Power factor

[Bugman1400]

Can I get someone to sign my CEU form for reading this thread so I can get credit to turn into the Board for renewal?

 

[mbrooke]

at 1/4 or 90 deg V is peak
but I is 90 deg out of phase = 0
power = V x I = peak x 0 = 0

Mathematics built around what is known to happen, but again, I still believe (though not stating with certainty) that when a cap is charging it is indeed KW. How do the laws of physics know if I am going to take the capacitor off line that exact instant to light a light bulb or let the cap drain its charge back into the source? Remember, to force a surplus of electrons on one plate to cause a depletion on the other takes energy.

 

[Besoeker]

Mathematics built around what is known to happen, but again, I still believe (though not stating with certainty) that when a cap is charging it is indeed KW. How do the laws of physics know if I am going to take the capacitor off line that exact instant to light a light bulb or let the cap drain its charge back into the source? Remember, to force a surplus of electrons on one plate to cause a depletion on the other takes energy.

Interesting question. Capacitors can store energy. Super capacitors and ultra capacitors can store quite a lot of it. Energy, not power.

 

[mbrooke]

Interesting question. Capacitors can store energy. Super capacitors and ultra capacitors can store quite a lot of it. Energy, not power.

Energy / power same thing for me Though it would technically be potential energy. Kinetic is when the bulb lights.

 

[Ingenieur]

Consider
230 kVAC 60 Hz
cap 100 MVA (think 100,000 HP)
So C = 5 uF
huge system

energy or work potential
31 nutritional calories 2 tsp sugar
36.7 watthr

dW/dt or power
0.00115 W

only upon discharge

 

[Ingenieur]

Energy / power same thing for me Though it would technically be potential energy. Kinetic is when the bulb lights.

Energy ~ work
power = time rate of work or dW/dt

 

[Sahib]

Mathematics built around what is known to happen, but again, I still believe (though not stating with certainty) that when a cap is charging it is indeed KW.

One problem is the 'KW' of capacitor can only be added vectorily and not algebraically to that across a resistor. So to avoid confusion it is called 'KVAR' instead of 'KW'.

 

[Besoeker]

One problem is the 'KW' of capacitor can only be added vectorily and not algebraically to that across a resistor. So to avoid confusion it is called 'KVAR' instead of 'KW'.

Avoid confusion?
kW and kVAr are simply different things. One is a measure of power, the other is not.

 

[mbrooke]

One problem is the 'KW' of capacitor can only be added vectorily and not algebraically to that across a resistor. So to avoid confusion it is called 'KVAR' instead of 'KW'.

Well look at it like this, normally KVAR is when a capacitor sends power back to its source, hence no actual work is being done. However, is it still KVAR if said capacitor stores a charge to be latter discharged through a resistive load? The laws of physics tell us that when we were charging that capacitor we were indeed doing some type of work otherwise we would not be able to see/measure energy being dissipated during the discharge. Just because it is being converted to potential energy (to be used latter) does not negate work having been done.

 

[winnie]

If you switch on a capacitor and then switch it off. the elapsed time may not be an integral multiple of 60 hz cycles. In that case some energy supplied by source is stored in capacitor with which you may do some useful work such as lighting a LED. But this does not work with an inductor. Why?

This absolutely does work with an inductor, if you do the switching correctly.

An inductor stores energy in a magnetic field, which requires current to flow. The energy in that magnetic field will cause the inductor to try to maintain _constant_ current flow. The proper way to 'disconnect' an inductor to store energy is to _short_ its terminals.

With practical materials and practical inductors used in power systems, this is not a very practical way to store energy, whereas it is _easy_ to do with a capacitor. But if you take a look at the technology of 'superconducting magnetic energy storage' you will see exactly this technique in use.

-Jon

 

[winnie]

Again, we are now arguing terminology.

If you consider _instantaneous_ power, then you have _watts_ flowing into the capacitor for 1/4 cycle, and then _watts_ flowing out of the capacitor for 1/4 cycle. Considering _instantaneous_ measurements watts and volt amperes are _equivalent_.

It is only when you are looking at time average measurements, averaged over entire half cycles, that it is meaningful to consider W and VA as different things. But now we are talking about the terms W and VA.

For time averaged measurements in ac systems, W describes (real) energy delivered to a load over time and VA describes (real) energy shuttling back and forth between source and load, with no net energy delivered. The energy shuttling between generator and capacitor is _real_ energy that could really do work, but instead gets swapped between the two components.

-Jon

 

[Besoeker]

An inductor stores energy in a magnetic field, which requires current to flow.

Typically, the dc coil of a relay or contactor will have a flywheel diode across it to allow the current to continue until the inductive energy is dissipated.

 

[Besoeker]

Again, we are now arguing terminology.

If you consider _instantaneous_ power, then you have _watts_ flowing into the capacitor for 1/4 cycle, and then _watts_ flowing out of the capacitor for 1/4 cycle. Considering _instantaneous_ measurements watts and volt amperes are _equivalent_.

It is only when you are looking at time average measurements, averaged over entire half cycles, that it is meaningful to consider W and VA as different things. But now we are talking about the terms W and VA.

For time averaged measurements in ac systems, W describes (real) energy delivered to a load over time and VA describes (real) energy shuttling back and forth between source and load, with no net energy delivered. The energy shuttling between generator and capacitor is _real_ energy that could really do work, but instead gets swapped between the two components.

-Jon

Since the title of the thread is power factor, one might reasonably assume that it isn't about instantaneous values.

 

[Ingenieur]

power factor can be both: instantaneous and average/fixed (but we already knew that)
for grid load control instantaneous is required
perhaps for a fixed load like a motor a constant is more appropriate

the study of real time power factor is interesting

 

[mbrooke]

Again, we are now arguing terminology.

If you consider _instantaneous_ power, then you have _watts_ flowing into the capacitor for 1/4 cycle, and then _watts_ flowing out of the capacitor for 1/4 cycle. Considering _instantaneous_ measurements watts and volt amperes are _equivalent_.

Terminology to understand :thumbsup:

It is only when you are looking at time average measurements, averaged over entire half cycles, that it is meaningful to consider W and VA as different things. But now we are talking about the terms W and VA.

For time averaged measurements in ac systems, W describes (real) energy delivered to a load over time and VA describes (real) energy shuttling back and forth between source and load, with no net energy delivered. The energy shuttling between generator and capacitor is _real_ energy that could really do work, but instead gets swapped between the two components.

-Jon

Perfect explanation

 

[mbrooke]

Since the title of the thread is power factor, one might reasonably assume that it isn't about instantaneous values.

Perhaps, but in order to understand what PF is in the first place it helps to look at it milli second by millisecond. "phantom power" is a misnomer and often adds to the mystery of what happens in a capacitive or inductive circuit. There is no phantom power, only voltage and current.

 

[Besoeker]

power factor can be both: instantaneous and average/fixed (but we already knew that)

Did we???