Motor Contribution to Fault Current

[jbond]

If you have a bus with:
6 income generators
3 big mill motors fed off of it
and 1 interconnector running through a transformer to another bus

And if there is a fault on that interconnector, then to calculate the fault level I understand that you calculate the contribution of the generators, but in respect to the motors, won't their contribution be negative?

The way that I'm thinking, is that they will draw current away from the bus in normal operation, which they convert to inertia in a certain direction of rotation. Now when that supply to the motor is withdrawn, won't the inertia of the load induce a current flow in the motor that is associated with that direction of rotation? (which of course is still in the same direction - out of the bus)

So, to calculate the fault level on that interconnector, won't i calculate the contribution of the generators and then subtract the magnitude of the contribution of the motors?

Thanks for your guidance,
Matt.

 

[alehman]

You're close, but the "direction of rotation" concept you suggest is backwards. When you remove the source or short it out, the inertia of the motor causes it to continue rotating. The residual magnetic field of the rotor will cause it to act briefly as a generator, feeding current into whatever load is there (normal loads or faults).

For AC motors, the directio of current flow is not related to the direction of rotation. A motor can rotate either direction while consuming current or generating current.

 

[electricpete]

alehman is correct.

Consider the simpler case that the motors are syncronous. The difference between being a generator and motor is not a reversal of direction, but just a small change in phase angle of rotor with respect to stator field (lags stator field for motor and leads stator field for generator). If you suddenly added a huge load to the system (like a fault) all the generators slow down slightly so the other machines (formerly motors) will now now have rotor leading stator field and act like generators.

From the standpoint of the stator, an induction motor is not much different. Let's say rotor is rotating at fr. Stator field is at fLine. Field induced into rotor is at frequency (fLine-fr). Net result is the rotor field as seen by the stator is at frequency fr + (fLine-fr) = fLine. (I probably was a little sloppy in not identifying the role of p = # poles). The main point is that from the standpoint of the stator, an induction motor rotor field looks just like a sync motor rotor. So the behavior of your induction motors during a fault is the same as described above for sync motors.

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[cuky2000]

As a general rule, it is fair to say that all rotating machine (generators, motors, synchronous condenser, etc) contribute to the SC fault even if they are connected thru a transformer or impedance.

 

[bofu2000]

The behaviour of motors is such that, in the event of a fault, they behave like generators for a few cycles, so the motor contribution is to be added, just as you do for the generators.

 

[jbond]

Thanks guys for your time and help,

Matt.