That's not right.
Take a specific example of a 100kVA generator with a 0.8pf.
Power is thus 80kW.
Or (80/0.746)HP
About 107HP
Your line 3 equation is incorrect.
generator output measurement
- Thread starter srinivasan
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That's not right.
Take a specific example of a 100kVA generator with a 0.8pf.
Power is thus 80kW.
Or (80/0.746)HP
About 107HP
Your line 3 equation is incorrect.
He multiplied, you divided ??
Somewhere in the options I changed it to show more posts per page. I get this entire thread (so far) on one page.
Indeed. One kW is more that one HP.
Not quite.
1hp=(0.8/0.746)*kVA =1.07kVA.
In your version, if the pf got worse, you'd need more hp.
You need to remove '1' in '1hp=(0.8/0.746)*kVA =1.07kVA' and simply state
hp=(0.8/0.746)*kVA =1.07kVA. Otherwise there is conflict with the definition of
1hp=(0.746/0.8)*KVA=0.9325*KVA. as given in my previous post.
kwired
Electron manager
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Why does everyone's formulas keep having a .80 factor in them. I am assuming this is a power factor. If we were talking about input power to a motor it makes sense (if the motor is 80% PF). I don't get why you all keep using it in this instance.
If we have a 5kW rated generator the output rating should be 5kW.
To convert to Hp divide by 746, and no additional factors included for loads with 100% power factor.
If you want to know if a generator will run a specific motor, you need to know the power factor of that motor, not all are 80%.
And this is all disregarding whether or not the generator has capability of across the line starting of a motor with a rating near the generator rating, or even using power factor correction to bring PF up to or near 100%
Am I missing something here?
Maybe OP is asking a general rule of thumb for how big of a motor can a specific generator run? Otherwise as worded I think it is just asking if a generator can be specified using Hp instead of kW. With no other details how can we suggest anything other then 1 Hp = .746 kW?
If we have a 5kW rated generator the output rating should be 5kW.
To convert to Hp divide by 746, and no additional factors included for loads with 100% power factor.
If you want to know if a generator will run a specific motor, you need to know the power factor of that motor, not all are 80%.
And this is all disregarding whether or not the generator has capability of across the line starting of a motor with a rating near the generator rating, or even using power factor correction to bring PF up to or near 100%
Am I missing something here?
Maybe OP is asking a general rule of thumb for how big of a motor can a specific generator run? Otherwise as worded I think it is just asking if a generator can be specified using Hp instead of kW. With no other details how can we suggest anything other then 1 Hp = .746 kW?
Sorry, you do. First please note that 0.8 factor here is the design power factor of the alternator. It is a conversion factor for alternator maximum output power in kW, given gen set power in KVA. The gen set of ordinary design can not be run at KVA=kW, due to limitation imposed by its prime mover i.e the diesel engine.
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kwired
Electron manager
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- NE Nebraska
Understandable if converting between kVA and kW. That was not brought up by the OP, it was simply a question of using HP instead of kW, both are true power units of measurement.
I disagree, at least if the prime mover and the alternator are a combined unit anyway. The output rating is going to be the maximum load that can be connected without overloading anything. Even if the alternator or the prime mover is capable of delivering more power, together they have one output rating. If that rating is 7460 watts then it is also 10 hp. Will same generator set have trouble with 7460 watts of resistance load vs an 80% PF inductive load - yes.
If OP is asking how big of a motor in HP can he run with a specific size genset, that is not same as asking how much HP can the same genset deliver.
No. The OP asked about the rating of gen set in hp, which is expressed in KVA.
If the rating of the gen set is expressed in kW, then the factor 0.8 has no role to play in arriving at the hp rating for the gen set.
The OP did not ask that.
kwired
Electron manager
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- NE Nebraska
No. The OP asked about the rating of gen set in hp, which is expressed in KVA.
If the rating of the gen set is expressed in kW, then the factor 0.8 has no role to play in arriving at the hp rating for the gen set.
The OP did not ask that.
OK, I did read the OP again, he was talking KVA instead of KW.
Generators are seldom rated in KVA so this is where the confusion on my part set in. So question for OP may be where did his KVA value come from, since the generator likely is rated in KW? To come up with KVA the power factor needs to be known.
Even though the gen sets are rated in kW, the design power factor of its alternator is always 0.8 so that its size in KVA =kW/0.8
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In my experience generators are rated in kVA as a rule.
For the majority of applications, they are fixed output voltage and, like most electrical machines, can handle up to a particular rated current at any power factor. The maximum output power may be limited by rated generator current or the maximum power of the prime mover.
JoeStillman
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In this discussion, the kVA and power factor are a distraction. Horsepower is the same thing as Watts - energy per unit tiime. If you want too know how much brake horsepower a genset has, you divide its kW rating by 0.746 kW/HP.
I always think of a "genset" as a set containing two basic elements, each with a separate rating; 1) the prime mover, rated in kW 2) the alternator, rated in kVA. The kW rating tells you how much real work the prime mover can do. The kVA rating is more of a measure of the thermal limits of the alternator (i.e. amps at the rated voltage.) The power factor rating of an alternator has more to do with the alternators ability to regulate voltage. Of more important concern is the power factor of the load.
If you want to talk about the total electric motor horsepower that can be connected to a genset, you have a lot more number crunching to do than just adding up brake horsepower. I'm not sure the art of generator sizing was the subject of the OP.
I always think of a "genset" as a set containing two basic elements, each with a separate rating; 1) the prime mover, rated in kW 2) the alternator, rated in kVA. The kW rating tells you how much real work the prime mover can do. The kVA rating is more of a measure of the thermal limits of the alternator (i.e. amps at the rated voltage.) The power factor rating of an alternator has more to do with the alternators ability to regulate voltage. Of more important concern is the power factor of the load.
If you want to talk about the total electric motor horsepower that can be connected to a genset, you have a lot more number crunching to do than just adding up brake horsepower. I'm not sure the art of generator sizing was the subject of the OP.
kwired
Electron manager
- Location
- NE Nebraska
Mostly my thoughts also. OP wasn't all that clear what he was asking about either. HP and KW are a measurment of same thing just in different standard units. OP wants to state the output rating of the generator in HP for some reason. Does he want to state the rated output power in HP or does he want to state what HP value of a motor it will run. A 1 HP rated motor will deliver 1 HP of power at the output shaft, but will require more than 1HP of input power, because it is not 100% efficient.
srinivasan
Member
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- Bangalore,India
SORRY for the long back. i just want to state the rated output of the generator in hp .. instead of kva.. actually we have 250kva , 415 v gen set.. i just want to state this in the unit of hp.. if we dont know the efficiency ....
srinivasan
Member
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- Bangalore,India
kwired
Electron manager
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- NE Nebraska
JoeStillman
Senior Member
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- West Chester, PA
It would be an easy calculation if we knew the kW rating of the engine. Since its for non-technical auditing purposes, it probably won't hurrt to make an assumptiion (something I would avoid otherwise.)
Most genset mfgrs I've seen publish their ratings according to kW, with 0.8 power factor. If we assume your 250 kVA genset is typical, then the kW rating would be 250 kVA x 0.8 = 200 kW. So the brake horsepower of the prime mover would be 200 kW/0.746 kW-per-HP = 268 HP.
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