THE PHYSICS OF... POWER

[wwhitney]

Amps_vector

What do you mean by the above? I'm not aware it's an expression that has any conventional meaning.

Anyway, let me try it this way:

Take an arbitrary, periodic waveform. It's average value (the usual average) is its DC component.

Subtracting this off, you have a periodic waveform that averages zero. I would call this the AC component of the waveform. If it is sufficiently smoth, it can be expressed as a (possibly infinite) sum of sinusoidal waveforms, the Fourier transform.

Pure DC would just be a constant value waveform. Pure AC would be either (a) a periodic waveform that averages zero or (b) a sinusoidal waveform (only one frequency present). I'm not sure of which terminology is usual.

Other waveforms, such as PWM DC, have both AC and DC components.

Cheers, Wayne

 

[FionaZuppa]

At any instant in time, amps is a real number, a scalar. The only direction involved is + or -, and a single real number captures that.

Cheers, Wayne

Amps itself cannot be negative, ± represents only a direction. Amps will be perpendicular to that plane and will be moving across that plane in a direction, perhaps notated as 90° is "-out" and 180° is "+in", the reference needs to be defined, etc.

 

[wwhitney]

Amps itself cannot be negative, ± represents only a direction.

Once you choose a convention that one direction is positive, the opposite direction is negative.

Wayne

 

[Besoeker]

i think you answered your own comment by using "AC". DC by definition is not the avg of the waveform. Next is "AC", so i will agree with "AC", in that DC PWM when used in LRC ckts will contain AC-like characteristics, but yet is not AC. AC implies Amps_vector has changing directions whereas DC implies that Amps_vector has one and only one direction relative to the source.

Take a simple uncontrolled rectifier bridge such as the input to most variable frequency drive. There is a six times fundamental frequency component in both the output current and voltage.
Power electronics circuits in general are rarely pure DC.

I think the lowest ripple application I got involved with was psophometric weighting on 48Vdc supplies for telecoms. As I recall, it was down to a few mV at certain frequencies.

 

[FionaZuppa]

Once you choose a convention that one direction is positive, the opposite direction is negative.

Wayne

:thumbsup:

 

[ggunn]

You know the old story with the politically incorrect name where there are tigers chasing the protagonist through the jungle? He climbs a tree and the tigers chase each other around the tree faster and faster until they turn into a circle of butter around the base of the tree.

This thread reminds me of that story.

 

[FionaZuppa]

You know the old story with the politically incorrect name where there are tigers chasing the protagonist through the jungle? He climbs a tree and the tigers chase each other around the tree faster and faster until they turn into a circle of butter around the base of the tree.

This thread reminds me of that story.

just the physics, its a physics oriented topic

but nice, the butter may be used as "real" food at that point.

 

[Besoeker]

Amps itself cannot be negative

Regenerative drives?

 

[GoldDigger]

Amps itself cannot be negative,

How about in a semiconductor where the current is carried by hole flow rather than electron flow? :angel:

Just joking....

 

[Carultch]

How about in a semiconductor where the current is carried by hole flow rather than electron flow? :angel:

Just joking....

Or human nerve cells, where positive sodium ions are current carriers.

 

[FionaZuppa]

How about in a semiconductor where the current is carried by hole flow rather than electron flow? :angel:

Just joking....

do holes carry charge ?

 

[GoldDigger]

do holes carry charge ?

Since a hole represents the absence of an electron, you can say that it has a positive charge.

More to the point, the current associated with the motion of a hole is in the opposite direction to the current associate with an electron moving in the same direction as the hole.

And I suppose that you could have a buildup of hole on the edge of a capacitor plate made up of the right type semiconductor.

 

[FionaZuppa]

Since a hole represents the absence of an electron, you can say that it has a positive charge.

More to the point, the current associated with the motion of a hole is in the opposite direction to the current associate with an electron moving in the same direction as the hole.

And I suppose that you could have a buildup of hole on the edge of a capacitor plate made up of the right type semiconductor.

ah, more physics. yep, in reality, DC batt, the Amps_vector is from batt-neg to batt-pos, but most look at it as pos to neg. holes or no holes, the mag field defines the directional component of true Amps_vector, common (but wrong) is notion that amps direction is from pos to neg.

now now, are these holes moving in a solid conductor? would make for an interesting wire if they were.
holes are for another day :weeping:

lets keep this thread to kVAr and imaginary power :huh:

 

[mbrooke]

While most of this is well over my noggin, I just want to say I've learned a TON reading this thread. Keep up the good work everyone!

 

[Sahib]

The whole concept of 'reactive power' comes from trying to extend concepts that are true only for _DC_ or _instantaneous_ measurements to AC circuits.

We take the concept of Amp, which applies to DC circuits, and come up with the measurement 'Amp RMS' and apply it to AC circuits.

We take Volt, which applies to DC circuits, and come up with 'Volt RMS' and apply it to AC circuits.

For _DC_ or _instantaneous_ measurements, Volts * Amps = Watts.

But V(RMS) * A(RMS) does _not_ equal Watts(time averaged). We introduce the concept of reactive power and power factor to maintain the utility of RMS measurements.

When you are not looking at time averages, there is no such thing as reactive power.

When you connect a power supply to an electromagnet, then _real_ power flows to create the magnetic field. _Real_ energy is stored in that magnetic field.

If you connect a load to that electromagnet and disconnect the power supply (something done with diodes all the time in switching power supplies), then _real_ power flows as the magnetic field collapses, and _real_ energy is delivered to the load.

If you connect a capacitor to an AC supply, then _real_ power flows to the capacitor for 1/4 cycle, and _real_ power flows from the capacitor back to the supply for 1/4 cycle. This is _real_ energy being shuttled back and forth. (Since this power flows for a fixed amount of time, it is a defined amount of energy.) If you were to intercept back and forth shuttling (another example: a voltage multiplying circuit using diodes and capacitors) then _real_ power will flow to a load. The same shuttling applies to an inductor connected to an AC supply.

It is only when looking at the time average values, where we want to account separately for the power flowing to the load and the power shuttling back and forth then we see 'reactive power'.

It may be reasonable to call energy stored in a magnetic field 'reactive energy', but that is _real_ energy which could be used to do real work. Only when that energy is returned to the supply on a cyclic basis to we call it 'reactive power'.

-Jon

If you consider all energy as real ie active, there would not be any need for power factor. The reactive energy is ,however,real in that it costs poco/consumer money.

 

[FionaZuppa]

If you consider all energy as real ie active, there would not be any need for power factor. The reactive energy is ,however,real in that it costs poco/consumer money.

reactive power √-1X = not real power, and no cost to poco
but, in real world, dealing with Amps is very real, so thata where jX bites you
but why all the fuss, if the unit has a PF less than 1 just add X_C or X_L as needed and be done with it. build to kW demand and adjust kVAr as needed, install those solenoid cap banks and call it a day, thus the imaginary jX "power" is nothing to worry about.

the use of the term "power" for jX is kinda wrong, but the power folks needed to place a name on it. "imaginary power",

 

[K8MHZ]

If posts in a thread go back and forth without accomplishing anything are they real or reactive?

:angel:

 

[K8MHZ]

If you consider all energy as real ie active, there would not be any need for power factor. The reactive energy is ,however,real in that it costs poco/consumer money.

Yes, there would be, PF is still needed to determine generation and distribution equipment size due to VA vs. watts used by the connected load.

 

[FionaZuppa]

If posts in a thread go back and forth without accomplishing anything are they real or reactive?

:angel:

hey now, your back & forth in that thread from 2011 about PF and jX was rather lengthy

 

[winnie]

If you consider all energy as real ie active, there would not be any need for power factor. The reactive energy is ,however,real in that it costs poco/consumer money.

Again, you _must_ distinguish between instantaneous measurements and time averages.

RMS is a type of time average. RMS current doesn't tell you how much current is flowing in a circuit at any given instant; it tells you something about how much current is flowing 'on average'.

All energy is actually energy. But in AC circuits, some energy will flow from the load back to the source for part of the AC cycle.

If you want to describe this using instantaneous measurements, you get graphs such as the one Carultch posted in #243.

If you want to describe this using time average measurements, then you need to use power factor.

-Jon