When you buy a standby generator, especially from a major manufacturer, you expect a minimum level of engineering has been done. That engineering should include consideration that the engine and generator would work well together. One of the methods of insuring this compatibility is the torsional vibration analysis.
An engine with its rotating crankshaft and reciprocating pistons and rods sets up a vibration unique to that engine alone; similar to all others of that model, but still unique to the individual engine. The generator that then bolts up to the engine's flywheel also sets up a unique vibration. If these vibrations align, an unacceptable harmonic can develop.
In order to anticipate this phenomenon, it is wise to calculate the torsional natural frequencies within the system and define the excitation forces expected over the planned range of speed and power output characteristics of the system. The coincidences of the excitation frequencies and the expected torsional natural frequencies may then be accommodated.
How important is this?
When I worked for a major machinery manufacturer, we offered an excellent 300kW generator set. This machine performed exceptionally well at the 300kW rating. After the model had successfully sold and performed well for a number of years, the manufacturer boosted the rating up to 350kW.
A company who already owned some of the units at the 300kW rating bought one of the new 350s and applied it to a rock crusher. A week later it threw the fan through the radiator.
Surprised with the failure, our service department repaired the equipment and delivered it back out to the crusher. A week later it was back with another broken fan drive.
They made another repair and sent it back out. A week later it was back. After the third radiator repair, we found that a torsional vibration analysis had not been run at the 350kW rating. We also found that we could make the fan drive live if we mounted the whole machine on old truck tires to absorb the vibration.
The factory ran a torsional vibration analysis on subsequent generators and was able to modify the equipment to allow the 350kW rating. Our original generator spent the rest of its life on truck tires.
The torsional vibration analysis noted in NFPA 110 is essential.
Generators and automatic transfer switches as well as their appurtenant devices employ high voltages that can hurt you. Do not attempt to work with this equipment unless you are qualified. Observe all rules and cautions found in the manufacturer's manuals as well as NFPA 70E, Standard for Electrical Safety in the Workplace.
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