I. Three-phase voltage imbalance
If the three-phase voltage is unbalanced, reverse-sequence current and reverse-sequence magnetic field will exist in the motor, generating a large reverse-sequence torque, causing an imbalance in the three-phase current distribution of the motor, and increasing the current in one phase winding. When the three-phase voltage imbalance reaches 5%, the phase current of the motor can exceed the normal value by more than 20%. The main manifestations of three-phase voltage imbalance are:
1. An abnormality occurs in one phase of the three-phase winding of the transformer, resulting in unbalanced power supply voltage.
2. The transmission lines are long, and the conductor cross-sections are uneven, resulting in different impedance voltage drops and causing voltage imbalance between phases.
3. Power and lighting are shared, with many single-phase loads, such as household appliances, electric furnaces, and welding machines, which are too concentrated on one or two phases, resulting in uneven distribution of electrical loads on each phase and an imbalance in power supply voltage and current.
II. Overload
When an electric motor is operating under overload conditions, especially during startup, the stator and rotor currents increase, causing heat generation. Prolonged overload can easily lead to winding current imbalance. Overload primarily manifests as:
1. The transmission mechanism, such as belts and gears, is too tight or too loose.
2. The coupling components are misaligned, and there are foreign objects stuck in the transmission mechanism.
3. The lubricating oil is dry, the bearings are stuck, and the machinery is rusted (including mechanical failures of the motor itself).
4. Voltage that is too high or too low increases losses.
5. Improper load matching; the rated power of the motor is less than the actual load.
III. Stator and rotor faults
Inter-turn short circuits, partial grounding, and open circuits in the stator windings can all cause excessive current in one or two phases of the rotor winding, resulting in a severe imbalance of the three-phase current. Faults in the rotor and rotor windings manifest as follows:
1. Dust, debris, or hard damage inside the bore caused a short circuit between the turns.
2. A phase of the stator winding is open-circuited.
3. The stator winding is damp, resulting in leakage current.
4. The bearings and rotor are damaged and deformed, and the rotor rubs against the walkie winding.
5. Broken bars in the squirrel-cage rotor windings cause weld cracks, resulting in unstable current.
IV. Improper operation and maintenance
The failure of operators to regularly inspect and maintain electrical equipment is a major cause of motor leakage, single-phase operation, and unbalanced current. Improper operation and maintenance manifest themselves primarily in the following ways:
1. The operator reversed the phase and neutral wires.
2. The incoming line is touching the junction box, causing leakage current.
3. The phase loss phenomenon is caused by reasons such as loose or oxidized connection switches and contacts.
4. Frequent starts, or starts that are too long or too short, can cause the fuse to break.
5. Long-term use without maintenance causes the motor to age and local insulation to deteriorate.
Current imbalance in a three-phase motor can lead to insulation breakdown. Whether breakdown occurs depends on the magnitude of the current in the motor windings. The starting inrush current is very large during motor startup, increasing the likelihood of breakdown, but this is not absolute; it depends on factors such as the current magnitude and insulation class. Three-phase current imbalance will definitely result in unstable motor torque.
I believe the main reason for the imbalance of the three-phase current in the motor is the imbalance of the three-phase windings, which is directly related to the motor's manufacturing process.
Three-phase current imbalance (in extreme cases, a single phase loss in the motor) is one of the main faults. Three-phase current imbalance can cause starting difficulties, noise during motor operation, and in severe cases, violent vibration and roaring. Increased current can also lead to motor winding burnout if the motor is not stopped in time.
