Power factor data listed on generator nameplate 2

[gtwy]

I do not have any experience with generators or calculating generator loads, but I just happened to notice the other day on a portable 3 phase generator nameplate a power factor. There was obviously a KVA rating but I don't remember if there was also a KW rating. How does this power factor relate to the output and or how it affects a load or type of load i.e. inductive, resistive etc?

 

[rbulsara]

pf=kw/kva

There would be either a kW or power factor rating on the gen name plate. You should not be exceeding either of the kW or kVA rating (in simpler terms do not exceed the rated current). The actual pf is the function of type of loads. You can have any inductive as long as you do not exceed above limits. Also the power factor implied is lagging (inductive) in your case.

For more, you need to brush up on the basic electrical power system by reading some good books or taking courses.

 

[gtwy]

Thanks for the response- I can see how the KW rating could be violated but not the KVA, w/ different power factors.

 

[alehman]

If you exceed the kW rating you will overload the engine. If you exceed the kVA rating you will overload the generator.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[gtwy]

Thank you for the responses. I just happened to be standing near this particular gen. and thought I'd read the nameplate for fun. The gen. is long gone, but I think now that I recall seeing the p.f., kva and kw listed. Knowing any two yields the other, so I was wondering if there was something more to it.

 

[cranky108]

There are two factors that make up the kVA and power factor rating on a generator. Both are limits, where the two meet is the powerfactor, at the rated kVA.
The two factors are the field limit, and the thermal limit of the machine.
You can get more power from the generator, but you have to have a better power factor, if the prime mover can handle it.
You can get more vars from the machine, but you must reduce the power output.

 

[waross]

The two limits on a generator are heating of the windings and the power of the prime mover.
The heating is determined by the current. There is a maximum safe current. When this current is multiplied by the rated voltage, you get the KVA. If the voltage on a generator is turned down, say from 240V to 208V the new KVA will now be the new voltage times the maximum allowable current which will not change.
The kW is a reflection of the power of the prime mover.
Most diesel generators are rated at 80% power factor to accommodate loads of varying power factor.
You should not exceed the rated current. On some machines you have to look at the voltage and the KVA on the nameplate and do the math yourself.
The other issue is kW. loading too much kW on a machine may have several results.
Some machines will over load the engine and start to slow down with just a small kW overload.
Some machines will handle some percent of overload, but the engine will overheat.
Some machines are overpowered and will handle a kW overload with no ill effects as long as the current (KVA) is at or below the rating. (This may arise if the load has a power factor above the machine's rated power factor.)

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

 

[gtwy]

Good info- I see why all the data was on the plate. I've spent the last 25 yrs in low power d.c.instrumentation where p.f. doesn't come up much. If all the loads were resistive then, the KW=KVA. The current supplied by the machine would be limited by the KW rating, even though the KVA rating is higher?

 

[gtwy]

Or i should have stated the closer the load is to unity power factor, the closer the generator limits shift to the KW rating.

 

[alehman]

Correct.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[waross]

If the set is rated for prime power, it will be allowed to output 110% of kW typically for one hour out of 12.
The same set rated as standby will typically be rated 10% higher than a similar prime rated set and no overloads are permitted.
That is, a prime rated set rated for 100 Kw will be allowed to output 110 kW one hour out of 12.
A similar set rated for standby will be rated at 110 kW with no overload permitted.
Although the prime and standby sets may have the same engine and alternator, the prime rated set may have more options such as an oil cooler, a larger oil sump, and/or a larger radiator.


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

 

[edison123]

How come I haven't seen the beer & suds analogy here yet ?

 

[rmw]

Where is Skogsgurra when you need him? Is he a beer and suds type of guy?

rmw

 

[ScottyUK]

Jeez, don't start that ball rolling! How come no one has posted a link to a capability chart? That shows everything you need to know about what the machine can do under steady-state conditions. Here is one of the first ones I found - any half-decent machines text will have some explanation of the chart. No beer, no suds!




----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[rmw]

And still no Skogs.

rmw

 

[waross]

Nice link, Scotty, thanks.
Skogs combines excellent technical knowledge with a lifetime of hands on field experience.
He may be blowing the foam off the beer in preparation for submitting a "Beer and Suds" post.
I'll give it a shot.
The cold glass represents the thermal capacity of the system.
The beer represents the energy.
I forget what the foam represents.
Blow off the foam and drink the beer.
Repeat,
"The cold glass repres.....
Repeat until you forget the question.
PS; For the greatest accuracy the foam must be blown at exactly right angles to the beer. However the glass may be at an angle "phase angle" to the beer and the foam.
If you understand the concept of power factor, you may understand the analogy.
If you don't understand power factor, now's the time to take action:
First blow the foam off the beer.
Drink the beer.
Repeat as needed.

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

 

[busbar]

I guess I learned another form of beer and suds, with the glass being kVA, the liquid beer as kW, and the foam being kVAR. [Campus security did not like bringing it with me to a basic electrical principles class ~18 years ago. My students didn’t like me pouring it out after the demonstration.]