The problem is that "Zs" is not single valued and depends on the magnetization state of the machine at the time the fault comes on.

In an induction machine the shaft rpm and frequency are related but not in step, there is a 'slip' as there is in an induction motor, and that frequency difference between revs*poles and the output waveform is the frequency of the AC that is circulating in the spinning armature. We like induction machines for pulse loads (things that have a 'firing pulse' nature) because the output frequency and the armature rate can drop almost at once, but power is still delivered, and  here you may get more current than I suggest.. However they do not like reactive loads, especially variable reactive loads, and are normal used only when the load is well known - floodlight trailers a commercial example that often use induction sets. (even so an impedance limit of 10 times FLC would be a heavy metal machine that is under-run for its FLC)


In the more common forcibly excited machine, (even one with no brushes, but an excitation winding and the rectifiers on armature), the shaft revs set the output mains frequency , and the voltage regulation is performed by winding the current in the rotating magnet up and down, which as a side effect makes the torque for the prime mover vary to scale with the amps. Fast faults happen on a time scale faster than the magnetic field can be changed, so the Zs of the moment is rather dependant on the current load, and the way you try to measure Zs..


All this is solved by having one or more earth fault relays that can be set to trip at several  amps, or even tens on a big set, but only after  300msec or 1000msec delay. For small faults fuses will work like normal, as will normal and S type RCDs. If a serious fault occurs and for whatever reason cannot be cleared, then the EFR will trip and serve like the main fuse for the branch it supplies.


Mike

The problem is that "Zs" is not single valued and depends on the magnetization state of the machine at the time the fault comes on.

In an induction machine the shaft rpm and frequency are related but not in step, there is a 'slip' as there is in an induction motor, and that frequency difference between revs*poles and the output waveform is the frequency of the AC that is circulating in the spinning armature. We like induction machines for pulse loads (things that have a 'firing pulse' nature) because the output frequency and the armature rate can drop almost at once, but power is still delivered, and  here you may get more current than I suggest.. However they do not like reactive loads, especially variable reactive loads, and are normal used only when the load is well known - floodlight trailers a commercial example that often use induction sets. (even so an impedance limit of 10 times FLC would be a heavy metal machine that is under-run for its FLC)


In the more common forcibly excited machine, (even one with no brushes, but an excitation winding and the rectifiers on armature), the shaft revs set the output mains frequency , and the voltage regulation is performed by winding the current in the rotating magnet up and down, which as a side effect makes the torque for the prime mover vary to scale with the amps. Fast faults happen on a time scale faster than the magnetic field can be changed, so the Zs of the moment is rather dependant on the current load, and the way you try to measure Zs..


All this is solved by having one or more earth fault relays that can be set to trip at several  amps, or even tens on a big set, but only after  300msec or 1000msec delay. For small faults fuses will work like normal, as will normal and S type RCDs. If a serious fault occurs and for whatever reason cannot be cleared, then the EFR will trip and serve like the main fuse for the branch it supplies.


Mike