influence of power fluctuations. The installation of UPS also guarantees stable operation if there are frequent fluctuations in the power grid. When installing and wiring high-precision motor equipment, ensure good grounding to avoid poor grounding causing current leakage or offset and leading to motor failure.
Misalignment Issues
The alignment of the motor and load shaft has a direct consequence on mechanical stability. Misalignment is a relative nonalignment of the motor and load shafts which can lead to imbalance axial and radial forces, produce extra torque, and increase wear on couplings and bearings. Since imbalance is responsible for misalignment in most cases, then it manifests in uneven vibration and noise, which accelerates fatigue failure in bearings and couplings.
Basically, misalignment faults take two forms: parallel misalignment and angular misalignment. In the case of parallel misalignment, the centerlines of the shafts are offset in a parallel direction. Angular misalignment of a non-zero angle is observed between the two shafts. Both misalignment types create an eccentric force on the rotor when in operation, leading to an unbalanced rotation with increased mechanical losses.
Alignment precision is very important during installation in practical application. It is recommended to install by using alignment instruments or laser alignment devices to ensure that the motor shaft and the load shaft are consistent to reduce the radial force effects that may prevent bearing wear due to misalignment. Especially in large motor equipment, the occurrence of misalignment faults may lead to serious mechanical vibration that affects motor life.
Rotor Damage
As the core component of , the performance of the rotor influences the whole performance of the motor directly. Most rotor failures are of the type of rotor bar fractures, wear of the rotor core, and detachment of the rotor end ring. In most cases, it is caused by sudden loads, overload operation, or mechanical stress. Long-time overload would raise the temperature through increasing the rotor electromagnetic losses, weakening the strength of rotor bars or end rings, and probably causing fractures.
Rotor damage leads to a decrease in the motor speed and output torque. Under extreme conditions, this will cause stalling or shutdown of the motor. A fractured rotor bar can develop unbalanced torque and creates an increase in vibration that will create further damage to the bearings. During local rotor core damage, the motor’s magnetic flux density decreases; this causes a reduction in the efficiency of the motor, lowers power factor, and creates localized core heating.
Avoid rotor damage by controlling motor workload and avoiding overload operation for a long time. Check the surface of rotors regularly to see if there is any integrity of rotor bars and end rings, so as to find and handle the early appearance of localized damages on the rotor bars. During installation, ensure that accurate rotor assembling is made; otherwise, inappropriate assembling may bring stresses and further faults to the rotor. Stop the motor right away for abnormal rotor conditions found, replace damaged rotor bars or end rings when necessary for the assurance of the normal running of the motor.
