Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
THE PHYSICS of… POWER
Introduction
A number of posts 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 some technicians are caught on the horns of a dilemma! In physics class 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, and something useful. For example: Hp; BTU/minute; lb(f)-ft per second; kg(f)-m per second; 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 became the rage. 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 instruments! More importantly, it eliminated “loss-talk!” But, more importantly, 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 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(ø)
Ø, Time-offset between V & I, the power-factor angle (in deg.).
VAr, Reactive-pwr = |V| x |A| x Sin (ø).
The Closing.
I believe the problem many 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. The following paragraph is the introduction to the “Armature Reaction” white paper I presented in Jan’07 (available on request). It lists terms or phrases used to describe “Reactive-Power” as well as my "hapless" goal to curtail use of inconsistent phrases:
THE PHYSICS of… ARMATURE REACTION
Adjectives describing Reactive-Power are plentiful, some even inventive, but most miss the point! There are some pairs 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 thought of earlier) the seemingly growing list of adjectives.”
FINAL POINT
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 electrical quantities, Volts and Amperes! 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” all over again!
BTW, interested in a one paragraph explanation of PF?
Regards, Phil Corso
Introduction
A number of posts 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 some technicians are caught on the horns of a dilemma! In physics class 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, and something useful. For example: Hp; BTU/minute; lb(f)-ft per second; kg(f)-m per second; 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 became the rage. 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 instruments! More importantly, it eliminated “loss-talk!” But, more importantly, 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 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(ø)
Ø, Time-offset between V & I, the power-factor angle (in deg.).
VAr, Reactive-pwr = |V| x |A| x Sin (ø).
The Closing.
I believe the problem many 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. The following paragraph is the introduction to the “Armature Reaction” white paper I presented in Jan’07 (available on request). It lists terms or phrases used to describe “Reactive-Power” as well as my "hapless" goal to curtail use of inconsistent phrases:
THE PHYSICS of… ARMATURE REACTION
Adjectives describing Reactive-Power are plentiful, some even inventive, but most miss the point! There are some pairs 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 thought of earlier) the seemingly growing list of adjectives.”
FINAL POINT
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 electrical quantities, Volts and Amperes! 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” all over again!
BTW, interested in a one paragraph explanation of PF?
Regards, Phil Corso
