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Induction motor fault current contribution vs motor load

Induction motor fault current contribution vs motor load

Induction motor fault current contribution vs motor load

(OP)
I was always under the impression that the asymmetrical current contribution of an induction motor to a system fault is due to the stored energy in the magnetic field between the rotor and stator (airgap flux). The way I see it this does not depend on the motor load, so whether a motor is 80% or 10% loaded the fault contribution is essentially the same.

Maximum fault contribution is given by LRA/FLA ~ 6p.u.

Is this correct?

Thanks.

RE: Induction motor fault current contribution vs motor load

When the mains clear but the motor is connected to the faulted feeder, the motor may become an induction generator and feed the fault until the rotation speed drops. When back feeding into a fault, I have seen the motor slow down faster than the load and we found pieces of the coupling all over the machine room. (400 HP wound rotor at speed when the fault occurred across the motor terminals.)
I may be wrong but my understanding is that the frequency must drop before the motor makes a contribution. But, stored magnetic energy and regeneration may both be factors.

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

RE: Induction motor fault current contribution vs motor load

As I understand it, assuming the line is disconnected due to the fault, the induction motor only regenerates into the circuit until the magnetic field collapses. Since the line is disconnected, that is a matter of cycles (some residual magnetism is still there, but is insignificant to this issue).

"Will work for (the memory of) salami"

RE: Induction motor fault current contribution vs motor load

(OP)
My understanding is that the motor contribution (regeneration) is not dependant on frequency but system voltage. A drop in system voltage will result in the stored magnetic energy in the motor collapsing into the fault. This is virtually guarenteed to happen for a DOL motor since the system volts collapses. Note also, that faults take at least 3 cycles to clear (breaker time) so this phenomenon is a near certainty for faults close to rotating plant.

I am quite certain that the motor load at time of fault does not affect the contribution of the motor to the fault but am open to other insights on this.

Regards.

RE: Induction motor fault current contribution vs motor load

The induction motor has an internal voltage when it is rotating at normal speed. When the terminal voltage drops below this internal voltage, (during a fault) the motor will contribute current into the fault. The magnetic field will decay rapidly. The frequency of the generated power may not be exactly equal to the system frequency, but it usually assumed to not decay significantly for the short duration of the motor contribution. The actual load on the motor is not generally taken into account - it could cause the rate of frequency decay to be faster (or slower) than for an uncoupled motor, but in practice we don't worry about this for typical short circuit calculations. So the short answer is that the actual load on the motor will not change the calculated short circuit current, unless you are conducting a very complex transient analysis. The short circuit current from the induction motor derives from the stored magnetic energy, and this is pretty independent of the motor load.

It's good to keep in mind that nearly all of the circuit analysis that is done in power engineering involves a lot of simplifying assumptions. The accuracy is sufficient for our purposes. It will never be "exact" unless we want to start from Maxwell's equations every time.

RE: Induction motor fault current contribution vs motor load

(OP)
Thanks for the confirmation dpc, appreciated.

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