P
Post Scope.
A number of postings related to electrical power generation contain misunderstood concepts, expressions, and definitions as related to generated power. My goal, for those members of this forum (especially newbies) that choose to accept, is to provide some enlightenment, without malice, without scorn, without prejudice! And, for those who don’t accept my goal, so be it!
Who cares as long as you’re happy!
Background.
Non-electrical engineers and technicians are caught on the horns of a dilemma! They are/were taught that power is defined as the rate of doing work. Since work is defined as force x distance, then power = force times distance divided by time. In other words power is something tangible, something measurable, something that is useful. For example: Hp; BTU/minute; lb(f)-ft/second; kg(f)-m/sec; Watts; etc. The dilemma is that when talking about synchronous generators conflicting terms have arisen. First, there is apparent-power referred to as Volt-Amperes (VA). Second, there is active-power called Watts (W). And last, there is reactive-power called Volt-Amps-reactive (VAr.) Confusion arises because neither the first expression VA, nor the third VAr, meets the physics definition of power!
History (at least how I remember it.)
In the early days of DC power generation it was easy to determine power - simply the product of Volts and Amps. Then, AC power brought with it terms like inductive reactance, capacitive reactance, amperes-in, amperes-out, lagging-current, leading-current, and power-factor! All of which muddied the waters of understanding. Non-electrical engineers struggled (some EEs still do!) Eventually, as generators were inter-connected to form networks it was realized that system improvement could be effected by manipulating a generator’s excitation, so that some of the networks’ lagging-current could be negated! Ah-ha, said the bean counters, “Perhaps we can sell power-factor improvement!” Still understanding faltered! This led to the adoption of the expression, Watt-less power. Now, a generating company could sell both Watts and Watt-less power! (Of course, only those-in-the-know, knew the latter was an oxy-moron!) Just imagine the confusion when the un-initiated were told, “Although
Watt-less, it still causes losses!” Eventually, the latter term morphed into the adumbrated entity called reactive-power! It was alive! It could even be measured by meter! More importantly, it eliminated “loss-talk!” And, if measurable it could be priced, and then sold!
Apparent-power, Active-power, and Reactive-power Relationship.
What then, is the difference between apparent-, active-, and reactive-power? Of course, most of you with a mathematical bent understand that voltage and current waves are sinusoidal in form, having like-frequencies but unlike-amplitudes. The use of vectors (those of you bothered by the term can use the newer term, phasors) was introduced to explain it. It is nothing more than a mathematical-artifice representing the time-relationship between corresponding points on the voltage and current waves, as follows:
S (apparent power) = |V| x |A|.
W (active-power) = |V| x |A| x Cos(f), with (f) the time-offset (in deg) between V & I, the power-factor angle.
VAr (reactive-power)= |V| x |A| x Sin (f).
The Closing.
I believe the problem many of the forum engineers, technicians, and others have is one of semantics! The watt-less term applies only to the reactive-element, i.e., inductor or capacitor in the circuit, not the source or supply. Also, for purposes of simplicity let’s ignore non-linear loads. Following is the introduction to the white paper on Armature Reaction I presented in Jan ’07. It lists terms or phases to describe “Reactive-Power” as well as my "hapless" goal to curtail the continued use of inconsistent phases:
“Adjectives that described Reactive-Power are plentiful, some even inventive, but most miss the point! Here are some pairs that were culled from A-List and Off-List responses: adds-subtracts; additive-subtractive; absorbs-produces; augments-negates; crowds-expands; decreases-increases; flows-in; flows-out; overcomes-replaces; overtakes-fights; magnetizes-demagnetizes; supports-opposes; strengthens-weakens; and swells-shrinks. There have been and certainly will be others! Thus far, no-one has used adjectives such as: encourage; discourage; thwart; or tweak! I hope this paper will curtail (hmm, a synonym I hadn’t noticed earlier) the seemingly growing list of adjectives.”
My final point is this - call the “various powers” whatever you want to if it works for you! But, always remember that a generator supplies only two quantities, volts, and amps! Whether a generator’s current is resistive, reactive, or some combination of the two, is determined by the phase-displacement of the generator’s line-current, relative to the generator’s terminal-voltage. Just remember it’s the story of the bear and the wall!
Regards,
Phil Corso, PE ([email protected])
A number of postings related to electrical power generation contain misunderstood concepts, expressions, and definitions as related to generated power. My goal, for those members of this forum (especially newbies) that choose to accept, is to provide some enlightenment, without malice, without scorn, without prejudice! And, for those who don’t accept my goal, so be it!
Who cares as long as you’re happy!
Background.
Non-electrical engineers and technicians are caught on the horns of a dilemma! They are/were taught that power is defined as the rate of doing work. Since work is defined as force x distance, then power = force times distance divided by time. In other words power is something tangible, something measurable, something that is useful. For example: Hp; BTU/minute; lb(f)-ft/second; kg(f)-m/sec; Watts; etc. The dilemma is that when talking about synchronous generators conflicting terms have arisen. First, there is apparent-power referred to as Volt-Amperes (VA). Second, there is active-power called Watts (W). And last, there is reactive-power called Volt-Amps-reactive (VAr.) Confusion arises because neither the first expression VA, nor the third VAr, meets the physics definition of power!
History (at least how I remember it.)
In the early days of DC power generation it was easy to determine power - simply the product of Volts and Amps. Then, AC power brought with it terms like inductive reactance, capacitive reactance, amperes-in, amperes-out, lagging-current, leading-current, and power-factor! All of which muddied the waters of understanding. Non-electrical engineers struggled (some EEs still do!) Eventually, as generators were inter-connected to form networks it was realized that system improvement could be effected by manipulating a generator’s excitation, so that some of the networks’ lagging-current could be negated! Ah-ha, said the bean counters, “Perhaps we can sell power-factor improvement!” Still understanding faltered! This led to the adoption of the expression, Watt-less power. Now, a generating company could sell both Watts and Watt-less power! (Of course, only those-in-the-know, knew the latter was an oxy-moron!) Just imagine the confusion when the un-initiated were told, “Although
Watt-less, it still causes losses!” Eventually, the latter term morphed into the adumbrated entity called reactive-power! It was alive! It could even be measured by meter! More importantly, it eliminated “loss-talk!” And, if measurable it could be priced, and then sold!
Apparent-power, Active-power, and Reactive-power Relationship.
What then, is the difference between apparent-, active-, and reactive-power? Of course, most of you with a mathematical bent understand that voltage and current waves are sinusoidal in form, having like-frequencies but unlike-amplitudes. The use of vectors (those of you bothered by the term can use the newer term, phasors) was introduced to explain it. It is nothing more than a mathematical-artifice representing the time-relationship between corresponding points on the voltage and current waves, as follows:
S (apparent power) = |V| x |A|.
W (active-power) = |V| x |A| x Cos(f), with (f) the time-offset (in deg) between V & I, the power-factor angle.
VAr (reactive-power)= |V| x |A| x Sin (f).
The Closing.
I believe the problem many of the forum engineers, technicians, and others have is one of semantics! The watt-less term applies only to the reactive-element, i.e., inductor or capacitor in the circuit, not the source or supply. Also, for purposes of simplicity let’s ignore non-linear loads. Following is the introduction to the white paper on Armature Reaction I presented in Jan ’07. It lists terms or phases to describe “Reactive-Power” as well as my "hapless" goal to curtail the continued use of inconsistent phases:
“Adjectives that described Reactive-Power are plentiful, some even inventive, but most miss the point! Here are some pairs that were culled from A-List and Off-List responses: adds-subtracts; additive-subtractive; absorbs-produces; augments-negates; crowds-expands; decreases-increases; flows-in; flows-out; overcomes-replaces; overtakes-fights; magnetizes-demagnetizes; supports-opposes; strengthens-weakens; and swells-shrinks. There have been and certainly will be others! Thus far, no-one has used adjectives such as: encourage; discourage; thwart; or tweak! I hope this paper will curtail (hmm, a synonym I hadn’t noticed earlier) the seemingly growing list of adjectives.”
My final point is this - call the “various powers” whatever you want to if it works for you! But, always remember that a generator supplies only two quantities, volts, and amps! Whether a generator’s current is resistive, reactive, or some combination of the two, is determined by the phase-displacement of the generator’s line-current, relative to the generator’s terminal-voltage. Just remember it’s the story of the bear and the wall!
Regards,
Phil Corso, PE ([email protected])