【2026 Motor Issues】Causes of unbalanced three-phase current in motors
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What are the causes of unbalanced three-phase current in motors? (1) Three-phase voltage imbalance ； (2) Poor welding or poor contact in a certain phase circuit inside the motor ； (3) Short circuit between turns of the motor windings, or short circuit to ground or between phases ； (4) Incorrect wiring.
The following motor knowledge is provided for reference: 1. When a single-phase transformer is operating without load, the current is out of phase with the main magnetic flux; there is a phase angle difference of aFe due to the existence of iron loss currents. The no-load current has a spike waveform because it contains significant third harmonics.
2. An alternating current also flows in the armature winding of a DC motor. However, direct current flows through its excitation winding. The excitation methods for DC motors include separately excited, shunt excited, series excited, and compound excited types.
3. The back EMF expression for a DC motor is E = CE·F·n； The electromagnetic torque expression is then Tem = CTFI.
4. The number of parallel branches in a DC motor is always in pairs. The number of parallel branches in the armature winding, on the other hand, is not necessarily fixed.
5. In DC motors, the elements of a single-layer winding are connected in series one on top of another. Whether it is a single-wave winding or a single-layer winding, the commutator connects all the components in series to form a single closed circuit.
6. An asynchronous motor, also known as an induction motor, is so named because the rotor current in such motors is generated through electromagnetic induction.
7、When an asynchronous motor is started with reduced voltage, the starting torque decreases, and it decreases in proportion to the square of the starting current in the windings.
8. When the amplitude and frequency of the primary side voltage remain constant, the degree of saturation of the transformer’s core remains essentially unchanged, as does the excitation reactance.
9. The short-circuit characteristic of a synchronous generator is a straight line; in the case of three-phase symmetrical short circuit, the magnetic circuit remains unsaturated； During a three-phase symmetric steady-state short circuit, the short-circuited circuit consists of a purely demagnetizing direct-axis component.
10. The current in the excitation winding of a synchronous motor is direct current, and the main methods of excitation include excitation by an excitation generator, excitation by a static rectifier, and excitation by a rotating rectifier.