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tubes, killawatt, current
tubes, killawatt, current
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Old 1st January 2018, 11:59 PM   #11
zigzagflux is offline zigzagflux  United States
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That can be true, but doesn't change the fact that you pay for fundamental only. Regardless of the source of harmonic being within the residence or in the distribution line, they furnish real power from one location only, and that is a generator. Any losses are either 'eaten' by them, or more generally incorporated into the rate structure.

The marketed purpose of the killawatt is to understand where your power consumption is, and how to potentially save money on your utility bill. They even claim to measure power the same as your utility. If that's true, it should be fundamental quantities only. I find no claim in their literature that harmonics are relevant, measured, or considered. Which is appropriate. Put down the true rms meter; it's not supposed to agree with the killawatt.

Last comment that is just as important; the presence of harmonics due to nonlinear loading presents a predominantly reactive load to the system. This could be in a lagging or leading mode, but the critical item is that the loading is not in real power, it is in reactive power. So the degree of harmonics will certainly change rms current readings, slightly alter rms voltage readings, and therefore significantly change VA readings. But the effect on watt (real power) readings should not change. The VAR (reactive power) readings will change with harmonics. Further understanding to explain why fundamental watt readings are appropriate.

Last edited by zigzagflux; 2nd January 2018 at 12:10 AM. Reason: VAR comment
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Old 2nd January 2018, 01:15 AM   #12
PRR is offline PRR  United States
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Quote:
Originally Posted by mikr View Post
...With tube amplfiers, when is it essential to use a true rms meter?.....
Uh, "never"?

I grew-up with two meters: needle (averaging) meters, and VTVMs (often peak on the AC ranges). Tube work is "never" so exact that we should be confused.

A special case: put both Average and Peak meters on a power amp output and turn up the level. At clipping, the Peak meter will stop rising, the Average keeps going up (because the Sine is going toward a Square with the same peak but more area over the waveform).

Mostly we "know" our wave-shape and, if unusual, can correct the reading.

The first time I saw "true RMS" was for calibrating theater dimmers. SCR/Triac dimmers make VERY ugly part-waves. One of the few waves so ugly that an Averaging meter didn't come close. (The Peak meter read 100% for anything over 50%.)
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Old 2nd January 2018, 08:02 AM   #13
6A3sUMMER is offline 6A3sUMMER  United States
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Outside the house:
I had never heard or thought that the electric company charges us only according to 60Hz power usage with no regard to harmonics (intended or realized). Whether it takes into account the phase or not is also interesting.

About 3 years ago, the power company replaced my ‘all electro-mechanical spinning wheel and
mechanical spinning numbers power meter’ with an ‘all digital meter’. If the new one only measures 60Hz power. It must have a couple of A/Ds. In addition it either needs: either 2 very effective low pass filters and a phase detector; or it has to run 2 FFTs, and phase detection.
Without phase detection, it certainly does not measure the true power dissipated.

Because the power company does not always deliver the same voltage, and because the customer load current varies, both the old electro-mechanical meter and the new digital meter must perform some kind of multiplication of voltage and current.

Inside the house:
The phase of current versus voltage varies according to the load(s), and current and voltage
may not be the same wave shape either.

In order to produce a Kill A Watt so that it only measures the 60Hz voltage and 60Hz current would also require either 2 very effective low pass filters, or it would require an FFT; and phase detection of the 60Hz (and only 60Hz) current and voltage (just like the electric power company’s meter above, if it does that).

My P4400.01 Kill A Watt cost about $20 or $25. I doubt that it measures it that way (FFTs or low pass filters).
It only displays one quantity at a time. But it must measure multiple factors at once to measure
Watts and Power Factor.
I am believing that it digitizes the waveforms, and does cycle-time integral of V, cycle-time integral of I, and does cycle-time integral of V*I (Watt).

The P4400 manual says: RMS Amps, RMS Volts, and Watts active power;
and Power Factor: Watts/(Vrms*Arms).
I do not see any claims that it only measures 60Hz, and not the harmonic contribution too.

The home Variac is an auto transformer, it may saturate, so it also may be a source of the harmonic distortion.

A power transformer with a high voltage secondary that allows choke input filter to get the desired B+ voltage, is what I use in my tube amps. It does not have nearly the current spikes of a cap input filter.

I can measure the real cycle rms of the current that the tube amp draws using the scope,
a differential probe setup, and a current sense resistor in the Neutral line. This allows me
to determine what value of slow blow fuse to use in the primary of the tube amp transformer.
This requires true rms measurement, a really bad current wave shape will fool any other measurement device (peak responding or average responding) from making an accurate determination of the amp rating of the slow blow fuse.

I can also use a True RMS voltmeter across the current sense resistor, and verify the scope reading.
In any case, the slow blow fuse responds according to true RMS current (and time versus overload value).
A DC current source and stopwatch will allow you to test fuses, and they are not always
very exact, they vary quite a bit. But once you test one, it is no good any more (get a box and check at least a couple).

I like PRR's comment that tube work is "never" so exact that we should be confused.
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Old 2nd January 2018, 09:50 AM   #14
Johnny2Bad is offline Johnny2Bad  Canada
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I don't know the details of the "smart meters" being deployed (and they are not all the same). I had assumed they only allowed simpler reading of power use (simpler to the Utility).

The old "rotor" meters to residential single-phase power cannot measure reactive elements; only 60 Hz power.* I suppose it's possible the newer smart meters could, but I have my doubts as it's still a single phase connection to the utility, and it seems to me you need three phase power to measure true power (power factor).

* This means that the Utility has no means to bill residential customers for the full power used via devices with a poor Power Factor, such as CFL bulbs. Recent increases in Utility rates are partly due to a widespread change to CFL bulbs in the home, as the Utility has no other means of accounting for the actual power used in single phase connections.
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Last edited by Johnny2Bad; 2nd January 2018 at 10:03 AM.
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Old 2nd January 2018, 03:21 PM   #15
zigzagflux is offline zigzagflux  United States
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Maybe a simpler generalized statement would help:

Real power in a real world harmonic rich system is largely (almost completely) 60 Hz only.

Reactive power in a real world harmonic rich system consists of the fundamental component and all harmonics.

This paper is a fun little read about how to measure VARs by various methods.
http://www.radianresearch.com/white_...icmetercal.pdf

Single phase vs three phase measurements really don't make any difference in measurement of power or power factor. The old time electromechanical meters used single phase stators on a common shaft. Sum of the shaft torques moved a register. A three phase meter was a combination of single phase devices. Read up on Blondel's theorem. The utility has no challenges with the measurement of power factor in single phase circuits; they simply by tradition did not charge residential customers for poor power factor (see post #9). Measuring VARs was simply phase shifting a quantity and using the same stator design. But all single phase.

I think the most interesting factoid coming out of the Radian paper is that measurement of VARs should perhaps be deprecated, and we should consider measuring real power (60Hz only is sufficient) and rms-VA (including all harmonics). Other desired values one might want to use for billing (power factor, VAR hours) would be calculated from those two quantities.
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Old 2nd January 2018, 07:35 PM   #16
6A3sUMMER is offline 6A3sUMMER  United States
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The 3 attached scope screen captures tell the story of the power at my house.

Plot # 15: 125Vrms 60.0076 Hz
The 60Hz fundamental is slightly less than 125V, (the wave shape includes the harmonics).

Plot # 17: The cursors show the relative amplitudes of the 60Hz and 180Hz.
The 180Hz 3rd harmonic is 36dB less than the 60Hz fundamental.
-36dB = 1.58% You can also see the 5th, 7th, and 9th harmonic.
All the other harmonics, odd and even are insignificant (too small to be of much effect).

I put the cursors just below the peaks of the spectral lines. That gives the same differential
result of 36 dB, but it allows you to see the peak of each special line. If I did not put them exactly the same distance below the peak, you can forgive me for a 1 dB error or so.
You will see the actual difference in the auto-crosshairs in screen shot #19

From this screen shot, I estimate the total harmonic distortion to be about 3%, maybe 4%.
But I am not going to measure all the harmonics. Then re-exponentiate them into voltages, and
calculate the THD from the root sum squares of the harmonic voltages; divided by the re-exponentiated fundamental voltage; and multiplied by 100 to get % THD.

As an example for you, suppose that there was only the 3rd harmonic, and the 5th harmonic, and that they were each -40dB. That means each harmonic is 1/100 of the fundamental voltage. 10^(-40dB/20) = 0.01
That is 1% 3rd, and 1% 5th.
But according to the (correct) root sum square method, that is 1.4%THD (not 2% THD).
Root ((1 squared) + (1 squared)) = Root (2) = 1.414.. 1.4%
A little rule of thumb works here for me, nobody pays me to make measurements anymore.

Plot # 19: Another set of cursors shows the frequency as well as the amplitudes (automatic crosshairs) of the 60Hz and 180Hz.
You will see the auto-crosshair on the 60Hz measures 41.4dB (that is 41.4dBV).
That calculates to 117.5Vrms of the 60Hz fundamental.

How does 117.5Vrms compare to the 125Vrms in screen shot #15?
The screen shots #15, versus screen shot #17 / #19, were captured at slightly different times.
The on and off of my electric heater, my water heater, the neighbor off the street transformer changed his power draw, etc., may all have affected this. These all can make the voltage (and distortion) jump up and down.
Attached Images
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File Type: bmp TEK0017.BMP (76.1 KB, 7 views)
File Type: bmp TEK0019.BMP (76.1 KB, 6 views)
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Old 2nd January 2018, 07:41 PM   #17
6A3sUMMER is offline 6A3sUMMER  United States
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Johnny2Bad,

Thanks for mentioning CFLs.
As I understand it, the purposes were to:
1. Reduce the use of Tungsten (expensive).
2. Increase the use of Mercury (toxic). Oh, was that on purpose, or just overlooked?
3. All in the name of saving energy.
Question: How much energy does it take to properly dispose of a single CFL?
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Old 3rd January 2018, 06:06 AM   #18
PRR is offline PRR  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
...Because the power company does not always deliver the same voltage, and because the customer load current varies, .... meter must perform some kind of multiplication of voltage and current.....
Suggested Read: Electric Power Metering, Knowlton, 1934, $13 reprint (but I have a 6th printing from WWII)

Gives a short history of attempts to meter electric power. The 1934 AC Watt-Hour meters are recognizably same as the ones I grew up with. And same as the one I have now except behind the gear-train is electronics which beam data back to the company (no meter-reader). Full treatment of that, and much more about PF, demand, and polyphase metering.

The classic rotating disk meter has a Voltage coil and a Current coil. Their magnetic fluxes combine as a Multiplication. Torque on disk == V*I. Permanent magnets provide a retarding effect proportional to speed. The result is that RPM == V*I. The number of revolutions in an hour, or a month, is proportional to Watt-Hours.

The V*I multiplication is essentially "instant". If you hung a pure capacitor on the meter, you would have Current at zero Voltage, and Voltage at zero Current. These four points would correctly read zero actual Power. Thinking now, midway between V and I zeros there is finite V with finite I. There may be some reading.(*)

I think the thinking is that NO customer would hang a PURE reactance on the line. It does the customer no good. Customer wants some work done. Customer needs Power. Some common Power-sucking loads also have reactance, Power Factor. The big one is large motors which run with significant lagging current. This heats the lines and also upsets system balance. Large factories etc submit to Power Factor metering, penalties for high PF, and may opt to add opposite reactance (capacitors for motors) to null their PF surcharge.

This was never done for residential customers. The early loads were filament lamps, unity PF. The bonus loads, electric cooking, ironing, heating, are also unity PF. By this time the domestic power industry was neck-deep in Practices and Tariffs, so changing the rules became very difficult. The next big domestic load was air-conditioning, now a major part of the load in some areas. A/C pump motors have significant PF. Apparently the electric companies just wrap it up in the relatively high rates they charge small distributed customers (residences). Residential work also requires significant over-investment to cover a few high-use days a year, line and transformer capacity which sits 90% idle 360 days a year. Compare to the rates a large factory working steady and hard can negotiate.

And no, you do not need 3-phase to have or meter PF. Just that we overlook PF for small loads, enforce it for large loads, typically large enough to justify the costs of 3-phase delivery. 3-ph is sorta a discriminatory line between little customers and big customers; we play on different rules.

I'm working at another house with an "opaque" meter. It has an LCD behind glass but no rotating disk. Such things could work any way they want. Even pull random numbers from a hat. (Some customer got a $800,000 bill, but was offered a 10-month payment plan with it. She thought she had connected her holiday lights wrong. A human at the company admitted it must be a mistake, her bill was $200.) We would hope some approximation of the classic disk meter. Even to the point that a disk versus an opaque-meter would give the same reading for any likely residential load (+/-$1).

(*)Knowlton says there is torque but both + and -, so the result is correct: zero.

Remember that frequency DOES vary. In modern gridded systems, not much; but this was considered in 1934. Knowlton shows that with unity PF a meter will read 0.5% high at 50Hz, 0% at 60Hz, and 1.6% low at 70Hz. Extrapolating unwisely, it may read 30% low at 120Hz. The coil reactance is not totally swamped and begins to matter at higher frequencies.
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Old 3rd January 2018, 06:19 AM   #19
PRR is offline PRR  United States
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Quote:
Originally Posted by 6A3sUMMER View Post
... CFLs. As I understand it, the purposes were to:...
No. Just efficiency. Incandescent is terrible efficiency. Fluorescent is better, but does not scale to small size (excess surface/volume ratio).

Mid-1970s, Ed Hammer at G.E. worked on small fluorescents, dimensions and gas mixes, phosphors, and devised the spiral tube. GE liked it but it would need a whole new factory. They passed on that but considered licensing. Meanwhile the idea leaked out. Philips did a magnetic ballast version. Osram did an electronic ballast CFL. Asian glassblowers huffed and puffed, along with their electronic ballast designers.

The cost of tungsten has never been an issue.

In retrospect, the savings from CFLs may have been in vain. We would not pay for good ones, and the cheap ones crapped-out too fast. CFLs in every room invites Mercury poisoning; also the tubes make far more shards of glass. H-D does not charge me to take-back CFLs, but safe disposal can't be free, and may not be good for the workers. I have been working to remove ALL the CFLs from my home; it is amazing how many we acquired in the decade they were dominant.
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Old 3rd January 2018, 07:59 AM   #20
6A3sUMMER is offline 6A3sUMMER  United States
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PRR,

Very informative about power measurement, thanks!

My comments about CFLs was more tongue in cheek. I never liked CFLs, and have been replacing with LEDs. I am thinking they may be less environmentally friendly (production to grave) than the old incandescents.
I believe my oven light will always be an incandescent, or the oven will just be dark.

When the cold and wind dies down, I need to take another walk around the block. I think I saw a big capacitor on a power pole in a residential area.

I use choke input filters on my B+ whenever possible. It does keep the B+ secondary cooler (and probably the primary as well), for the same current output.
The disadvantages are DC = 0.9 * rms, and magnetic spray from the choke.
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