i would like to know what is the safest draw from a generator, that is, what is the minimum capacity of its pwer that i should draw (output) so that the generator does not get damaged. i.e. if i draw only 30% of the generator capacity during off peak hours, and almost 70% during peak hours, does this mean i am shortening the life of the generator?
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generator load 2
- Thread starter frodo1
- Start date
Skogsgurra
Electrical
Is it a normal AC generator you are talking about? A plain vanilla 50 or 60 Hz generator?
If that is the case, there is no problem at all letting it run idle for as long as you want. What damage are you thinking of?
Gunnar Englund
If that is the case, there is no problem at all letting it run idle for as long as you want. What damage are you thinking of?
Gunnar Englund
- Thread starter
- #3
The generator won't care if the load is low, but the engine will. Most diesel engines are not happy with very light loads. 30% power rating should be ok, but long periods of idling or very light laoding are best avoided. Unburnt fuel and oil builds up in the stack, and the cylinder bores become glazed. Probably other bad things too, but I'm not a reciprocating engine specialist.
Bear in mind that the generator is rated in kVA and the engine in kW: in theory you could have a generator at or near its kVA rating with almost zero kW loading if you had a particularly poor load power factor or if the excitation was adjusted incorrectly. This is invisible to the engine, which only sees real power.
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I don't suffer from insanity. I enjoy it...
Bear in mind that the generator is rated in kVA and the engine in kW: in theory you could have a generator at or near its kVA rating with almost zero kW loading if you had a particularly poor load power factor or if the excitation was adjusted incorrectly. This is invisible to the engine, which only sees real power.
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frodo1,
Running a generator with ligh load is not a problem at all as you think. The simple caution and the most common problem should be focus on overloading that results to overheating as stressing the generator's winding.
Other thing is on economic point of view while operating the machine on light load. It is impractical to operate the machine, If the genset is large, the consideration should be pointed to fuel consumption, lube oils, auxiliary equipments, etc. This thing would hinder you to run the machine, come to think of it!
scot, the unburnt fuel and oil stacks are insignificant problem. Evidently, we run it as a hot reserve power to supply a grid for peak load without experiencing the latter you stated, for as long the machine is well maintained.
Tell your mechanics to scraped out the sediments or recondition the machine parts, otherwise scrap them out.
Running a generator with ligh load is not a problem at all as you think. The simple caution and the most common problem should be focus on overloading that results to overheating as stressing the generator's winding.
Other thing is on economic point of view while operating the machine on light load. It is impractical to operate the machine, If the genset is large, the consideration should be pointed to fuel consumption, lube oils, auxiliary equipments, etc. This thing would hinder you to run the machine, come to think of it!
scot, the unburnt fuel and oil stacks are insignificant problem. Evidently, we run it as a hot reserve power to supply a grid for peak load without experiencing the latter you stated, for as long the machine is well maintained.
Tell your mechanics to scraped out the sediments or recondition the machine parts, otherwise scrap them out.
TheBlacksmith
Mechanical
Two stroke cycle diesel engines dislike light loading, there are legions of reports documenting abnormal wear on military engines abused in this way. But if the set is regularly getting 70% load every 8-12 hours out of 24 hours, as the original poster alludes, that is virtually the cyclic operation most OEM's recommend to counteract light loading periods.
Skogsgurra
Electrical
To conclude: No problem running the generator without load. There might be a problem with the prime mover - but only if it is a two stroke diesel engine (must confess, never heard of such things before - thought all diesels were four stroke). Is it a two stroke diesel?
Gunnar Englund
Gunnar Englund
" - thought all diesels were four stroke"
Actually I think that the very ubiqutous GMC -71 series engines are 2 stroke. There are many 2 stroke diesels out there. They are not common in newer installations because, compared to modern 4 strokes, they burn very dirty.
Ian Rines
Harris Corporation
Palm Bay,FL
Actually I think that the very ubiqutous GMC -71 series engines are 2 stroke. There are many 2 stroke diesels out there. They are not common in newer installations because, compared to modern 4 strokes, they burn very dirty.
Ian Rines
Harris Corporation
Palm Bay,FL
-
1
- #9
TheBlacksmith
Mechanical
skogsgurra,
Just because all the diesels made in Skövde are four stroke... The General Motors 53, 71, 92 and 149 series were all two stroke - exhaust valves in the head and intake ports with a pressurized airbox in the cylinder walls. Fairbanks Morse used opposed pistons and two cranshafts, one leading the other with the bottom piston uncovering exhaust ports and the upper piston uncovering the intake ports and again a pressurized airbox. However, emmissions and fuel economy have driven them out of the marketplace.
Just because all the diesels made in Skövde are four stroke... The General Motors 53, 71, 92 and 149 series were all two stroke - exhaust valves in the head and intake ports with a pressurized airbox in the cylinder walls. Fairbanks Morse used opposed pistons and two cranshafts, one leading the other with the bottom piston uncovering exhaust ports and the upper piston uncovering the intake ports and again a pressurized airbox. However, emmissions and fuel economy have driven them out of the marketplace.
In regards to safe minimum loads I would say yes that there are such ratings. However, as many have suggested, it is dependant on the generator system that you are operating. Stand-by (emergency gens.) contin. Power and or peak power operations.
Minimum loads for the most part are requirements for engine maintenance (carbon build up). It is typical that many generator manufacturers (Onan, Kohler...) will suggest maintenance guidlines that suggest units be run monthly at 30% of rated capacity and anually at 100% of rated capacity. In contrast to that some manufacturers of units (Waukesha)designed for use in Cogen situations may state that generators not be operated at loads less than 60% of rating. Reasons for these minimum ratings usually have nothing to do with damage that may occur from low load operation but more to do with performance issues. (frequency stabilization)
Minimum loads for the most part are requirements for engine maintenance (carbon build up). It is typical that many generator manufacturers (Onan, Kohler...) will suggest maintenance guidlines that suggest units be run monthly at 30% of rated capacity and anually at 100% of rated capacity. In contrast to that some manufacturers of units (Waukesha)designed for use in Cogen situations may state that generators not be operated at loads less than 60% of rating. Reasons for these minimum ratings usually have nothing to do with damage that may occur from low load operation but more to do with performance issues. (frequency stabilization)
Skogsgurra
Electrical
Thanks Blacksmith, I will tell the Skövde guys next time I go there. I actually have a few HV supplies for core drying that I shall return there before the holidays. Oh? Holidays already over us? I'll do it in August then.
Gunnar Englund
Gunnar Englund
-
1
- Moderator
- #13
I have a lot of sets running with very light loads and I have experienced light loading problems first hand.
First I must state that this is a problem with new engines. I have discussed the problem with the engineers from two vendors.
The problem occasionally arises with a new engine in which the rings are not seated perfectly.
When the engine is opperated at a reasonable load, the combustion pressure forces the rings to seat in. Once the rings are properly seated there is usually no further problem.
I had a problem with one set, only one of many of the same make, where the engine was lightly loaded and pumped the crank case oil out the exhaust until the engine shut down on the low oil pressure safety switch. The set was then restarted two more times and shut down again on the low oil pressure safety switch both times.
Then they called me. After this abuse the rings were so well seated that the engine never burned or pumped another drop of oil.
The problem with the second set was similar. The problem was discovered when dirty oil started leaking out of a seam in the muffler onto the new engine. This was a different make of engine, but again only one problem out of many installed sets.
I put a load bank on the set for about 5 days and there was no more trouble.
The third engine did not have a problem, but the vendor was concerned with the loading pattern. The size of the set was dictated by one large motor which had to be started once or twice a day. Normal load was about 10% of rated load.
The vendor was threatening to void the warranty on the basis of the light loading. We reached a compromise; We put a load bank on the set for about 5 days to ensure that the rings would seat properly and then the vendor agreed to continue the warranty.
An interesting note on the earlier comment re KVA vs Kilowatts.
The set with the warranty issue had been in service for a year or two with no problems of any kind and some idiot decided that it needed more load. To that end, he disconnected all the power factor correction capacitors in the plant. The current on the set went up but the load on the engine remained about the same.
The power factor penalties went up alarmingly when the plant was on utility power.
Back to the original question;
No problem with the generator.
Problems with the engine are most likely during the first hours, days or weeks of it's service life.
Once the rings are properly seated there are no further problems.
Still, a light load on a diesel engine is something that I don't like but that I often have to accept.
Glazing of the cylinder walls is something that is possible but I suspect that it is rare and that poorly seated rings may often be mis-diagnosed as glasing. I have not yet experienced glazing.
Two cycle diesels;
EMD (Electro-Motive Division) built larger engines on the same general pattern as the GM two cycle diesels. They were widely used to power diesel electric locomotives.
The GM numbering series was the number of cylinders followed by the cubic inches displacement per cylinder.
A v was interposed for "V" type engines. The ubiquitous 6-71 was a straight six engine but there was also a 6V-71 engine. The 71 series ranged from the 2-71 to the 16V-71.
The 6-71, 6V-71, 8V-71 6V-53, 6V-92 (and less often, the 4-71 and the 12V-71) were widely used in heavy trucks.
The exhaust frequency was twice that of a comparable 4 cycle engine. The GM engines were widely know as "Screaming Jimmies".
There was also a 6-110 built probably in the 40s, possibly into the 50s.
Respectfully
First I must state that this is a problem with new engines. I have discussed the problem with the engineers from two vendors.
The problem occasionally arises with a new engine in which the rings are not seated perfectly.
When the engine is opperated at a reasonable load, the combustion pressure forces the rings to seat in. Once the rings are properly seated there is usually no further problem.
I had a problem with one set, only one of many of the same make, where the engine was lightly loaded and pumped the crank case oil out the exhaust until the engine shut down on the low oil pressure safety switch. The set was then restarted two more times and shut down again on the low oil pressure safety switch both times.
Then they called me. After this abuse the rings were so well seated that the engine never burned or pumped another drop of oil.
The problem with the second set was similar. The problem was discovered when dirty oil started leaking out of a seam in the muffler onto the new engine. This was a different make of engine, but again only one problem out of many installed sets.
I put a load bank on the set for about 5 days and there was no more trouble.
The third engine did not have a problem, but the vendor was concerned with the loading pattern. The size of the set was dictated by one large motor which had to be started once or twice a day. Normal load was about 10% of rated load.
The vendor was threatening to void the warranty on the basis of the light loading. We reached a compromise; We put a load bank on the set for about 5 days to ensure that the rings would seat properly and then the vendor agreed to continue the warranty.
An interesting note on the earlier comment re KVA vs Kilowatts.
The set with the warranty issue had been in service for a year or two with no problems of any kind and some idiot decided that it needed more load. To that end, he disconnected all the power factor correction capacitors in the plant. The current on the set went up but the load on the engine remained about the same.
The power factor penalties went up alarmingly when the plant was on utility power.
Back to the original question;
No problem with the generator.
Problems with the engine are most likely during the first hours, days or weeks of it's service life.
Once the rings are properly seated there are no further problems.
Still, a light load on a diesel engine is something that I don't like but that I often have to accept.
Glazing of the cylinder walls is something that is possible but I suspect that it is rare and that poorly seated rings may often be mis-diagnosed as glasing. I have not yet experienced glazing.
Two cycle diesels;
EMD (Electro-Motive Division) built larger engines on the same general pattern as the GM two cycle diesels. They were widely used to power diesel electric locomotives.
The GM numbering series was the number of cylinders followed by the cubic inches displacement per cylinder.
A v was interposed for "V" type engines. The ubiquitous 6-71 was a straight six engine but there was also a 6V-71 engine. The 71 series ranged from the 2-71 to the 16V-71.
The 6-71, 6V-71, 8V-71 6V-53, 6V-92 (and less often, the 4-71 and the 12V-71) were widely used in heavy trucks.
The exhaust frequency was twice that of a comparable 4 cycle engine. The GM engines were widely know as "Screaming Jimmies".
There was also a 6-110 built probably in the 40s, possibly into the 50s.
Respectfully
Skogsgurra
Electrical
All this is very interesting. And I have learnt a lot. Waross gave an impressing overview from a corner of technology I hardly knew existed. But, the original poster, frodo, never said anything about combustion engines. His concern seems to be the generator. And we all seem to agree that he needn't worry about that.
What about other prime movers? Are two stroke diesels the only ones that don't like light loads? Or are there others?
Gunnar Englund
What about other prime movers? Are two stroke diesels the only ones that don't like light loads? Or are there others?
Gunnar Englund
Skogsgurra,
Waross gave a correct analysis concerning the importance of loading an engine/generator for the purpose of seating the rings. However, that process should be done on New engine/generators during the initial start and test but is still only an option that carries an expence to the customer. The full issue concerning excessive light or continuous no load conditions has to do with "Wet Stacking" and carbon build up. With these situations it really doesn't matter if the engine is a two stroke or four stroke but is more related to fuel type. A diesel will be more suseptable to carbon build up than a NG or propane.
BTW, the two stoke Detriot engines were the perfect prime movers for power generation as they were very fast responders to load change. California was the first to make a ban on the two stroke diesel. The biggest mistake was viewing all two storke diesels in one category (i.e. trucks, boats, generators....).
Waross gave a correct analysis concerning the importance of loading an engine/generator for the purpose of seating the rings. However, that process should be done on New engine/generators during the initial start and test but is still only an option that carries an expence to the customer. The full issue concerning excessive light or continuous no load conditions has to do with "Wet Stacking" and carbon build up. With these situations it really doesn't matter if the engine is a two stroke or four stroke but is more related to fuel type. A diesel will be more suseptable to carbon build up than a NG or propane.
BTW, the two stoke Detriot engines were the perfect prime movers for power generation as they were very fast responders to load change. California was the first to make a ban on the two stroke diesel. The biggest mistake was viewing all two storke diesels in one category (i.e. trucks, boats, generators....).
- Moderator
- #16
Another interesting point that has been alluded to but not stated specifically. When a lightly loaded set is loading up the exhaust system with lube oil, the exhaust temperature is below the ignition temperature of the oil. If the set has been pumping oil for some time and then the load increases the exhaust temperature may rise above the ignition temperature of the oil in the exhaust system. Depending on the circumstances this may result in clouds of black smoke, flames, and/or burning lube oil being expelled from the stack.
Truckers have been known to pour a quart of old oil down the vertical exhaust stack of another drivers truck, either maliciously or as a practical joke. Victims have seen the clouds of smoke coming out the exhaust and jumped to the conclusion that they had serious engine problems. They would go directly to the repair shop, but by the time they got there the oil would have usually all burned out and the mechanics would find nothing wrong with the engine.
Truckers are a fun loving bunch. There is quite a list of little mostly harmless tricks they play on each other's trucks.
Respectfully
Truckers have been known to pour a quart of old oil down the vertical exhaust stack of another drivers truck, either maliciously or as a practical joke. Victims have seen the clouds of smoke coming out the exhaust and jumped to the conclusion that they had serious engine problems. They would go directly to the repair shop, but by the time they got there the oil would have usually all burned out and the mechanics would find nothing wrong with the engine.
Truckers are a fun loving bunch. There is quite a list of little mostly harmless tricks they play on each other's trucks.
Respectfully
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