KVA, KW and PF of a Generator 2

[Sergejs]

Why do we need to specify PF of a generator? I mean if we indicate kW then we should indicate the worst power factor so that not to exceed the total current generator can handle. But if we specify kVA, PF shouldn't be important (I mean it will be important when we start looking at the load, but it will depend on the load, generator maximum allowed current can be calculated knowing only kVA)

Please tell me if I don't understand something.

 

[catserveng]

There was a recent thread discussing this, here are a couple of links that may be helpful in understanding the issue,

http://www.frontierpower.com/library/generatorratings.htm

http://pdf.cat.com/cda/files/4252018/7/demystifying

https://www.cumminsgeneratortechnologies.com/

http://www/en/download/ratingsbook/RatingsBook.pdf

Hope that helps, Mike L.

 

[waross]

KVA is derived from the Ampere rating of the generator and the voltage. If the voltage is changed, for example from 208 Volts to 240 volts then the KVA rating will change. The kW rating describes the prime movers ability to produce power.
Gen-sets are typically supplied with generator ends of 25% greater cpacity than the prime mover.
eg: An 80 kW prime mover will typically drive a 100 KVA generator end.
Hence the ratings:
100 KVA
80 kW
PF 80%
If your expected power factor will be above 80% then size the set by KW.
If your expected power factor will be 80% or below then size the set by KVA.
Don't worry about specifying a power factor. The set will be rated at 80% PF.
If your ordering be aware:
A Prime rated set will allow a 10% overload for one hour in twelve or twenty four.
A standby set will not allow any overload.
Although a prime set may have more accessories than a standby set, a 100 KVA prime rated set and a 110 KVA standby set will often be the same basic engine and generator end.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[proelect8717]

dear waroos,

Thank you for your comment. I want to clarfiy one thing you mention, you say if voltage is increased from 208V to 240V, the kVA will change, I agree. But impact it will have on alternator current carrying capacity? as current carrying capacity is determined by size of winding conductor & cooling. What will happen if I run an alternator at rated current on rated voltage (say 208V) and what if at rated current on higher voltage than rated (say 240V)?

 

[waross]

The current rating doesn't change.
The nominal 120 Volt windings can normally be operated up to over 138 Volts. This allows series operation at 277 Volts for 277/480 Volt wye generation.
If you need more than rated KVA out of an existing 120/208 Volt generator you can push the voltage up to 138 or 140 Volts and then use auto transformers to drop the voltage back down to 120/208 volts.
This will gain almost 17% more KVA. You may run out of kW, depending on your power factor and the actual power of the prime mover rather than the rated power. There may often be some excess power in the prime mover of a 120/208 Volt generator. The extra power may be sitting there waiting to be used or you may have to get a diesel mechanic to adjust the dead-rack setting.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[catserveng]

Bill,

Your patience in explaining this over and over in many threads is amazing, I appreciate the time you take in trying to make this understandable for those not "getting it"

LPS for you, Mike L.

 

[waross]

Thanks Mike. Have a beer on me, after you blow the foam off. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Sergejs]

Dear waross,

I should have indicated that this question came up when I was specifying synchronous generator in SKM power tools. I've put voltage as 690V and got the following results:

1) 100kVA 0.8pf = 83.7A
2) 100kVA 1pf (or any pf) = 83.7A
3) 80kW 0.8pf = 83.7A
4) 80kW 1pf = 66.9A

I do understand that generators kW will be <= to prime movers kW since we cant generate more real power than real power input. But why does SKM let you specify p.f. (if you put in kVA before) that does not change anything?

Sergejs

 

[waross]

Although the 80% PF is almost universal for generator ratings above about 15 KVA, as generator size drops down from 15 KVA, generator set rating at 100% power factor becomes more common.
Also, you may want to specify the actual power factor of the load rather than the rated power factor.
Remember, the load determines the actual power factor. A generators rated power factor describes the ratio between the KVA rating and the kW rating.

Bill
--------------------
"Why not the best?"
Jimmy Carter