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Earth Fault Loop Impedance with Safety Generator
Former Community Member
Do I take this into consideration? If so how do I size wire in a building with a safety generator? What is my "Ze" when the supply switches from DNO to the safety generator? How do diesel generators behave during faults?
My concern is that ProMbrooke seemed to be mixing quoted fault levels from various sources and was not aware of the huge effect that the generator type and design has on the transient output characteristics. It may be that larger async machines are more popular in Germany.
Flywheel moment of inertia is normally calculated large enough to store enough energy to smooth over between the firing of successive cylinders with acceptably low torque ripple (ripple p-p perhaps 1 to 3% of the average torque, probably a bit less on a Rolls-Royce job, and perhaps more vibration allowed on a lawnmower where a large diameter wheel would be hard to fit in, and anyway you may prefer it to stop dead if it hits something hard. ).
So you could say it stores the energy of maybe 30 to 100 revs at nominal motor output power. So if the PM dies, it will run down to 1/e1/2 of original speed in that many rotations.
edit sorry divide by no of cylinders that fire per rev.
In practice in the case discussed the shaft speed will be well on the way down in a fraction of a second.
My concern is that ProMbrooke seemed to be mixing quoted fault levels from various sources and was not aware of the huge effect that the generator type and design has on the transient output characteristics. It may be that larger async machines are more popular in Germany.
Flywheel moment of inertia is normally calculated large enough to store enough energy to smooth over between the firing of successive cylinders with acceptably low torque ripple (ripple p-p perhaps 1 to 3% of the average torque, probably a bit less on a Rolls-Royce job, and perhaps more vibration allowed on a lawnmower where a large diameter wheel would be hard to fit in, and anyway you may prefer it to stop dead if it hits something hard. ).
So you could say it stores the energy of maybe 30 to 100 revs at nominal motor output power. So if the PM dies, it will run down to 1/e1/2 of original speed in that many rotations.
edit sorry divide by no of cylinders that fire per rev.
In practice in the case discussed the shaft speed will be well on the way down in a fraction of a second.
