Question re generators for those in the know...

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Walter105
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Question re generators for those in the know...

Unread post by Walter105 » Wed Mar 20, 2019 9:09 pm

Hi everyone

We've been told that our 500kVA diesel generator that runs our office and production facility is only being used to around 75% of its total generating capacity and that due to it running often at these reduced loads it is actually causing damage to the generator.

Advice is to either pull more power off it to run it up to full capacity or reduce the running times.

This seems odd to me. Why would running it below full capacity for extended periods (between 4 and 12 hours) cause damage to it?
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Re: Question re generators for those in the know...

Unread post by OG » Wed Mar 20, 2019 9:28 pm

Does not harm the alternator, but it is not good for the engine.
Running at a lower load for extended periods will cause the piston rings to not expand to full seal, and hot gasses can leak past the rings, glazing the sleeves, and depositing excessive carbon into the sump, prematurely contaminating the oil and causing excessive wear. Also, unburnt fuel will cause builup of carbon on the injector nozzles and turbo. Working the engine to full capacity will burn off these deposits.
Be careful, if ridges have built up in the sleeves because of the carbon deposits, running the engine up to max can cause the piston rings to break.
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Re: Question re generators for those in the know...

Unread post by Induna » Wed Mar 20, 2019 9:29 pm

Hi,

75% load is actually fine. it still gives you spare capacity of 25% to absorb any additional inrush load. Also note that the diesel engine power is de-rated for altitude (assuming you are up on the reef), which means the generator can produce the rated 500kVA but the engine would probably not be able to produce enough power to supply the full rated 500kVA.

A ball park figure of anything between 50-80% rated power will be ok.
Last edited by Induna on Wed Mar 20, 2019 9:31 pm, edited 1 time in total.
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Re: Question re generators for those in the know...

Unread post by kwest » Wed Mar 20, 2019 9:29 pm

In many respects the same principle as running an aero engine. High loads at constant parameters. Diesel engines particularly like to run under optimum load otherwise they say the cylinders "glaze" or as I understand form a residue in the cylinders that limits optimum performance over time. ?. Also, purely economic , why invest capital at only 75 % return ?
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Re: Question re generators for those in the know...

Unread post by Ugly Duckling » Wed Mar 20, 2019 9:45 pm

Many gensets have dummy loads which are energised to keep the engine running at optimum rating. They produce huge amounts of heat so proper cooling is required.
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Re: Question re generators for those in the know...

Unread post by Walter105 » Wed Mar 20, 2019 11:28 pm

kwest wrote:
Wed Mar 20, 2019 9:29 pm
Also, purely economic , why invest capital at only 75 % return ?
Sunk cost. Was bought and paid for a number of years ago to run a bigger facility. Restructuring of operations moved it to a smaller facility, and with current loadshedding now the issue and hence the question.

Many thanks to all for the answers.
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Re: Question re generators for those in the know...

Unread post by Roger » Thu Mar 21, 2019 2:42 pm

Busy installing another 100kva into one of our offices. Was told 60-80% of capacity is the best range to be operated at.
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Re: Question re generators for those in the know...

Unread post by vanjast » Thu Mar 21, 2019 9:29 pm

As mentioned, full load is generally not a good idea (unless engine and alternator are rated well above the 'rated' load).

- The engine always requires services intervals anyway - running at light loads and full load would usually require shorter service intervals.

- Although alternators are rated at xyz KVA, most people mistake this for KW - so A 75% load (Calculated) is a good ballpark figure that will keep it within limits.

- A lot of generator alternators I dug into are wired to just within rated limits. This means they will cook sooner than later. Even production of the reputable one's have been farmed off to cheaper production facilities, with 'cheaper' results.

- If a genny is unable to give it's 'rated' power, and the alternator is ok, the engine will need it's internals redone. A clean engine makes a big difference ;)
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Re: Question re generators for those in the know...

Unread post by heisan » Thu Mar 21, 2019 9:55 pm

An interesting document from Caterpillar on the various generator types and how they are rated (I see that it ties to an ISO specification, so these ratings should be quite uniform):

http://s7d2.scene7.com/is/content/Caterpillar/C10282446
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