Re: [Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
The power factor case is semantically different than the SWR case in the following way: With SWR, energy is flowing in both directions at near the speed of light. If the reflected signal is the exact same frequency as the forward signal (as in the case where it is a reflection of a signal at a particular frequency, then there will be points along the line where the forward and reflects will be in phase and point along the line where they will be out of phase. SWR is a ratio of the max voltage to the min voltage. If there is no reflection, there is nothing to add or subtract, and so the max and min voltage will be the same. The ratio (SWR) is therefore 1:1. If all energy is reflected, the minimum will be zero, so that ratio of the max to the min will be infinite. Now if I simultaneously measure a power line at 10 different point in a mile, I would detect no significant difference in power factor between the points. This is because the voltage anywhere on the wire is the same (san some IR drop), and the current into one end in the same as the current flowing out the other end. Therefore, the angle between the voltage and current waveforms wil be the same all aong the run. ...Well, not exactly: 60 Hz has a period of 16.7ms. On quarter wavelength is roughly where transmission line effects become significant. One quarter of a period is at 60Hz is 4.17ms. If c=2.98x10^8 meters per second, it would travel 1241.7km in a quarter cycle. (Actually, it is slightly less due to the relative delectric constant of air being slightly higher than that of a vacuum.) If the distance between the source and the load is significantly less than this, no significant steady-state effects will be observable. (Transient effects such as spikes could be seen if a power line broke in a windstorm.) If I were to measure our power line at points much further apart (say thoudands of miles) we could indeed measure a difference because what happens is sufficiently delayed with respect the other points. Thus the adjective semantically. When we talk about power factor the tacit assumption is that the same voltage exists at all locatons of the wires and that the current in equals the current out (i.e. that we are talking about distances much much smaller than a quarter wavelength). When we talk about SWR (or more correctly VSWR) we tacitly assume we are talking about transmission lines of the order of or longer than a quarter wavelength. Jeff Condit
[Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
Very interesting theory. I am teaching SWR at present to my third year college students. Could be a good discussion point since they have already studied power factor. However, SWR can exist with a purely resistive mismatched load, so it needs a bit of modification to take all into account. 73 Nigel ve3id --- In Repeater-Builder@yahoogroups.com, Bob Witte K0NR [EMAIL PROTECTED] wrote: --- In Repeater-Builder@yahoogroups.com, kb9bpf kb9bpf@ wrote: Since I'm way more into RF than industrial power distribution, I've always been able to think of power factor on the electrical power grid in terms similar to antenna system reflections, which are commonly measured in terms of SWR. After all, both are AC systems where the voltage and current bear a phase relationship to each other. When they are perfectly in phase the power factor is 1.0, and a 60-Hz SWR meter would measure 1:1. When they are out of phase (power factor 1) that SWR meter would read greater than 1:1. I suspect, though I haven't done the math or looked up the specific matahematical definition of power factor, that it would be direcly proportional to the reciprocal of the power factor. And as we know, when that happens the power generating end has more difficulty delivering power efficiently to the load. A while back I was doing some analysis of power factor to understand it better and I found that it has a lot in common with SWR. Both are focused on the issue of power transfer, so I guess we shouldn't be surprised. The thing they really have in common is for max power with AC signals, the voltage and current need to be in phase (phase angle of zero). For linear systems with nice sine waves, PF = cos (phase angle) where phase angle = the angle between voltage and current sinusoids Wikipedia has a good explanation of PF at http://en.wikipedia.org/wiki/Power_factor Play around with some typical circuits and you'll find that an SWR of 1 also has voltage and current in phase. Again, not a surprise since it represents the best power transfer. This is from memory, so the usual disclaimers apply :-) 73, Bob K0NR
[Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
Good point Nigel. Back in the '60s I designed a video distribution amplifier for receiver video to tape recorders, O scopes and the like and I matched the input impedance to prevent reflections on the video line from distorting the signal. The output impedance of my amplifier was as low as I could get it, and it worked very well. I did not design the output impedance to be 50 ohms, the impedance of the cable we used to distribute the video. I depended on the load to be 50 ohms resistive to prevent any reflection. On the other hand, the audio distribution amplifiers were not matched on the receive end. I also designed the output impedance of an audio amplifier to be as low as I could get it, and was able to switch multiple loads on the output without changing the level. The input impedance of the audio circuits was always 600 ohms or higher. The power distribution grid is more like the audio and video situation than the SWR problems we see at RF. 73 - Jim W5ZIT --- In Repeater-Builder@yahoogroups.com, Nigel Johnson [EMAIL PROTECTED] wrote: Very interesting theory. I am teaching SWR at present to my third year college students. Could be a good discussion point since they have already studied power factor. However, SWR can exist with a purely resistive mismatched load, so it needs a bit of modification to take all into account. 73 Nigel ve3id
RE: [Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
Yes standing waves can exist with a pure resistive load on a line but the mismatch of the load with the line impedance creates a reactance (depending on line length). A transmission line must be long enough (wavelength wise) for standing waves to exist. A short line (wavelength wise) like an audio cable, will not exhibit standing waves because it is too short for them to exist. Even a short (wavelength wise) RF cable will not exhibit standing waves. Keep in mind that the typical SWR and power meters that we use to measure SWR with are NOT really measuring SWR. They are measuring impedance mismatch of the internal impedance that the meter is set for, with a scale on the meter that converts the impedance ratio to a would be SWR. A very long power transmission line can have standing waves on it if the power factor problems are not kept in check. So power factor could be thought of like SWR but only on long distribution lines. Keep in mind that short transmission lines whether it be power, audio or RF do not have standing waves on them when they are very short wavelength wise. 73 Gary K4FMX -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:Repeater- [EMAIL PROTECTED] On Behalf Of Nigel Johnson Sent: Saturday, September 20, 2008 7:26 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Does anyone else think of Power Factor like SWR? Very interesting theory. I am teaching SWR at present to my third year college students. Could be a good discussion point since they have already studied power factor. However, SWR can exist with a purely resistive mismatched load, so it needs a bit of modification to take all into account. 73 Nigel ve3id --- In Repeater-Builder@yahoogroups.com, Bob Witte K0NR [EMAIL PROTECTED] wrote: --- In Repeater-Builder@yahoogroups.com, kb9bpf kb9bpf@ wrote: Since I'm way more into RF than industrial power distribution, I've always been able to think of power factor on the electrical power grid in terms similar to antenna system reflections, which are commonly measured in terms of SWR. After all, both are AC systems where the voltage and current bear a phase relationship to each other. When they are perfectly in phase the power factor is 1.0, and a 60-Hz SWR meter would measure 1:1. When they are out of phase (power factor 1) that SWR meter would read greater than 1:1. I suspect, though I haven't done the math or looked up the specific matahematical definition of power factor, that it would be direcly proportional to the reciprocal of the power factor. And as we know, when that happens the power generating end has more difficulty delivering power efficiently to the load. A while back I was doing some analysis of power factor to understand it better and I found that it has a lot in common with SWR. Both are focused on the issue of power transfer, so I guess we shouldn't be surprised. The thing they really have in common is for max power with AC signals, the voltage and current need to be in phase (phase angle of zero). For linear systems with nice sine waves, PF = cos (phase angle) where phase angle = the angle between voltage and current sinusoids Wikipedia has a good explanation of PF at http://en.wikipedia.org/wiki/Power_factor Play around with some typical circuits and you'll find that an SWR of 1 also has voltage and current in phase. Again, not a surprise since it represents the best power transfer. This is from memory, so the usual disclaimers apply :-) 73, Bob K0NR Yahoo! Groups Links
[Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
--- In Repeater-Builder@yahoogroups.com, kb9bpf [EMAIL PROTECTED] wrote: Since I'm way more into RF than industrial power distribution, I've always been able to think of power factor on the electrical power grid in terms similar to antenna system reflections, which are commonly measured in terms of SWR. After all, both are AC systems where the voltage and current bear a phase relationship to each other. When they are perfectly in phase the power factor is 1.0, and a 60-Hz SWR meter would measure 1:1. When they are out of phase (power factor 1) that SWR meter would read greater than 1:1. I suspect, though I haven't done the math or looked up the specific matahematical definition of power factor, that it would be direcly proportional to the reciprocal of the power factor. And as we know, when that happens the power generating end has more difficulty delivering power efficiently to the load. A while back I was doing some analysis of power factor to understand it better and I found that it has a lot in common with SWR. Both are focused on the issue of power transfer, so I guess we shouldn't be surprised. The thing they really have in common is for max power with AC signals, the voltage and current need to be in phase (phase angle of zero). For linear systems with nice sine waves, PF = cos (phase angle) where phase angle = the angle between voltage and current sinusoids Wikipedia has a good explanation of PF at http://en.wikipedia.org/wiki/Power_factor Play around with some typical circuits and you'll find that an SWR of 1 also has voltage and current in phase. Again, not a surprise since it represents the best power transfer. This is from memory, so the usual disclaimers apply :-) 73, Bob K0NR
[Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
Normally all this is taken into the design and operation of the distribution grid. It's quite possible to have generation plants simply coast portions or all of their equipment on the grid as correction factor in hydro operation sometimes known as motoring the prime mover. In the very much smoke and mirrors grid operation I've seen here in Northern California... we would receive out of area energy purchased at a much reduced cost and pay the local plant a minimal amount to simply motor the line and correct the area PF. Although less common these days there are/were also a fair number of dyna-motor facilities set upon the grid to address distribution line problems. Most of the required functions are now address by fixed electronics (power correction capacitors and in some cases inductors). Transformers are normally thought of as RF Shunts and along the distribution path (grid) you have a fair number of low RF Impedance devices paralleled. Whale tale stories... KW hour cost for very old Hydro Plants was fixed for decades at about 2.5 mils of a cent. Last 1995 era cost of Nuclear energy from Diablo Canyon was informally quoted at 11 cents kwh. So of course in the Enron Days they'd motor over a hundred mega-watts of low cost local (clean) Sierra California hydro plants (price set under an old 1957 contract) so PGE could cost justify the cash-cow Nuclear Plant at the higher cost. go figure... cheers, s. There was a case I think in Canada where they ran into this problem. The transmission lines from one side of the continent to the other was an odd multiple of a quarterwave (1,250 km). Since a quarter-wave acts as a impedance transformer, when they put a low impedance load at the far end, the cable transformed this to a high impedance at the generator end and they couldn't get power into it. The solution? Either fit impedance correction devices along the cable, or transmit DC power and convert it back to AC at the load end (they do that with a power cable running between the mainland of Australia and the island of Tasmania).
Re: [Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
also known as 'rotary condenser' operation Gary - Original Message - From: skipp025 To: Repeater-Builder@yahoogroups.com Sent: Thursday, September 18, 2008 11:24 AM Subject: [Repeater-Builder] Re: Does anyone else think of Power Factor like SWR? Normally all this is taken into the design and operation of the distribution grid. It's quite possible to have generation plants simply coast portions or all of their equipment on the grid as correction factor in hydro operation sometimes known as motoring the prime mover.
[Repeater-Builder] Re: Does anyone else think of Power Factor like SWR?
To answer the question, I don't. Look at it this way. The AC supply to your house, for example, is a low impedance; it can supply hundreds of amps (the transmitter). The 100 watt lamp (antenna/load) in your house is a relatively high impedance compared to the supply. There is no impedance match between supply and load, not even close. SWR is very high. Do we care? No, we're not trying to transfer the maximum amount of power here as we do in an antenna system, only the amount of power required by the lamp. Hope that helps. Laryn K8TVZ
