A diesel generator question

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T.M.Haja Sahib

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A diesel generator takes load up to a particular limit.Its speed drops excessively,when the load increases beyond this limit,even though the increase in load is within the rating of the gen set.What may be the causes?Thanks.
 
If the generator bogs down it is either too much load or a problem with the gen set. Bad fuel is always a possible culprit. So is poor maintenance.

When choosing a generator it is always important to consider future needs. I hope such considerations were accounted for.

Was the generator sized to accommodate actual loads or anticipated loads? It seems as though the generator of which you speak was not up to modern design specifications.
 
ActionDave said:
If the generator bogs down it is either too much load or a problem with the gen set. Bad fuel is always a possible culprit. So is poor maintenance.

When choosing a generator it is always important to consider future needs. I hope such considerations were accounted for.

Was the generator sized to accommodate actual loads or anticipated loads? It seems as though the generator of which you speak was not up to modern design specifications.
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The engine is of Escorts make and alternator is of Crompton Greaves make and of adequate size.

The new gen set was lying idle for almost two years.The filters were changed.New lubrication oil was provided.

Yet when the gen set was tested on resistance load,its speed drops excessively beyond a certain limit of permissible load.
 
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T.M.Haja Sahib said:
The engine is of Escorts make and alternator is of Crompton Greaves make and of adequate size.

The new gen set was lying idle for almost two years.The filters were changed.New lubrication oil was provided.

Yet when the gen set was tested on resistance load,its speed drops excessively beyond a certain limit of permissible load.
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Most of the generators I've come across are rated in kVA at a cos phi of 0.8 and the prime mover (engine in this case) rated accordingly. Thus a 10kVA unit would be limited to 8kW by the prime mover.
Could that be the issue?
 
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Besoeker said:
Most of the generators I've come across are rated in kVA at a cos phi of 0.8 and the prime mover (engine in this case) rated accordingly. Thus a 10kVA unit would be limited to 8kW by the prime mover.
Could that be the issue?
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Thanks for your reply.But that is not the issue here.Resistive load was sized also taking into account the factor 0.8 for gen set KVA.
 
masterinbama said:
Does this Generator have windings in parallel? I have once before had a set bog down below rated load due to a having one winding that was not connected properly.
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In the subject alternator,there are two coils per phase in the stator.The two coils are connected in series in STAR formation.I checked all the three phases of the alternator with a clamp meter for any abnormal current,when the speed of the gen set dropped excessively as the load was put on within its rating.There was none.
 
iwire said:
Has it been checked out by a generator technician?
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Yes.
He is suggesting to derate the gen set down to load where the engine speed does not drop excessively.

I think this is not correct.Because the gen set only remained unused for almost two years since purchase.
 
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T.M.Haja Sahib said:
Thanks for your reply.But that is not the issue here.Resistive load was sized also taking into account the factor 0.8 for gen set KVA.
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If you got rated gen set kVA with a resistive load then you would likely have overloaded the prime mover which would account for the symptoms.
 
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Besoeker said:
If you got rated gen set kVA with a resistive load then you would likely have overloaded the prime mover which would account for the symptoms.
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Even after taking into account the factor 0.8?
 
No.It is not that way.Say there is 30 KVA gen set. Apply 0.8 to it.The gen set should be capable of supplying 30x0.8=24kW i.e 24kW of resistive load.In the present case the gen set is capable of supplying only 20kW of resistive load due to some internal problem in it which I am trying to find out with the help of the members of this forum.
 
T.M.Haja Sahib said:
No.It is not that way.Say there is 30 KVA gen set. Apply 0.8 to it.The gen set should be capable of supplying 30x0.8=24kW i.e 24kW of resistive load.
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If that's what was done, then I agree. I took your comment "gen set KVA" in an earlier post to infer that you meant rated gen set kVA.

T.M.Haja Sahib said:
In the present case the gen set is capable of supplying only 20kW of resistive load due to some internal problem in it which I am trying to find out with the help of the members of this forum.
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In your original post you mentioned that the speed drops excessively.
One simple and obvious conclusion is the prime mover is being loaded beyond its capability and that causes it to slow.
Do you have any nameplate details of ratings etc that you can provide?
 
Was this generator connected to the exact same load before it went into storage? If so, and it performed as desired, then it's likely that something has become damaged during storage.

The piston rings in the engine may have got stuck. There may be problems with the fuel system from having fuel sit so long it evaporated. There may be problems with the rubber seals. Sitting for long periods can be fatal for IC machines.

Just curious, what is the max rated load for the generator and what are you trying to run with it?

If you want to try to fix this thing, you may need some special equipment.

You will need to take pressure readings for the fuel system and failing any progress there, compression tests of the cylinders. Did the generator tech that was there take any readings of the sort?
 
I've had that problem before with natural gas generators, the pipe fitters installed too small of gas line to the unit. Once it got past a certain load, it would shut down due to fuel starvation. Could be a clogged fuel system, but it looks like the generator tech would have checked that, but then the gas unit I had problems with the generator tech missed that. The customer had us monitor the load because the tech said we had a short circuit in our load wiring. The load was staged with two transfer switches, and as soon as the second transferred, the generator would die. After the test, I went out to look at the generator (100 kva three phase) and saw the 3/4" low pressure line ran to it. They replaced it with the proper size, no more problem.
 
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