hi, i wonder if we have plate cos phi value of 0,8 does it mean that a 1000 kVA generator will give nominally 800 kwatt? is there any drawback if we operate this generator with a load of 1000 kwatt? ?f we could do it, so why do we have cos phi = 0,8 values on generators ups's etc...greetings..
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of generator cos phi
- Thread starter turkoman
- Start date
At 0.8pf 1000kVA your generator may only be rated for 800kw, so if you load kw over this, your generator shaft may not be stong enough.
Looking at your prime mover (engine, turbine etc) again may not give any more power than 800kw
Use of 0.8pf seems to be historic, since it gives a margin above what is normally used. Any historians out there?
Looking at your prime mover (engine, turbine etc) again may not give any more power than 800kw
Use of 0.8pf seems to be historic, since it gives a margin above what is normally used. Any historians out there?
davidbeach
Electrical
What is the power rating of you prime mover? The generator certainly can't produce more power out than the shaft power in. Assuming you could get 1000kW out of the prime mover, the generator could supply 1000kW at unity power factor. By the time load power factor gets down to 0.8 you can only get 800kW out of the generator without exceeding the 1000kVA rating.
Typically a 800kW prime mover will be coupled to a 1000kVA generator so that you can get the rated 800kW as long as your load power factor is better than 0.8. If you had a 1000kW machine connected to a 1000kVA generator you'd be very disappointed to find that you can only get your 1000kW at unity power factor.
Typically a 800kW prime mover will be coupled to a 1000kVA generator so that you can get the rated 800kW as long as your load power factor is better than 0.8. If you had a 1000kW machine connected to a 1000kVA generator you'd be very disappointed to find that you can only get your 1000kW at unity power factor.
- Thread starter
- #4
what i wanna learn is a point of contest between davidbeach and hoxton i think..
So does a 1000 KVA generator is capable of giving 900 or 950 or 1000 whatever (more than 800 kva) KVA out of its terminals electrically if this generator is rated 1000 KVA and cos phi = 0,8 on its plate?
So does a 1000 KVA generator is capable of giving 900 or 950 or 1000 whatever (more than 800 kva) KVA out of its terminals electrically if this generator is rated 1000 KVA and cos phi = 0,8 on its plate?
Your prime mover supplies only real power and it can only give out what its rated for, which in this case would be 800kW max. Your alternator is a diff. story. For example, at maximum of its rated capacity, your generator will provide 800kW (prime mover max) + 600kVAR (alternator max) = 1000kVA (generator max). For other limitations, you need to look at manufacturer’s generator capability curves.
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
davidbeach
Electrical
Prime mover supplies the watts, excitation system supplies the vars. For any given combination of watts and vars you can not exceed the kVA rating of the alternator. The alternator nameplate will provide the kVA rating, the prime mover nameplate will provide (or maybe at least imply) the kW rating. The alternator will probably be able to supply more watts at unity power factor than the prime mover can supply.
Do you want a prime mover/alternator combination that can supply the rated kW over a broad range of power factor or do you want a prime mover/alternator combination that can provide full alternator rated kVA at unity power factor? You can't economically have both, and the general preference is that the alternator be sized to provide full prime mover kW over a broad range of power factors.
Do you want a prime mover/alternator combination that can supply the rated kW over a broad range of power factor or do you want a prime mover/alternator combination that can provide full alternator rated kVA at unity power factor? You can't economically have both, and the general preference is that the alternator be sized to provide full prime mover kW over a broad range of power factors.
The basis of the ratings may help you.
The current that a generator is capable of producing is determined by the heating effect of the current. If you draw more current than the generator is rated for it may burn up.
The voltage rating is generally the system voltage.
Multiply the volts times the amps and you get VA. Divide by 1000 for KVA.
That is the capability of the generator.
Loads tend to be at less than unity power factor. The prime mover is usually rated at less kW than the KVA. This generally gives good service.
0.8 or 80% is pretty standard for a PF rating on generators.
But, change the voltage and the KVA changes but the kW of the motor stays the same. Take a machine rated at 40 kW and 50 KVA at 240 volts and trim it down to 120/208 volts. The KVA is now based on 208 rather than 240 and drops to 43.3 KVA. The prime mover will still deliver 40 kW.
If the same generator is reconnected for single phase, the KVA drops to 33.3 KVA but the engine will still develop 40 kW. Care must be exercised so as to not overload the generator. It will have enough power to easily damage itself if not properly protected.
Bill
--------------------
"Why not the best?"
Jimmy Carter
The current that a generator is capable of producing is determined by the heating effect of the current. If you draw more current than the generator is rated for it may burn up.
The voltage rating is generally the system voltage.
Multiply the volts times the amps and you get VA. Divide by 1000 for KVA.
That is the capability of the generator.
Loads tend to be at less than unity power factor. The prime mover is usually rated at less kW than the KVA. This generally gives good service.
0.8 or 80% is pretty standard for a PF rating on generators.
But, change the voltage and the KVA changes but the kW of the motor stays the same. Take a machine rated at 40 kW and 50 KVA at 240 volts and trim it down to 120/208 volts. The KVA is now based on 208 rather than 240 and drops to 43.3 KVA. The prime mover will still deliver 40 kW.
If the same generator is reconnected for single phase, the KVA drops to 33.3 KVA but the engine will still develop 40 kW. Care must be exercised so as to not overload the generator. It will have enough power to easily damage itself if not properly protected.
Bill
--------------------
"Why not the best?"
Jimmy Carter
Dear turkoman.
You have an 1000kVA and cosf = 0.8 (Pg=800kW, Qg=600kVAr), ofcourse, and you want to get more power than 800kW output energy because of your loads were over, I think. Is that correct?
If that, your generator will be heated as you know ('cause U R electrical engineer
). To increase the kW output, you must decrease the kVAr output because of the generator's ratings.
1. For example, the load is 900kW and cosf=0.8 hence Q>600kVAr, you have to consider the rating of prime mover, if should, increase the prime mover speed. If not, you should increase the excitation current of the generator.
2. If the load has cosf <> 0.8, you should have manufacture's guide for reference.
You have an 1000kVA and cosf = 0.8 (Pg=800kW, Qg=600kVAr), ofcourse, and you want to get more power than 800kW output energy because of your loads were over, I think. Is that correct?
If that, your generator will be heated as you know ('cause U R electrical engineer
). To increase the kW output, you must decrease the kVAr output because of the generator's ratings. 1. For example, the load is 900kW and cosf=0.8 hence Q>600kVAr, you have to consider the rating of prime mover, if should, increase the prime mover speed. If not, you should increase the excitation current of the generator.
2. If the load has cosf <> 0.8, you should have manufacture's guide for reference.
- Thread starter
- #9
thank you my friends i am sure that you all are good engineers but what i ask is not a theory but a simple question, just tell me pls if i have a 1000 kVA diesel-generator on the nameplate of which writes cos phi = 0,8 THE MOTOR SHAFT which drive the alternator is meant to have a 800 KW output ?..
electricpete
Electrical
I don't know a lot about generator ratings and I didn't read the earlier responses. But one problem I see is that generator efficiency wouldn't be accounted for. If generator output is 1000kva at 0.8pf = 800kw, then generator input power needs to be a little higher. Right?
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electricpete
Electrical
By the way, this is a "motor generator" application ?
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- Thread starter
- #13
Skogsgurra
Electrical
I learned in these fora that there is no such thing as a "diesel motor". To the English speaking world, it is a "diesel engine".
None of us can guess what your generator can deliver under all circumstances. You really have to consult specs for the diesel engine (for real kW) and the capability curve for the generator itself.
What you consider "theories" are plain facts that are valid also for your generator. But real limits are only found in the generator documentation.
The specific case with 800 kW output and 1000 kvar needs 800/efficiency kW from the diesel engine - as Pete and others have pointed out.
Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
None of us can guess what your generator can deliver under all circumstances. You really have to consult specs for the diesel engine (for real kW) and the capability curve for the generator itself.
What you consider "theories" are plain facts that are valid also for your generator. But real limits are only found in the generator documentation.
The specific case with 800 kW output and 1000 kvar needs 800/efficiency kW from the diesel engine - as Pete and others have pointed out.
Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
I agree with electricpete. If we are talking about one integral device (diesel electrical aggregate), then, according to the nameplate, generator is capable to deliver 800kW electrical. Therefore, because of losses, motor shaft must have more mechanical power than 800kW. How much exactly, the manufacturer should know, directly, or by generator efficiency. What is the purpose? You want to disintegrate the aggregate and use motor for something else?
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