Reasons for unbalanced three-phase current when the motor is running

1. Reasons for unbalanced three-phase current when the motor is running

(1) The three-phase voltage is unbalanced, causing the three-phase current to be unbalanced;

(2) The motor is short-circuited between phases or turns, and the current between short-circuited phases or short-circuited turns increases, causing unbalanced three-phase current;

(3) Wiring error: When one phase is connected in reverse, the currents between the three phases are not equal and are much larger than the normal value;

(4) The starter has poor contact, causing partial circuit breakage of the motor coil.

II. Solutions

1. Three-phase voltage unbalance

If the three-phase voltage is unbalanced, there will be reverse current and reverse magnetic field in the motor, which will generate a large reverse torque, causing the three-phase current distribution of the motor to be unbalanced, and increase the current of a certain phase winding. When the three-phase voltage imbalance reaches 5%, the motor phase current can exceed the normal value by more than 20%. The main reasons are:

(1) An abnormality occurs in one phase of the three-phase winding of the transformer, resulting in an asymmetric power supply voltage.

(2) The transmission line is long, the conductor cross-sections are uneven, and the impedance voltage drops are different, resulting in unbalanced voltages between phases.

(3) Power and lighting are shared, with most single-phase loads, such as electrical appliances, electric furnaces, welding machines, etc., which are too concentrated on one or two phases, resulting in uneven distribution of power loads on each phase, making the power supply voltage and current unbalanced.

2. Overload

In overload operation, especially when starting, the stator and rotor currents of the motor increase and generate heat. If the time is slightly longer, the winding current imbalance phenomenon is very likely to occur. The main reasons are:

(1) The transmission mechanism such as belts and gears is too tight or too loose.

(2) The coupling parts are skewed and foreign objects are stuck in the transmission mechanism.

(3) Lubricating oil is dry, bearings are stuck, and machinery is rusted (including mechanical failure of the motor itself).

(4) The voltage is too high or too low, which increases the loss.

(5) The load is improperly matched and the rated power of the motor is less than the actual load.

3. Motor stator and rotor winding fault

If there is a short circuit between turns, local grounding, or open circuit in the stator winding, it will cause excessive current in one or two phases of the stator winding, causing serious imbalance in the three-phase current. The main reasons are:

(1) There is dust, debris, or hard damage in the inner chamber, causing a short circuit between turns.

(2) A phase of the stator winding is open circuit.

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

(4) The bearings and rotors are damaged and deformed, and the rotor and the running windings rub against each other.

(5) The squirrel cage rotor winding breaks and welds, generating unstable current.

4. Improper operation and maintenance by personnel. Operators fail to regularly inspect and maintain electrical equipment, causing leakage and phase loss in electrical equipment:

(1) The operator and installer connected the phase and neutral wires in reverse.

(2) The incoming line touches the junction box, causing leakage current.

(3) Phase loss is caused by reasons such as loose connection switches and contacts or oxidation.

(4) Frequent starting, starting time is too long or too short, causing the fuse to break.

(5) Long-term use and lack of maintenance will cause the electrical equipment to age and local insulation to degrade.