For any electric motor, as long as the actual current of the electric motor operation does not exceed the rated motor, the electric motor is relatively safe.
when the current exceeds the rated current, the electric motor windings are in danger of being burned up.
In the three-phase motor fault, the lack of phase is a relatively typical type of fault, but with the emergence of electric motor operation protection devices, this type of problem has been relatively well circumvented.
However, once the three phase motor is lack of phase, the winding will have a very short period of time to appear regularly burned fault appearance, different connection, the winding is burned in different patterns, triangle connection of the electric motor winding.
When the phase problem occurs, there will be a phase winding is burned and the other two phases are relatively intact; and star winding, there are two phases of winding burned, the other phase is basically intact
For the burned winding, the fundamental reason are because the current it withstands exceeds the rated current, but how much this current is, is a lot of netizens are very concerned about the problem, we are attempting to understand quantitatively through the specific calculation formula.
On this aspect of the analysis, there are also a number of experts to carry out thematic analysis, but there are always some immeasurable factors in different calculation and analysis, which will lead to a larger current shift, which has also become a topic of debate.
When the electric motor starts and runs normally, the three phase AC power is symmetrical load, and the three-phase current is equal in size and less than or equal to the rated value. When there is a phase disconnection, so that one or two phase line current is zero, the rest of the phase line current will increase the situation.
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 break.
For the Delta connected electric motor, When it is working under the rated value. The phase current of each group of winding is 1/1.732 times of the rated motor current (line current).
When one phase is disconnected, that is, two phase windings are connected in series and then in parallel with the other phase.
The winding current which bears the line voltage alone will be more than 2.5 times of the rated current.
It will cause the winding to be burned in a short time, while the other two phases winding current is smaller and will be in good condition in general.
For the star-connected electric 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 disconnected phase current is zero, and the current of the other two phase windings increases to more than two times the rated current, so that the two phase windings overheat and burn up.
But from the analysis of the whole process of phase loss, different windings, different quality state of the winding, the actual situation of the load and other factors, will lead to the change of current is relatively complex, can not be calculated and analyzed from a simple formula, we can only roughly analyze from some limit state and ideal mode.
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