kW vs. kVA and power factor 1

[jfriddell]

EE's:

I'm an agricultural engineer that is working on the sizing of generators for center pivots (motor starting applications). I'm a little confused about kW vs. kVA and the power factor. Is the power factor value something that is associated with the motors, the generator, or both? Should the kVA value or the kW value be used to size the driver for the generator? I'm familiar with the 3-phase equations such as:
kVA = V*1.73*I/1000 and kW = kVA * PF
I'm just unsure when to use kW or when to use kVA, etc.

Also, is there a general rule of thumb about how efficient generators are in converting the mechanical energy into electrical energy? I would imagine this varies by the degree of load on the generator.

Thanks for any help.

Jeremiah

 

[rbulsara]

Prime mover only supplies kW, the real power.
Refer to FAQ faq237-766, posted by me.

As for efficieny, I have seen, for the engine rated 1400HP a 1000kVA generator or 2800HP engine for 2000kW genset or in that range.

 

[jbartos]

Suggestion to jfriddell (Agricultural) Nov 17, 2003 marked ///\\I'm a little confused about kW vs. kVA and the power factor. Is the power factor value something that is associated with the motors, the generator, or both?
///With both. In fact many other loads and sources exhibit power factor, e.g. capacitors, inductors and combinations with resistors are basic electrical passive elements. Power supplies, electronic devices, etc. are another categories.\\ Should the kVA value or the kW value be used to size the driver for the generator?
///Essentially, kWs should be used.\\ I'm familiar with the 3-phase equations such as:
kVA = V*1.73*I/1000 and kW = kVA * PF
I'm just unsure when to use kW or when to use kVA, etc.
///kWs can be used when the active power in Watts or energy in kWhr are to be results or results related. kVAs and kVAr are often used when the electricity or devices are dealt with generally, or when power factor is close to the unity. Also, the harmonic content, THD, somewhat changing the equation that you are using, which deals with pure sinusoidal waves, no harmonic distortions.\\\
Also, is there a general rule of thumb about how efficient generators are in converting the mechanical energy into electrical energy?
///One thing is the generator efficiency and another thing is gen-set efficiency. The generators efficiency increases with the generator kW or kVA rating. The larger machines may be as much as 98% to 99% efficient, 100MVA to 1000MVA in size, respectively.\\ I would imagine this varies by the degree of load on the generator.
///Yes. The efficiency versus generator load is an monotoniously increasing curve. Visit

http://www.kato-eng.com/oveview.pdf

http://www.energ.polimi.it/dipartimento/Schedario/campanari/ASME99aTST.pdf

for: Figure 4
etc. for more info\\\

 

[alehman]

Basically kW should be used to select your prime mover, but other factors such as motor starting performance, ability to sustain fault current and frequency regulation enter into the prime mover sizing process.

Full-load generator efficiency ranges from 90% to 98%, depending on size (and a lot of other thigns).

 

[60hzhmmm]

jfriddel: In my limited experience there is more to this than efficency. I have never seen an engine used at it's "rated" horsepower as a prime mover for a generator. I believe that the usual engine ratings are peak momentary output. (typical specsmanship) If you were to run at that power level the engine would fail quickly. What I'm saying is the generator spec calls for X horsepower for it's expected output this includes efficency. Then you look at the engine spec for maximum "continous" horsepower. This needs to be higher than the generators requirement.
Then as alehman mentioned response and frequency regulation may require more horsepower yet.

 

[mikhe]

Your first port of call is to understand your load (motor/pump). The motor will have a kW rating and a starting load. The starting load is dependant on the type of starter you are using, for example if it is Direct On Line you will get a very high starting current, where-as soft starters limit this. Your supplier of the motor or pump will be glad to supply any data and graphs that you may require. Once you have determined what you are trying to support with the genset you can quantify your genset load requirement.
You don’t have to spec the genset at the greatest step load as most manufacturers design their equipment to handle short overload conditions.
Go to newage-avkseg.com web site and you will be able to download a selection of generator data sheets with their associated load capabilities.
You then need to consider the gensets block load acceptance capability and this is determined by the engines ability to withstand a sudden load without stalling. Again the only people that can give you this information are the manufacturers of your chosen engine, and they will be glad to help.
Careful consideration will save you lots of money, either over specing or under specing will greatly increase your cost of ownership