THE PHYSICS OF... POWER

[Sahib]

the power is complex
you have P and Q components
before and after it moves
you don't have all Q before it moves, then all P after
the Q is constant like a motor and the P increases with movement, but the Q does not disappear or get 'converted'

like a transformer or motor you have a var inrush and then real power to operate or generate torque

if it required var a DC relay would not operate
if it were var the meter would not spin with a rack of relays connected

What about ideal case with no losses?

 

[Ingenieur]

What about ideal case with no losses?

you still do work, force (accel mass) x distance
var can't do net work
but are required to establish the field so work can be performed by the real power

 

[Carultch]

a capacitive motor ?? or the labels were switched?

Corrected plot:

 

[GeorgeB]

However, the engine driving the generator must be rated 1kW, corresponding to 0.746Hp!

Phil

I'm many years out of engineering school, but something doesn't look correct; or is my memory of 1 Hp=746 Watts more faulty than I thought?

1 kW corresponds to about 1.34 Hp doesn't it?

 

[Carultch]

I'm many years out of engineering school, but something doesn't look correct; or is my memory of 1 Hp=746 Watts more faulty than I thought?

1 kW corresponds to about 1.34 Hp doesn't it?

Yes, you are correct. 1 kW = 1.34 HP

 

[Besoeker]

I'm many years out of engineering school, but something doesn't look correct; or is my memory of 1 Hp=746 Watts more faulty than I thought?

1 kW corresponds to about 1.34 Hp doesn't it?

I agree. Hence my post.

 

[FionaZuppa]

I agree. Hence my post.

1HP_mechanical = 745.7 watts
so yes, 1kW = 1.34HP_mechanical

i suspect phil goofed his #'s in the post

 

[winnie]

The real power moving armature is derived from magnetic field energy which ,before movement ,is purely reactive energy.

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

 

[FionaZuppa]

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.


-Jon

hmmm, not so true. what about DC PWM, it behaves much like the equations you are using for AC, yet the current has one direction only.

i think what you might say is, use the equations for AC everywhere, but in some cases things may collapse and/or drop out of the math. in general, as the observation becomes more constant the math becomes simpler yet the underlying physics still apply.

 

[Besoeker]

1HP_mechanical = 745.7 watts
so yes, 1kW = 1.34HP_mechanical

i suspect phil goofed his #'s in the post

No need for the subscript.

 

[FionaZuppa]

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.

-Jon

actually no. you have +amps_vector for 50% of the current cycle, and -amps_vector for the other 50% of the current cycle. a pure cap has no real power, we went over this during the last 20 pages or so. the amplitude of the current varies, has same period as voltage, but has angle offset from voltage by 90°

 

[winnie]

The whole concept of 'reactive power' comes from trying to extend concepts that are true only for _DC_ or _instantaneous_ measurements to AC circuits.
[...]
When you are not looking at time averages, there is no such thing as reactive power.

hmmm, not so true. what about DC PWM, it behaves much like the equations you are using for AC, yet the current has one direction only.

First, DC PWM is not DC, but AC with a large DC component

If you have DC PWM, then you have a choice: you can look at the instantaneous values of voltage and current, and analyze your circuit as these values change over time, or you can analyze using time averaged values.

If you have DC PWM into a pure resistive load, then the power delivered to the load is the RMS voltage times the RMS current, just like AC.

If you have DC PWM feeding a resistor in series with an inductor (make sure you have a diode so that current can keep flowing even when the PWM switch is off!!) then, then the power delivered to the load is _not_ Vrms * Arms! (Hmm, I guess it depends on where you are measuring and what components are in parallel between the measurement points; I'll need to think on this...)

But in the case of a switched system (DC PWM) then you have energy being stored in the inductor and then being delivered to the load, not returned to the source.

-Jon

 

[wwhitney]

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

. . .

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.

+100 to everything winnie wrote in the post referenced above. FZ, the above are both true statements.

Cheers, Wayne

 

[winnie]

actually no. you have +amps_vector for 50% of the current cycle, and -amps_vector for the other 50% of the current cycle. a pure cap has no real power, we went over this during the last 20 pages or so. the amplitude of the current varies, has same period as voltage, but has angle offset from voltage by 90°

And again, this is only true when you are looking at time average values.

Instantaneous Amps is a scaler value, not a vector value.

One problem in my description that you quoted is that the term 'real' has two meanings and I didn't clearly distinguish them.

We use 'real' and 'imaginary' to name the axes of the complex plane, where 'real' current is differentiated from 'reactive' or 'imaginary' current.

We use the complex plane to describe time average current in AC circuits, where 'real' current delivers power to the load and 'reactive' current does not. The reactive current is still actual motion of electrons, and is associated with actual movement of energy, simply no _net_ energy delivered to the load over time.

The energy delivered to an inductor over a 1/4 cycle is 'real' in the sense that it is energy stored in the magnetic field which could be used to do some work, say by lifting a weight. It is real in the sense that it exists and can be used. But it can only be treated as 'real' when you are looking at a fraction of the cycle, not over the long term time average.

I'll jump to the non-electron analogy: dollars.

Say I give you 1$ today. Tomorrow you give me 1$ back. The next day you give me 1$. The next day I give you 1$, and again the following day. We keep this up, every day moving 1$, changing directions every second day. My _net_ payment to you is zero. Some days you are up $1, some days I am up $1, but net over time is zero. The $ is still a real dollar, and if you break the cycle on the day you are up 1, then you have a $ to spend.

Power factor is like this. I will use 'actual' rather than 'real'. Actual energy shuttles back and forth. Examined in the instantaneous case, sometimes that energy in in the magnetic field of the load, sometimes it is back at the source.

-Jon

 

[FionaZuppa]

First, DC PWM is not DC, but AC with a large DC component

-Jon

i not sure what you mean, where does the current Amps_vector change direction with DC PWM? take a inductor, DC PWM will create a mag field that follows PWM shape/period yet the mag field N/S never swaps to S/N.

We use 'real' and 'imaginary' to name the axes of the complex plane, where 'real' current is differentiated from 'reactive' or 'imaginary' current.
-Jon

still not correct. there is no √-1 Amps. but there is √-1 Power :thumbsup:, its the Amps associated with the √-1 Power that the poco does not like to provide even though their avg of that √-1Power=0 (+ some losses of course).

 

[Besoeker]

Power factor is like this. I will use 'actual' rather than 'real'. Actual energy shuttles back and forth. Examined in the instantaneous case, sometimes that energy in in the magnetic field of the load, sometimes it is back at the source.

-Jon

I don't greatly disagree with anything you posted.

We talk about RMS volts and RMS amps and power factor. Instantaneous values have been mentioned a few times but that is somewhat irrelevant in this context.

 

[FionaZuppa]

Instantaneous Amps is a scaler value, not a vector value.
-Jon

huh? being a delta t=0 observation does not make it a scalar. at any instant in time the Amps has a amplitude AND direction, Amps is s defintion of charge flow which always has amplitude AND direction, and has none of those when Amps hits zero-crossing

 

[wwhitney]

i not sure what you mean, where does the current Amps_vector change direction with DC PWM?

Any non-constant waveform has an "AC" component. The DC component is just the average value of the waveform (usual average, not RMS). Once you subtract that off, you get a waveform with average value zero, and it obviously changes sign over time.

Cheers, Wayne

 

[wwhitney]

huh? being a delta t=0 observation does not make it a scalar. at any instant in time the Amps has a amplitude AND direction

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.

When you use phasors to represent a current waveform, that's an artificial construct to make the math easier. Phasors just let you keep track of the magnitude of the waveform and its phase shift simultaneously, and conveniently many of the rules for manipulating complex numbers gives results corresponding to the analogous operations on the waveforms themselves. You can do all the physics of AC without using phasors and just deal with the waveforms directly, it would just be more cumbersome for computations.

So the "direction" of the current phasor is just a mathematical representation of the phase shift. And phase shift is a concept that only applies to waveforms, i.e. complete current information over some time interval. When speaking of instantaneous values only, there is no such thing as phase shift.

Cheers, Wayne

 

[FionaZuppa]

Any non-constant waveform has an "AC" component. The DC component is just the average value of the waveform (usual average, not RMS). Once you subtract that off, you get a waveform with average value zero, and it obviously changes sign over time.

Cheers, Wayne

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.