IEC 60909 motor contribution

[radug]

Hello,

I have some questions that I think are related to thread238-289927 but it is closed.

squasher,

In your first post, shouldn't it be used Ith instead of Ik for short-time withstand current in 3 seconds?

It is a question I have about IEC 60909 because if Ith should be used, then I do not know what is the use of Ik related to switchgear rating. I think it would be used only for protective device coordination purposes.

And about motor contribution, IEC 60909 says that they must be considered when doing maximum short-circuit calculations and discarded when doing minimum short-circuit calculations. Correct me if I am wrong, but I think that:
-Maximum short circuit: use ip, Ith(1sec or 3sec) for busbar rating, and addionaly Ib for circuit-breaker breaking current rating. So, no use for Ik.
-Miniumum short circuit: Ik for protection device coordination, but as with mimumum motors are neglected it does not calculate motor contribution so when setting motor-protection relays, Ik of the maximum short circuit should be used.

Last, I have been looking for a reference in the standard or other place about asynchronous motor behaviour during short-circuit: why consider contribution for maximum short-circuit and not for minimum? during how much time does the motor contribution last during short-circuit? if it stops, why? because it breaks, because it self-stops?

 

[ijl]

To your last question regarding asynchronous motors:
Maximum current: running motors that can contribute to the current.
Minimum current: stopped motors, cannot contribute to the currents.

Motors do not stop or break. The currents from the motors decay towards zero because there is no field winding. The decay is much faster than the decay of short circuit current of synchronous machines. The decay of asynchronous motors is determined by the internal time constants, that depend on the resistances and reactances of the windings. The current typically decays in a fraction of a second, in some cases even during a couple of cycles.

 

[radug]

ijl,

From your explanation, then current contribution from motor becomes zero and then the motor starts to behave again as a motor (consuming power)?

I have done many short-circuit calculations and always wondered why do motors contribute or behave as generators in short-circuits.

Do you know about an specific reference that explains it?

 

[ijl]

current contribution from motor becomes zero and then the motor starts to behave again as a motor

Well, not quite. If the (three phase) fault is at the motor, then then voltage is zero, and current decays to zero. But if the fault is elsewhere, then the behaviour of the current is more complicated. It is generally assumed in short circuit calculations that the rotating speed stays constant during the period considered.

One reference is Nasser Tleis, Power system modelling and fault analysis, theory and practice (not very easy reading but there are others.

But why do motors behave as generators in short circuits? It is complicated, and best answered with lots of equations. But _very_very_ roughly, the reason is similar to the inrush current of inductances: When the voltage at the motor changes suddenly, a DC component will be added to the currents in the windings, because current cannot change suddenly. The rotating magnetic field generated by this current will then induce more AC currents.

 

[veritas]

radug

The way I explain asynchronous motor contribution is that under normal running conditions there is a magnetic field estbalished in airgap between rotor and stator. There is thus reactive power in this field.

With the occurance of a fault, this energy is discharged into the fault and for a very short period (up to 5 cycles max) the motor acts as a generator. This is because the motor acts as an inductor and inductors do not like change in current through them - they will oppose it and try to maintain the status quo.

This is simply put.

With regards to Ith, you may wish to visit thread238-312247.

Regards.