What is the cause of the unbalanced three phase current of the motor

Update:03-11-2020
Summary:

One: Three-phase voltage unbalance If the three-phase v […]

One: Three-phase voltage unbalance

If the three-phase voltage is unbalanced, there will be a reverse sequence current and a reverse sequence magnetic field in the motor, which will produce a large reverse sequence torque, which will cause an unbalanced three-phase current distribution of the motor and increase 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. The three-phase voltage imbalance is mainly manifested in:
(1) An abnormality occurs in a phase of the three-phase winding of the transformer, and an asymmetric power supply voltage is transmitted.
(2) Long transmission lines, uneven conductor cross-sections, and different impedance voltage drops result in unbalanced phase voltages.
(3) The power and lighting are mixed and shared. Among them, there are many single-phase loads, such as household appliances, electric furnaces, welding machines, etc., which are too concentrated in one phase or two phases, resulting in uneven distribution of power load of each phase, which makes the power supply voltage, The current is unbalanced.


Two: overloaded


The motor is in overload operation, especially when starting, the stator and rotor currents of the motor increase and generate heat. The time is slightly longer, it is easy to appear winding current imbalance. The overload is mainly manifested in:
(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 jammed by foreign matter.
(3) The lubricating oil is dry, the bearing is jammed, and the machinery is rusted (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, motor rated power is less than actual load.


Three: Stator and rotor warp group failure


Interturn short circuits, local grounding, and open circuits in the stator windings will cause excessive current in one phase or two of the stator windings, causing serious imbalance of the three-phase currents. The faults of the mover and rotor winding are manifested in:
(1) There are dust, debris, and hard wounds in the inner chamber, causing short circuits between turns.
(2) A certain phase of the stator winding is open.
(3) The stator winding is damp and there is leakage current.
(4) The bearing and rotor are damaged and deformed, and the rotor rubs against the winding of the mover.
(5) The squirrel-cage rotor winding is broken and welded, resulting in unstable current.


Four: improper operation and maintenance


The inability of operators to do regular inspection and maintenance of electrical equipment is the main factor that artificially causes motor leakage, phase loss, and unbalanced current.
Improper operation and maintenance are mainly manifested in:
(1) The operator installer reversed the phase and neutral wires.
(2) The incoming line collides with the junction box, and there is leakage current.
(3) Phase loss caused by various connection switches, loose contacts, oxidation and other reasons.
(4) Frequent starting, too long or too short starting time, causing the fuse to open phase.
(5) Long-term use and lack of maintenance will make the motor age and local insulation degradation.


The unbalance of the three-phase motor current 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 higher, but it is not absolutely correct. This is related to the current size and insulation level. The unbalance of the three-phase current will definitely produce the instability of the motor torque. The cause of the unbalance of the three-phase current of the motor is personally believed to be mainly caused by the unbalance of the three-phase winding of the motor, which is directly related to the manufacturing process of the motor.

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