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Three-phase voltage unbalance
If the three-phase voltage is unbalanced, there will be reverse-sequence current and reverse-sequence magnetic field in the Motor, resulting in a large reverse-sequence torque, resulting in an unbalanced distribution of the three-phase current of the motor and increasing the current of a certain phase winding. When the three-phase voltage unbalance reaches 5%, the motor phase current can exceed 20% of the normal value. Three-phase voltage imbalance is mainly manifested in:
An abnormality occurs in one of the three-phase windings of the transformer, and an asymmetrical power supply voltage is delivered.
The transmission line is long, the cross-section of the conductor is uneven, and the impedance and voltage drop are different, resulting in an unbalanced voltage of each phase.
Power and lighting are mixed and shared, in which there are many single-phase loads, such as: household appliances, electric furnaces, welding machines, etc. are too concentrated on a certain phase or a certain two-phase, resulting in uneven distribution of power load on each phase, making the power supply voltage and current. unbalanced.
The motor is in an overloaded operation state, especially when starting, the stator and rotor currents of the motor increase and generate heat. The time is slightly longer, and the phenomenon of winding current imbalance is very likely to occur. Heavy load is mainly manifested in:
Transmission mechanisms such as belts and gears are too tight or too loose.
The coupling parts are skewed, and the transmission mechanism is stuck by foreign objects.
The lubricating oil is dry, the bearing is stuck, and the machine is rusted (including the mechanical failure of the motor itself).
The voltage is too high or too low, which increases the loss.
Improper load matching, the rated power of the motor is less than the actual load.
Stator and rotor group failure
The occurrence of inter-turn short circuit, partial grounding, open circuit, etc. in the stator winding will cause excessive current in one phase or two of the stator windings, resulting in serious imbalance of the three-phase current. The faults of the rotor and rotor windings are manifested in:
There is dust, sundries and hard wounds in the inner chamber, which will cause short circuit between turns.
A phase of the stator winding is disconnected.
The stator winding is damp and there is leakage current.
The bearing and rotor are damaged and deformed, and the rotor and the rotor winding are rubbed.
The squirrel-cage rotor winding is broken and welded, resulting in unstable current.
Improper operation and maintenance
The operator can’t do the inspection and maintenance of the electrical equipment regularly, which is the main factor that causes the electric motor to leak electricity, run out of phase, and generate unbalanced current. Improper operation and maintenance are mainly manifested in:
The installation personnel will reverse the phase and neutral lines.
The incoming line collides with the junction box, and there is leakage current.
The lack of phase is caused by various connection switches, loose contacts, oxidation, etc.
Frequent starting, too long or too short starting time, resulting in fuse phase failure.
Long-term use and lack of maintenance will cause the motor to age and local insulation to deteriorate.
Three-phase motor current imbalance may cause insulation breakdown of the motor. Whether the breakdown depends on the current in the motor windings. When the motor starts, the starting inrush current is very large. At this time, the possibility of breakdown is high, but it is not absolute. It is related to the size of the current and the insulation level. The unbalance of three-phase current will definitely produce the instability of motor torque.
The reason for the imbalance of the three-phase current of the motor is that I personally think that it is mainly caused by the imbalance of the three-phase winding of the motor, which is directly related to the manufacturing process of the motor.
Among them, the three-phase current imbalance (the extreme case is the motor phase loss) is one of the main faults. Unbalanced three-phase current may cause difficulty in starting. The motor makes noise when it is running. In severe cases, the motor will vibrate and roar violently. If the current increases, it may also cause the motor winding to burn if it is not stopped in time.
COPY from: https://anelectricmotor.com/why-is-the-three-phase-current-of-the-motor-unbalanced/
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