1. 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 large reverse sequence torque, resulting in unbalanced three-phase current distribution of the motor and increasing the winding current of a phase. When the unbalance of three-phase voltage reaches 5%, the phase current of the motor can exceed 20% of the normal value. The three-phase voltage unbalance is mainly manifested in the following aspects:
(1) In the three-phase winding of transformer, one phase is abnormal, and the asymmetrical power supply voltage is transmitted.
(2) Long transmission line, uneven conductor cross section size and different impedance voltage drop result in unbalanced voltage of each phase.
(3) The power and lighting are mixed and shared, among which there are many single-phase loads, such as household appliances, electric furnace, welding machine, etc., which are too concentrated in one or two phases, resulting in uneven distribution of power load in each phase and unbalanced supply voltage and current.
When the motor is in overload operation state, especially when starting, the current of stator and rotor of the motor increases and heats up. If the time is a little long, the unbalance of winding current is easy to appear. Overload is mainly manifested in the following aspects:
(1) Belt, gear and other transmission mechanisms are too tight or loose.
(2) The coupling parts are skewed, and the transmission mechanism is jammed by foreign matters.
(3) The lubricating oil is dry, the bearing is jammed, and the machinery is rusted (including the mechanical failure of the motor itself).
(4) If the voltage is too high or too low, the loss will increase.
(5) The rated power of the motor is less than the actual load.
3. Stator and rotor fault
Stator winding inter turn short circuit, local grounding, open circuit, etc., will cause excessive current in one or two phases of the stator winding, which makes the three-phase current seriously unbalanced. The faults of stator and rotor windings are as follows
(1) There were dust, debris and hard trauma in the inner chamber, which caused turn to turn short circuit.
(2) One phase of stator winding is open circuit.
(3) The stator winding is damp and has leakage current.
(4) The bearing and rotor are damaged and deformed, and the rotor rubs with the stator winding.
(5) The broken bars of squirrel cage rotor winding are welded and cracked, which results in unstable current.
4. Improper operation and maintenance
Operators can not do a good job in the inspection and maintenance of electrical equipment on a regular basis, which is the main factor that causes the motor leakage, lack of phase operation and unbalanced current.
Improper operation and maintenance is mainly manifested in:
(1) Operators and installers connect the phase and zero lines reversely.
(2) There is leakage current when the incoming line collides with the junction box.
(3) Each connection switch, contact loose, oxidation and other reasons cause the phenomenon of phase loss.
(4) Frequent start-up, too long or too short start-up time, resulting in fuse phase failure.
(5) Long term use, lack of maintenance, make the motor aging, local insulation degradation