Why does the winding of a three-phase motor burn out when it is phase-deficient?

How large can the current be in star connection and delta connection?

For any motor, as long as the actual operating current of the motor does not exceed the rated current, the motor is relatively safe. When the current exceeds the rated current, the motor winding is at risk of being burned out. Among the faults of three-phase motors, phase deficiency is a relatively typical type of fault. However, with the emergence of motor operation protection devices, such problems have been better avoided.

However, once a phase deficiency problem occurs in a three-phase motor, there will be a fault manifestation that the winding is regularly burned out within a very short period of time. For different connection methods, the rules of the winding being burned out are different. For the motor winding in delta connection, when a phase deficiency problem occurs, one phase winding will be burned out while the other two phases are relatively intact; for the star-connected winding, two-phase windings will be burned out and the other phase is basically intact.

For the burned-out windings, the fundamental reason is that the current they bear exceeds the rated current. But how large this current is exactly is a question that many netizens are very concerned about. Everyone tries to have a quantitative understanding through specific calculation formulas. There have also been many experts who have carried out special analysis on this aspect. However, in different calculation and analysis, there are always some immeasurable factors that will cause a large deviation in the current, which has also become a topic of endless debate.

When the motor starts and operates normally, the three-phase alternating current is a symmetrical load, and the three-phase currents are equal in magnitude and less than or equal to the rated value. When one phase is broken, the current of one or two phase wires becomes zero, and the current of the remaining phase wires will increase. We take the load during the electric operation process as the rated load, and qualitatively analyze the current situation from the distribution relationship of the winding resistance and torque after the phase break.

For a motor in delta connection, when it operates normally at the rated value, the phase current of each group of windings is 1/1.732 times the rated current (line current) of the motor. When one phase is disconnected, that is, the situation where two-phase windings are connected in series and then in parallel with the other phase occurs. The current of the winding that bears the line voltage alone will reach more than 2.5 times the rated current, which will cause this winding to be burned out within a very short time. The currents of the other two-phase windings are relatively small, and they are generally in a good state.

For a motor in star connection, when a certain phase is disconnected, the other two-phase windings are connected in series to the power supply. When the load remains unchanged, the current of the disconnected phase is zero, and the current of the other two-phase windings increases to more than 2 times the rated current, causing the two-phase windings to overheat and burn out.

However, from the analysis of the entire process of phase deficiency, various factors such as different windings, different quality states of the windings, and the actual situation of the load will lead to a relatively complex change in the current. It is impossible to calculate and analyze it from a simple formula. We can only conduct a rough analysis from some extreme states and ideal models.