Relationship between Generator specified power factor and effeciency
Relationship between Generator specified power factor and effeciency
Almost all generators above about 10 KVA will be rated at 0.8pF.
So many loads are inductive that the overall power factor will be less than unity. It is common practice to size the prime mover in kW at 80% of the KVA rating of the generator. So if your alternator is rated for 100 KVA the prime mover will be rated at 80 mechanical kW, (plus a little for losses).
The load determines the power factor. It is what it is. Are you concerned with alternator efficiency, engine efficiency, overall efficiency of converting fuel into Watt-hours, or some other figure?
Loading has a lot of effect on efficiency, power factor little if any, depending on the conditions.
Consider a generator supplying a single family dwelling. During the evening when there is no load but the generator must run regardless, the overall efficiency and the electrical efficiency will be zero. Fuel in, no energy out.
With a small load the electrical efficiency will improve and may be quite good depending on the excitation demands relative to the load but the fuel to energy efficiency (which determines the operating cost) may still be very low. It could be 20%, 10% or less depending on the load.
So, what do you want to know?
So many loads are inductive that the overall power factor will be less than unity. It is common practice to size the prime mover in kW at 80% of the KVA rating of the generator. So if your alternator is rated for 100 KVA the prime mover will be rated at 80 mechanical kW, (plus a little for losses).
The load determines the power factor. It is what it is. Are you concerned with alternator efficiency, engine efficiency, overall efficiency of converting fuel into Watt-hours, or some other figure?
Loading has a lot of effect on efficiency, power factor little if any, depending on the conditions.
Consider a generator supplying a single family dwelling. During the evening when there is no load but the generator must run regardless, the overall efficiency and the electrical efficiency will be zero. Fuel in, no energy out.
With a small load the electrical efficiency will improve and may be quite good depending on the excitation demands relative to the load but the fuel to energy efficiency (which determines the operating cost) may still be very low. It could be 20%, 10% or less depending on the load.
So, what do you want to know?
Bill
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"Why not the best?"
Jimmy Carter






RE: Relationship between Generator specified power factor and effeciency
The generator does not have a power factor in itself. It is the load's power factor that decides.
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Relationship between Generator specified power factor and effeciency
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If we learn from our mistakes I'm getting a great education!
RE: Relationship between Generator specified power factor and effeciency
RE: Relationship between Generator specified power factor and effeciency
RE: Relationship between Generator specified power factor and effeciency
Yours Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Relationship between Generator specified power factor and effeciency
Reduced losses in the rotor I can understand, but lower I2R losses in the rotor is not intuitive to me. Why does this happen?
RE: Relationship between Generator specified power factor and effeciency
Conversely, a lagging power factor requires greater excitation, the AVR increases its output and the rotor losses increase.
Don't forget, however, that the efficiency of the diesel engine in a diesel set will change at different loadings and the overall efficiency change may, at times, overshadow the electrical efficiency changes.
Hang around generators for awhile and this may become intuitive also.
Bill
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"Why not the best?"
Jimmy Carter
RE: Relationship between Generator specified power factor and effeciency
I'm also wondering whether we're all starting to talk about leading vs lagging power factor from different perspectives? Operating a generator at a leading power factor implies reduced excitation, and will tend to depress the system voltage, not drive it up.
Or maybe I just haven't had enough sleep this week!
RE: Relationship between Generator specified power factor and effeciency
The same KVA load at a higher PF will make little difference to the stator losses as the current and the I2R losses will remain the same.
Rotor: Field: Anything which requires more field current will cause more I2R losses in the rotor.
Power factor, importing and exporting VARs. In an islanded set, the load sets the power factor.
In a networked set, there may be several reasons to run a set at other than the load PF. VARs may be exported to increase and control the voltage with a relatively small station at the end of a long transmission line. There are occasions when the utility will buy VARHrs.
But any condition that causes a reduction in excitation current in the rotor will result in reduced I2R losses in the rotor and improved efficiency.
In regards to the efficiency graph posted by Cuky, this graph may or may not include the exciting losses. At those efficiencies it may not include excitation. I may be wrong.
And, the result of an increase of PF from 0.8 to unity improves the efficiency by 1%.
To put it into perspective, while we are discussing 1% improvement in the stator and possible much less in the rotor, winter comes and we switch to winter diesel fuel. An instant 10% drop in overall efficiency. (KWHrs per Gallon of fuel.)
Bill
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"Why not the best?"
Jimmy Carter
RE: Relationship between Generator specified power factor and effeciency
RE: Relationship between Generator specified power factor and effeciency
I have seen exceptions with diesel generators in the under 1 Mega-Watt range.
These were prime power sets going in to remote areas. The engine was oversized by 25% so that rebuild intervals could be extended. The theory was that with extra power available, the engine could continue in service with valve seals leaking and blow-by past the rings for a year or so more before a rebuild and still have enough power to supply full power to the alternator. When these sets were new they could supply full rated KVA at unity power factor.
I worked on one install and in the course of that saw the specs for several others. I wasn't around long enough to find out if the theory played out in practice.
Bill
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"Why not the best?"
Jimmy Carter