The characteristics of a motor phase loss fault are manifested as increased vibration, abnormal noise, high temperature, reduced speed, and increased current. When starting, there is a strong humming sound and the motor cannot start.
The causes of a motor phase loss fault are problems with the power source or connections, such as poorly selected fuses or loose connections, burnt fuses, poor contact of switches, loose or disconnected connectors, or one phase of the motor winding being disconnected.
After a motor is burnt out due to a phase loss fault, the direct fault symptoms of the winding are regular burn marks and similar degrees of damage. The fault point position is severely burned in phase-to-phase, phase-to-ground, or inter-turn faults, and the extent of the damage is relatively lighter as it spreads outward. This is a feature that distinguishes phase loss faults from other faults.
When a motor has a phase loss, the rotating magnetic field of the stator becomes severely unbalanced, which generates negative-sequence current in the stator. The negative-sequence magnetic field interacts with the rotor to produce a voltage of approximately 100Hz, causing a sharp increase in current and heat generation in the rotor. The motor's carrying capacity decreases during a phase loss, leading to a sharp increase in stator current and heat generation. This is the most direct manifestation. Due to the severe unevenness of the magnetic field in the motor, the motor vibrates severely, causing damage to the bearings. If the motor continues to run with a phase loss, it will stop rotating instantly, leading to motor burnout. To prevent this problem, most motors have phase loss protection.
During normal startup or operation, the three-phase current is balanced and equal in size, and is less than or equal to the rated value. After one phase is disconnected, the three-phase current becomes unbalanced or too large.
During startup with a phase loss, the motor cannot start and the winding current is 5 to 7 times the rated current. The heat generated is 15 to 50 times the normal temperature rise, which quickly exceeds the allowable temperature rise and causes the motor to burn out.
When a fully loaded motor has a phase loss, it enters an overcurrent state, with the current exceeding the rated current. The motor goes from being under-loaded to being over-loaded, causing the line current of the undisconnected phases to increase further and the motor to burn out quickly.
When a lightly loaded motor has a phase loss, the current in the undisconnected windings increases rapidly, causing the winding to burn out due to overheating.
Phase loss operation is very harmful to squirrel cage motors that are used for long-term continuous operation. About 65% of motor burnout incidents are caused by phase loss operation. Therefore, phase loss protection for motors is very important.
Advantages and Disadvantages of Variable Frequency Power Supply for Motors
Variable frequency motors and AC motors are two completely different product series, but using AC motors with variable frequency is a common practice. Regardless of whether it is the motor manufacturer or the supporting equipment manufacturer, this simple and convenient method may be the most economical and quickest. However, it carries great risks, including shortened motor life or direct burnout.
Since AC motors are often used with variable frequency, there must be a reasonable side to it. The speed of an induction motor is directly related to the frequency of the power supply: the higher the frequency, the higher the speed (not considering slip). The output torque and current size are related to the load (including "dynamic load" and "steady-state load or static load"). If the impact of harmonic components is not too serious, the increase in current during rated load is not too large. The variable frequency range during operation is not less than 30% of the rated frequency. Many situations meet these conditions, and the actual operating speed is not too low to affect ventilation and heat dissipation, and the increase in current is not too large, so the motor can work as expected.
However, the "extremely high risks" are due to the fact that most end users are not aware that the variable frequency converter will output a large amount of high-frequency harmonics during variable frequency operation, and the motor may resonate at a certain speed. Therefore, during rated load, the motor may be overloaded (current increase is too large), high-frequency harmonics may break through the wire insulation, and the variable frequency operation may cause howling or violent vibration, and even cause the power module of the variable frequency converter to explode.
Differences in Performance Parameter Selection
For ordinary induction motors, the main performance parameters considered during design are overload capability, starting performance, efficiency, and power factor. However, for variable frequency motors, since the critical slip rate is inversely proportional to the frequency of the power supply, the motor can start directly when the critical slip rate is close to 1. Therefore, overload capability and starting performance do not need to be considered too much. The key problem to be solved is how to improve the motor's adaptability to non-sinusoidal power sources. Therefore, the measures taken during design mainly include:
● Reducing the resistance of the stator and rotor as much as possible. Reducing the stator resistance can reduce the copper loss of the fundamental wave and compensate for the increase in copper loss caused by high-frequency harmonics.
● Increasing the inductance of the motor appropriately to suppress high-frequency harmonics in the current. However, the rotor slot leakage resistance is large and its skin effect is also large, which increases the copper loss of high-frequency harmonics. Therefore, the size of the motor leakage reactance needs to be considered in the context of the entire speed range to ensure reasonable impedance matching.
● Designing the main magnetic circuit in a non-saturated state. This is because high-frequency harmonics will deepen the saturation of the magnetic circuit, and in order to increase the output torque at low frequencies, it is necessary to appropriately increase the output voltage of the variable frequency converter.
Design Strategies for Structures
During structural design, fully consider and evaluate the impact of non-sinusoidal power source characteristics on the insulation structure, vibration, noise, and cooling methods of variable frequency motors. The following methods can be used to ensure that the product meets the requirements:
● Using an insulation level of F or higher, and increasing the strength of the ground insulation and coil insulation, especially considering the ability to withstand impact voltage. Some motor manufacturers use thick film magnetic wire, while regular motor manufacturers use special magnetic wire for variable frequency motors. From the performance indicators of magnetic wire, the insulation used in variable frequency magnetic wire is different from that of ordinary magnetic wire. Using thicker wire can have some effect, but it does not solve the fundamental problem. However, from a cost and price perspective, variable frequency magnetic wire is more expensive than ordinary magnetic wire.
● Considering the stiffness of the motor components and the entire motor to effectively avoid vibration and noise problems in the motor, and increasing the inherent frequency of the motor to avoid resonance with various force waves.
● For motors with a capacity greater than 132kW, bearing insulation measures should be taken. This is because variable frequency motors are prone to magnetic field asymmetry, which can cause axial current during motor operation. When other high-frequency components are combined, the axial current will increase significantly, leading to bearing damage. Therefore, insulation measures are generally taken.
● For constant power variable frequency motors, when the speed exceeds 2P motor synchronous speed, high-temperature-resistant special grease should be used to ensure the lubrication effect of the bearings during operation, and special high-temperature bearings should be used if necessary.
In conclusion, motor phase loss faults can cause serious damage to the motor and have significant impacts on its performance. The symptoms of a phase loss fault include increased vibration, abnormal noise, high temperature, reduced speed, and increased current. The causes of a phase loss fault can be due to power source or connection issues. During operation, a phase loss fault can lead to negative-sequence current, which can interact with the rotor and cause a voltage of approximately 100Hz, leading to a sharp increase in current and heat generation in the rotor. It is important to have phase loss protection for motors to prevent burnout caused by phase loss faults. Additionally, when using AC motors with variable frequency power supply, it is important to consider the performance parameter selection and structural design to ensure the motor's adaptability to non-sinusoidal power sources and prevent potential risks.