Three-phase load imbalance causes - News - Global IC Trade Starts Here Free Products

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1. Three-phase voltage imbalance

If the three-phase voltage is unbalanced, there is a reverse sequence current and a reverse sequence magnetic field in the motor, which generates a large reverse sequence torque, which causes the three-phase current distribution of the motor to be unbalanced, so that the current of a certain phase winding increases. When the three-phase voltage unbalance is 5%, the motor phase current can exceed 20% of the normal value. The main reason is:

(1) An abnormality occurs in one of the three-phase windings of the transformer to deliver an asymmetric power supply voltage.

(2) The transmission line is long, the wire cross-section is not uniform, and the impedance voltage drop is different, resulting in voltage imbalance of each phase.

(3) Power and lighting mixing, in which single-phase load is too much, such as: electrical appliances, electric furnaces, welding machines, etc. are too concentrated in one phase or two phases, resulting in uneven distribution of electrical load of each phase, so that the supply voltage and current unbalanced.

2. Overloaded

In the overload operation state, especially when starting, the motor stator and rotor current increase heat. The time is slightly longer and the winding current imbalance is prone to occur. The main reason is:

(1) Transmission mechanisms such as belts and gears are too tight or too loose.

(2) The Coupling mechanism is skewed, and the transmission mechanism is stuck with foreign matter.

(3) The lubricating oil is dry, the bearing is stuck, and the mechanical rust is dead (including the mechanical failure of the motor itself).

(4) The voltage is too high or too low to increase the loss.

(5) Improper load matching, the rated power of the motor is less than the actual load.

3. Motor stator and rotor winding failure

The occurrence of inter-turn short circuit, local grounding, and open circuit in the stator windings may cause one phase or two of the currents in the windings to be excessively large, causing a serious imbalance of the three-phase current. The main reason is:

(1) Dust, debris, and hard wounds are set in the interior, causing a short circuit between the turns.

(2) One phase of the stator winding is broken.

(3) The stator winding is damp and there is leakage current.

(4) The bearing and rotor are damaged and the rotor is rubbed against the winding of the rotor.

(5) The squirrel-cage rotor winding is broken and cracked, resulting in unstable current.

4. Improper operation and maintenance

The operator can not regularly check and maintain the electrical equipment, artificially causing leakage of electricity and phase loss of the electrical equipment:

(1) The operation installer will reverse the phase and neutral lines.

(2) The incoming line collides with the junction box and has leakage current.

(3) The phase loss phenomenon is caused by various connection switches, loose contacts, oxidation, and the like.

(4) Frequent starting, the starting time is too long or too short, causing the fuse to be out of phase.

(5) Long-term use, lack of maintenance, aging of electrical equipment, and degradation of local insulation.