Power factor was originally understood and defined by the power triangle, where PF = W/VA. Back in the day where nonlinear loads were a fraction of linear loads, the net result was that the current (drawn by load) was a replica of the voltage (fixed sinusoid as generated by the source). So current was a 60 Hz sinusoid with very little harmonic content.
Therefore, you could also assume that the power factor was equal to the cosine of the angle between voltage and current. This is only true in the absence of harmonics, and is an example of how a rule-of-thumb works its way into law. But power factor was never defined to be angle between voltage and current.
Alas, people propagated the misnomer to the point it became understood in this manner, so the industry created a new type of power factor to satisfy tradition. They called it 'displacement power factor', now defined as the cosine of the angle between fundamental voltage and fundamental current. This allowed them to also keep the original definition of power factor (W/VA) and called this the 'true power factor'. Side note: analog power factor meters found on switchboards measure displacement power factor.
Again, in a linear circuit, displacement and true power factors are the same. Nonlinear, not so.
In a traditional power supply where a rectifier feeds a large DC storage capacitor, the fundamental current and voltage are in phase, so displacement power factor is nearly 1.0. But due to the presence of harmonics in the highly peaked current waveform, true power factor is quite poor, maybe 0.6. The reason for this is on a true-rms basis, aggregate real watts are a fraction of aggregate volt-amperes (I use the term aggregate to clarify the true-rms calculation includes all harmonics). Essentially, the higher order harmonics contribute additional losses and VAR flow back and forth between the source and load, so do not result in real power conversion. If your FFT resolved the relative phase angle of each harmonic in addition to their magnitude you would see this.
So the goal of Active Power Factor Correction is to correct true power factor. The only way of doing this is to eliminate the VAR-leaning harmonics, by forcing current to be drawn as a sine wave. It actually does very little to displacement power factor, as it needed no correction in the first place.
Does this help?
Therefore, you could also assume that the power factor was equal to the cosine of the angle between voltage and current. This is only true in the absence of harmonics, and is an example of how a rule-of-thumb works its way into law. But power factor was never defined to be angle between voltage and current.
Alas, people propagated the misnomer to the point it became understood in this manner, so the industry created a new type of power factor to satisfy tradition. They called it 'displacement power factor', now defined as the cosine of the angle between fundamental voltage and fundamental current. This allowed them to also keep the original definition of power factor (W/VA) and called this the 'true power factor'. Side note: analog power factor meters found on switchboards measure displacement power factor.
Again, in a linear circuit, displacement and true power factors are the same. Nonlinear, not so.
In a traditional power supply where a rectifier feeds a large DC storage capacitor, the fundamental current and voltage are in phase, so displacement power factor is nearly 1.0. But due to the presence of harmonics in the highly peaked current waveform, true power factor is quite poor, maybe 0.6. The reason for this is on a true-rms basis, aggregate real watts are a fraction of aggregate volt-amperes (I use the term aggregate to clarify the true-rms calculation includes all harmonics). Essentially, the higher order harmonics contribute additional losses and VAR flow back and forth between the source and load, so do not result in real power conversion. If your FFT resolved the relative phase angle of each harmonic in addition to their magnitude you would see this.
So the goal of Active Power Factor Correction is to correct true power factor. The only way of doing this is to eliminate the VAR-leaning harmonics, by forcing current to be drawn as a sine wave. It actually does very little to displacement power factor, as it needed no correction in the first place.
Does this help?
Not at all. PFC cleans the load up very nicely. This will NOT FIX the distorted power but will not make it worse. If all loads had a PF of 1 there would be not be any distortion.
All 3 PCs here have PFC power supplies with a power factor of .99 and will start and operate down below 70 Volts (120 system). CFLs will eventually be required to have PFC as well though how long we use them is questionable (newer technology).
BTW the last place I worked at had several hundred older VTRs with no PFC and the AC power waveform was WAY worse than the OPs. A plain clamp on ammeter would read 83 AMPs while an RMS meter read 97.
G²
Last edited:
This is just a math representation. In reality, there is not a physical "angle" between voltage and curent. It's just time.
You are wrong. PF is just a delay between voltage and current, has absolutelly nothing to do with distortion. I can have an AC device that has 0.5 PF but alomst no distortion (mostly any small asynchron or a synchron motor with low excitation) and a device that has PF close to 1 with lots of distortion (mostly any fluorescent tube lamp with electronic ballast).
Last edited:
Who used the word 'physical' when discussing angle
In a sense, you are right, but time all by itself is also meaningless to describe lag/lead without specifying frequency. Like being on a merry-go-round, people towards the outside edge have faster speed than people on the inside edge, yet their angular speed is the same. Angle representation is a better way to discuss these issues. The entire power industry speaks in terms of angle, not time, even though they have the luxury of assuming constant frequency. Please do not complicate or confuse this clearly misunderstood subject by being pedantic. Angle is the best way to describe lag/lead behavior, and will certainly be the only hope for people in the audio realm to begin to understand power systems. Once you consider how a generator produces three phase voltage at 120 degrees apart on each leg (regardless of frequency) you begin to see that 'physical angle' is actually not all that far off from the truth. When you realize that a mho distance relay is set in ohms magnitude and phase angle in degrees. When the maximum torque angle of a negative sequence directional element is set in degrees. When a sync check relay is set in degrees. When a transmission line phase angle shifting transformer's taps are described in volts/step and angle of lead/lag. When ANSI/IEEE standards describe transformer HV/LV delta wye phase shift in degrees. When the utility measures VARs by phase shifting the excitation voltage by exactly 90 degrees to a watt-hour meter. Time in seconds is completely irrelevant in these cases of the power system. Time is used to coordinate one device to another (who trips first), never to describe voltage/current relationships.Absolutely correct. It has been mentioned a few times in this thread, and still gets overlooked or misunderstood.
I hope you don't try to explain that to me. I have several degrees in electrical power engineering
Time is the only measurable thing in AC. We choose to raport it to mains frequency just for ease of understanding. Even the devices that measure the "PF" measure time delays.
Anyway, my point was that transients that are shown in this tread don't have nothing to do with mais frequency. PF doesn't have nothing to do with the deformation of the AC sin wave.
If I don't dwell into those "boring" details, people that don't realize what... they don't know and will imagine all kind of fantastic explanations.
"Earth is flat" concept came from that.
Time is the only measurable thing in AC. We choose to raport it to mains frequency just for ease of understanding. Even the devices that measure the "PF" measure time delays.
Anyway, my point was that transients that are shown in this tread don't have nothing to do with mais frequency. PF doesn't have nothing to do with the deformation of the AC sin wave.
If I don't dwell into those "boring" details, people that don't realize what... they don't know and will imagine all kind of fantastic explanations.
"Earth is flat" concept came from that.
Last edited:
They do not, that's what I'm trying to explain. A power factor meter is constructed by using two coils arranged perpendicular to one another (90 degrees). One is a current coil, the other a voltage coil. Given their electrical polarities, they are PHYSICALLY arranged 90 degrees. Mechanical torque is developed by the orthogonal arrangement such that it is proportional to the angle relationship between them. Time in seconds is completely irrelevant.
Proof of this is is to compare the behavior of the power factor meter at 59Hz and 61Hz. Apply exciting signals at 10 degrees apart. By simple math, the time delays between these two cases are different, would you not agree? But the power factor meter correctly displays 10 degrees in each case. Reason is simple; the meter doesn't measure time; it measures (directly) electrical angle. *Time is not the only thing measurable in AC* No doubt the more recent electronic meters measure either time or generate a PWM signal that is proportional to angle, but this is a choice to obtain an answer based on what's in the tool box- and they still have to factor in the frequency to get the meaningful value, angle. The relationships between watts, vars, and va is inextricably tied to angle, time being a result of math (and also being irrelevant). There absolutely is such a thing as an angle between voltage and current in an AC system.
It is a frustrating thing to experience on multiple occasions audio people misrepresenting power systems. There is a dearth of solid power knowledge on this forum, and the funny thing is how the truth gets presented, then argued as wrong.
I have also tried explaining the root cause, to no avail. The problem is harmonics generated by a nonlinear load causing voltage drop across the system. Net result is a distorted voltage. You have claimed
which is patently false. Active power factor correction draws current as a sine wave, therefore produces no harmonics, therefore does not cause the voltage drop, therefore improves the quality of the voltage. It may be a small improvement, since there are a multitude of other devices in your house (and on the aggregate grid) that continue to produce harmonics, but it does improve nonetheless. The greater percentage of nonlinear load that is upgraded to APFC, the less distortion the voltage will have.
Perhaps a more fundamental problem. An analog power factor meter's movement operates on electrical torque. What is the equation for electrical torque?
If you don't believe there is a physical "angle" between voltage and current, how does this power factor meter work?
Do you believe the meter develops torque based on time?
There is no physical angle between voltage and curent in the circuit. It is just one of our mental abstract representation of a delay (reported to the period of the alternating signal).
We do measure the delay between the U and I with a meter that has two coils at 90 degree to convert the voltage induced current to force, but also have an inductor series with one of them (that provides the necessary delay). We do write on the dial cos(fi) but again, that is just one math abstraction.
Another math representation is done using the complex imaginary numbers with help of "j=sqrt(-1)"... Again, there is no physical "j".
We do measure the delay between the U and I with a meter that has two coils at 90 degree to convert the voltage induced current to force, but also have an inductor series with one of them (that provides the necessary delay). We do write on the dial cos(fi) but again, that is just one math abstraction.
Another math representation is done using the complex imaginary numbers with help of "j=sqrt(-1)"... Again, there is no physical "j".
Last edited:
An angle does not have to be 'physical' in order to be real. I can define the angle between two objects in an abstract linear vector space by using inner products. This angle has exactly the same meaning as the physical angle between two mechanical force vectors, so it is just as real.
The number 2 is real (in both senses of the word); it it not merely a label for how many apples I have.
The number 2 is real (in both senses of the word); it it not merely a label for how many apples I have.
Abstact space is not equal with real space. It is just a figment of our immagination.
A picture of you is not the real you.
No, abstract vector spaces really exist. Not to those who can only do concrete reasoning of course. I am not talking about a representation, although many of them can also represent others too.
Issues like orthogonality (a right angle between vectors) now crop up in EE areas like COFDM broadcasting and CDM used for some mobile phone systems. Unfortunately most EEs are never taught about vector spaces so struggle to relate to how a right angle can crop up when looking at Fourier analysis or correlation functions. I remember hearing about orthogonal codes in my EE masters course, and realising that most of the students (and some of the lecturers?) would not be able to fully appreciate that we were really talking about an inner product.
Issues like orthogonality (a right angle between vectors) now crop up in EE areas like COFDM broadcasting and CDM used for some mobile phone systems. Unfortunately most EEs are never taught about vector spaces so struggle to relate to how a right angle can crop up when looking at Fourier analysis or correlation functions. I remember hearing about orthogonal codes in my EE masters course, and realising that most of the students (and some of the lecturers?) would not be able to fully appreciate that we were really talking about an inner product.
that's interesting, I'm a computer engineer and had 5 pure math courses at the university. and a lot of it at other courses that weren't pure math.
the real problem is that most engineering students have this view that math is useless and there only to annoy you (they're somehow right but not to the extent they think) and continue to look at things like that for their entire career.
I couldn't help but remember what someone here said on the subject a while ago. it was something along the lines "I'm not saying that the university is totally useless but being taught something the wrong way can certainly be a handicap".
Last edited:
When I first learnt about linear vector spaces the application was Fourier theory and then quantum mechanics. I did not realise that over 20 years later the same ideas would crop up when learning about things like orthogonal codes in telecommunications. Inner products defined on vector spaces seems too impossibly abstract to be worth teaching to 'practical' people like engineers, yet such ideas are the foundation of some modern technology.
A few years ago the head of one of our engineering institutions complained that UK engineers were taught too much "pure maths". I fear he was showing his own ignorance, as most courses here contain far too little maths. The little maths that is taught is only taught as a calculation tool, rather than as an underlying principle.
A few years ago the head of one of our engineering institutions complained that UK engineers were taught too much "pure maths". I fear he was showing his own ignorance, as most courses here contain far too little maths. The little maths that is taught is only taught as a calculation tool, rather than as an underlying principle.
well my impression is that it mostly depends on the teacher. there are good ones and there are bad ones.
I guess it's an issue all around the world, some of them force students into thinking that physics equals a lot of formulas and the need to understand what lies beneath is optional.
but in the end you can't blame them and that is because IMO there's not much of a need for mathematically proficient engineers nowadays. as complexity in all fields of engineering grew I feel that nowadays there's more of a need for "qualified workers" as one of my bosses once put it than for real engineers that really "get it". and I'm of the opinion that for these current needs there is way too much math being taught it's a sad state of affairs if you ask me but nonetheless real.
please excuse my rant.
I guess it's an issue all around the world, some of them force students into thinking that physics equals a lot of formulas and the need to understand what lies beneath is optional.
but in the end you can't blame them and that is because IMO there's not much of a need for mathematically proficient engineers nowadays. as complexity in all fields of engineering grew I feel that nowadays there's more of a need for "qualified workers" as one of my bosses once put it than for real engineers that really "get it". and I'm of the opinion that for these current needs there is way too much math being taught
it's a sad state of affairs if you ask me but nonetheless real.please excuse my rant.
I've hijacked this thread enough, so I'll quickly put out a brief thought and then shut up. Posting a cute gif of v, i, and p does nothing to answer the question about torque.
Maxwell's equations, which are deceptively simple, and the Lorenz force law, describe the interactions of current carrying conductors in magnetic fields and the forces applied or received. There are these annoying things like line and surface integrals, and a curious thing called a dot-product. Point being, torque behaves in a geometric fashion. A dot-product doesn't care about time, it acts on the basis of angle. Further, these equations apply to both AC and DC systems. It is easy enough to understand angle interactions when dealing with magnets and DC current, but the same angle exists with sinusoidal exciting sources.
Or perhaps sonic could explain how time is a variable in the torque applied to an induction disk.....I seem to recall this trick called the right hand rule. Geometry, anyone?
Maxwell's equations, which are deceptively simple, and the Lorenz force law, describe the interactions of current carrying conductors in magnetic fields and the forces applied or received. There are these annoying things like line and surface integrals, and a curious thing called a dot-product. Point being, torque behaves in a geometric fashion. A dot-product doesn't care about time, it acts on the basis of angle. Further, these equations apply to both AC and DC systems. It is easy enough to understand angle interactions when dealing with magnets and DC current, but the same angle exists with sinusoidal exciting sources.
Or perhaps sonic could explain how time is a variable in the torque applied to an induction disk.....I seem to recall this trick called the right hand rule. Geometry, anyone?
no hijacking really.
for me it's hard to see exactly where the confusion lies and if it's more than semantics. but it's not that important as I'm mostly concerned about the effects.
Actually I agree with you. We need to revive the technical colleges which teach practical engineering, as well as the universities teaching the hard theory properly. Instead, we make everyone get a 'degree' with just enough maths to confuse them but not enough to be really useful.mr_push_pull said:
I like that! Maxwell's equations contain far more than people think.zigzagflux said:
I've always had the sensation that these issues were fixed everywhere else
I've maintained for a long while that the system has a high inertia and some things will need to change sooner or later as the university as we know it does not have a precisely defined product for the current economic realities.
This just showed-up in my mail-box:
What’s The Difference Between Reactive Power Factor And AC-DC Supply Power Factor?
Why ac/dc conversion generates harmonics and how that relates to reactive power factor
What?s The Difference Between Reactive Power Factor And AC-DC Supply Power Factor?
What’s The Difference Between Reactive Power Factor And AC-DC Supply Power Factor?
Why ac/dc conversion generates harmonics and how that relates to reactive power factor
What?s The Difference Between Reactive Power Factor And AC-DC Supply Power Factor?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.