For any motor, as long as the actual running current of the motor does not exceed the rated motor, the motor is relatively safe, and when the current exceeds the rated current, the motor windings are in danger of being burned. In three-phase motor faults, phase loss is a typical type of fault, but with the emergence of motor operation protection devices, such problems have been better avoided.
However, once there is a phase loss problem in a three-phase motor, the windings will be regularly burned in a short period of time. Different connection methods have different rules for the burning of the windings. The motor windings of the delta connection method will have a phase loss problem. When it happens, one phase winding will be burnt and the other two phases are relatively intact; while for the star-connected winding, the two-phase winding will be burnt and the other phase will be basically intact.
For the burnt winding, the fundamental reason is that the current it withstands exceeds the rated current, but how big this current is is a problem that many netizens are very concerned about. Everyone tries to quantitatively understand it through specific calculation formulas. There are also many experts who have conducted special analysis on this aspect, but in different calculation and analysis, there are always some inestimable factors, which will lead to a large deviation of the current, which has also become a topic of constant debate.
When the motor starts and runs 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 disconnection occurs, the current of one or two-phase lines will be zero, and the current of the remaining phase lines will increase. We take the load during electric operation as the rated load, and analyze the current situation qualitatively from the distribution relationship of winding resistance and torque after phase failure.
When a delta-connected motor operates normally at rated values, 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, the two-phase windings are connected in series and the other phase is connected in parallel. The winding current that bears the line voltage alone will reach more than 2.5 times the rated current, which will cause the winding to be burned in a very short time, and the other two-phase winding currents are small and generally in good condition.
For a star-connected motor, when a phase is disconnected, the other two-phase windings are connected in series with 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 twice the rated current, causing the two-phase windings to overheat and burn.
However, from the analysis of the whole process of phase loss, various factors such as different windings, different quality states of windings, and actual conditions of the load will lead to complicated changes in current, which cannot be calculated and analyzed from simple formulas. We can only A rough analysis is made from some limit states and ideal modes.
Post time: Jul-15-2022