Line to Line/earth fault evaluation

[Gyo]

What are the things or calculation you would consider to determine if your motor could operate under fault conditions (e.g line-to-line or earth fault)?

I am assuming that if a phase fault happen, you would get higher negative sequence current as a result of unbalance 3-phase motor. Therefore the motor would vibrate even more, and losses would be higher.

Where would the current go for each of the fault conditions?? It is correct to assume that the motor current will be flowing towards the fault area, and if the two phase lines are normal, current will flow towards the motor?

How would you know how long will your motor will be able to operate under such fault?

 

[waross]

The motor will probably fail completely. It may fail faster than I can answer your question in detail.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gyo]

What is the mechanism that would make it fail?

 

[DRWeig]

Melted windings?

Best to you,

Goober Dave

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[dpc]

Are you talking about a fault inside the motor or an external fault?

 

[Gyo]

I am more interest in an external fault.

 

[dpc]

Normally an external fault is going to result in the the interruption of power to the motor, so motor damage is not an issue. For a sustained ground fault that is not cleared, the motor will fairly quickly overheat due to loss of phase voltage that results in high negative sequence currents. The bigger risk to the motor is loss of a phase due to a blown fuse, downed line, etc. This is not generally cleared quickly and can cause significant motor damage.

 

[waross]

OK external fault. The motor acts as a generator due to the back EMF. The motor generates a back EMF with equal voltages, just a little below normal line voltage and at equal angles. It wants to see balanced voltages and equal phase angles. If, due to an external fault, the voltages and/or phase angles are not equal, there will be a difference between the line and the motor back EMF. This difference in voltages and/or phase angles will cause heavy currents to flow in the motor windings.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Gyo]

Typically, how much current is expected to be drawn to or flow from the motor?

 

[waross]

The current unbalance may be 10 times the voltage unbalance.

Bill
--------------------
"Why not the best?"
Jimmy Carter