Power and Energy meters

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offgridQLD
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Power and Energy meters

Post by offgridQLD » Fri, 26 Jun 2015, 12:53

Discussion around good quality vs inexpensive AC energy meters. Following on from the off topic discussion in the PIP4048 thread.
viewtopic.php?title=pip4048ms-inverter& ... 332#p57600 [Link edited to go to correct post]

The conclusion seemed to be that the Powermate brand of AC energy meters (also re badged and sold under a few other brands like clipsal) are much better than your typical sub $50 meters that have looser overall accuracy specs, resolution, features and don't take power factor into account.

Powermate offer a few models ranging from the entry level 10A lite version (2% accuracy) A standard 10A version with (1% accuracy) then a HD 10A version (capable of short term 15A) They also have a 15A version. Both the 10A and 10A HD are also available with data logging output and PC software. 15A isn't available with DL.

My pick would be the 10 HD with data logging. Though it would be interesting to ask them if the 15A version will be available in the future with DL.

Kurt



Last edited by weber on Fri, 26 Jun 2015, 05:51, edited 1 time in total.

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Post by weber » Fri, 26 Jun 2015, 14:42

Thanks for starting the new thread, Kurt. I was avoiding further responses. I like how you said it was "starting" to get off topic in the PIP thread. Very funny.

But I note that the discussion was primarily about power meters, not energy meters. If you agree, you can change the subject heading by editing the first post.

I've edited your link in the first post, so it takes the reader directly to the first post on power meters in the PIP thread, instead of merely the start of the page. You can obtain the direct link for any post by clicking on the blue dot (or sometimes yellow star) at the top left of the post.

I also edited my post just before the digression in the PIP thread, to add a link, so those coming late to the PIP thread can skip the digression.

Re the EM1000 energy meter (not really a power meter): It meets an international 1% accuracy standard and I can't see the electricity companies giving away power just because it's spikey. And I read stuff about energy meter chips with 7 kHz bandwidth and 16 bit resolution with 256 samples per mains cycle, so I don't think it will miss 1% duty cycle currents, particularly since they will tend to be rounded by the inductance of the cabling.

Re the Power-Mate 10A Heavy Duty Serial: I wouldn't be surprised to learn that the Power-Mate 10A Heavy Duty differed from the Power-Mate 15A only in its power cables and plug and socket, in which case the Power-Mate 10A Heavy Duty Serial might easily be modified to become a Power-Mate 15A Serial. You should ask them.
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Power and Energy meters

Post by offgridQLD » Fri, 26 Jun 2015, 15:31

Happy to have the title changed.

The EM1000 energy meter (The little white digital Meter the utility companies use on grid connected homes) As you mentioned viewing the front LCD display its more of a energy meter. As it gives accumulative KWH (bidirectional) Though your not going to see instantaneous watts, amps, volts and so on via the display. I have one at my grid connected home yet I know little about them. A quick Google search shows that you can extract some power meter style data (watts) from them. Via usb http://www.mogilowski.net/lang/en-us/20 ... ith-cacti/ I found another web page where one guy was reading the optical pulse (going about it in a DIY way)http://diydeveloper.io/tech/2014/05/19/ ... der-part1/

Though I think a more usefull meter as a testing tool is one like the powermate that is a power meter and energy meter in the one device. We want to know what the load is in watts and perhaps log the accumulative energy over time.


Good point Weber regarding the 10A HD vs the 15A Powermate. I wondered the same thing and I think its likely they would be the same internals as fare as the 15A rating is concerned yet they most likely make the statement that its only good for short term 15A load due to the cable (covering themselves). In fact in general having a 10A cable installed would be more practical. Paralytically on the male end of the plug as you would need to use a adapter any time you wanted to plug the device into a 10A socket with the 15A powermate.

Hopefully this isn't going off topic again. Regarding power factor. My limited understanding of PF is for example. You had a 50% PF 1000w load. The load although shown as 1000W and billed as 1000w load over 1 hr.(in a simplistic way) The loads on the components would be the equivalent of a 1500w load. So the utility company's can charge penalty's to commercial businesses based on there PF load to cover the real loads on there wires - transformers.
So is it correct to assume in a offgrid system (battery - inverter) If you run loads with a bad PF (motors, fluro lights) Then you will increase the strain on your wiring and inverter but the strain (load) on the battery side will not be effected.

Kurt



Last edited by offgridQLD on Fri, 26 Jun 2015, 05:35, edited 1 time in total.

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Post by weber » Fri, 26 Jun 2015, 16:06

As the creator of the thread, you can change the title too, just by editing the first post.
offgridQLD wrote:Regarding power factor. My limited understanding of PF is for example. You had a 50% PF 1000w load. The load although shown as 1000W and billed as 1000w load over 1 hr.(in a simplistic way) The loads on the components would be the equivalent of a 1500w load. So the utility company's can charge penalty's to commercial businesses based on there PF load to cover the real loads on there wires - transformers.
So is it correct to assume in a offgrid system (battery - inverter) If you run loads with a bad PF (motors, fluro lights) Then you will increase the strain on your wiring and inverter but the strain (load) on the battery side will not be effected.

That's correct, except it's worse than you said. A 1000 W load with a PF of 0.5 has an rms current the same as a 2000 W load with a PF of 1, and hence with a linear load like an induction motor the stress on the wires and inverter is double. With a nonlinear load like a rectifier, the peak currents can be higher still.
Last edited by weber on Fri, 26 Jun 2015, 06:34, edited 1 time in total.
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Post by Rusdy » Fri, 26 Jun 2015, 18:47

Hi All,
Has anyone know that the power mate is also good to measure non sinusoidal power factor?

As I've seen in some comments that some current readings by other power meters are a bit off. My guess is, this is due to some power meter only can read sinusoidal waveform.

However, non sinusoid load requires FFT (Fast Fourier Transform) analysis to properly calculate the correct Power Factor:
Image
Taken from Here
Last edited by Rusdy on Fri, 26 Jun 2015, 08:49, edited 1 time in total.

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Post by weber » Fri, 26 Jun 2015, 19:44

Hi Rusdy. Yes the Power-Mate is good for non-sunusoidal waveforms. You don't need to do a fourier transform to calculate true power or power factor. Your utility meter (and presumably the Power-Mate) does it by taking simultaneous samples of voltage and current more than 100 times per mains cycle and multiplying them together, then averaging (low-pass filtering) the results.
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Post by coulomb » Fri, 26 Jun 2015, 21:03

offgridQLD wrote: So is it correct to assume in a offgrid system (battery - inverter) If you run loads with a bad PF (motors, fluro lights) Then you will increase the strain on your wiring and inverter but the strain (load) on the battery side will not be affected.

Essentially, yes. For an inductive or capacitive load (inductive is far more common), the instantaneous power essentially circulates between the battery and the reactive elements in the load. Inductors can store magnetic energy, and capacitors can store electrical energy. In an inverter, or just connected to the mains, they can return energy almost as effectively as they can take it.

I happen to have been doing some of these power calculations at work yesterday. Here is a classic power triangle:

Image

This represents the situation of a 0.8 power factor load. Let's say the mains is 250 V today; it's often close to that and it makes the maths a little easier. So we see 6 A flowing in the cables; that appears to be 6 x 250 = 1500 W, but actually there is only 900 W of real work being done. We say that the apparent power is 1500 VA (Volt Amps), and the real power is 900 W. VA have the same units as real power (Watts), but we like to distinguish them because they act so differently. The "discrepancy" is due to the 900 VAR (Volt Amperes Reactive) in the inductance of this load. I've drawn the 900 W at a right angle to the 1200 W, because these powers add according to Pythagoras's theorem (S^2 = P^2 + Q^2). Note that 1200 + 900 = 2100, which is not the same as 1500 at all. The reason for this is that the voltage across the inductance and the resistor representing the real load are at ninety degrees as well. It's a bit beyond the scope of this post, but you can think of these lines as being snapshots of vectors that rotate at the mains frequency (50 times per second).

So while there is 250 V flowing at the mains, there is effectively only .8 x 250 = 200 V that is across the resistance of the load, in phase with the mains voltage, doing real work (mechanical load, heat, and so on). There is a reactive voltage of .6 x 250 = 150 V across the equivalent inductor, representing the reactive part of the load. (Think if the load as an ideal inductor in series with an ideal resistor; they form a voltage divider.)

The acute angle in the diagram is the inverse cosine of 1200/1500 (simple geometry; my daughter does this stuff in grade 10 maths). It's about 37 degrees. Sometimes you see this angle called "phi", which is the greek letter ø or Φ (I hope that shows up OK). The cosine of this angle is the power factor, which is just the ratio of the real power to the apparent power. You might see "cos phi" or "cos ø" on the spec plate of a motor, for example. This is the electrical angle between the voltage and the current. As this angle approaches 90 degrees, the real power approaches zero (cos(90°) = 0). As this angle approaches 0 degrees, the real power approaches the apparent power (cos(0) = 1.0 or 100%).

Without that inductance, we could have delivered the 1200 W of power to the load using only 1200/250 = 4.8 A, or 80% of the actual current. So there is 100% x 6 / 4.8 = 125% as much current as we really need. There is some copper loss associated with the extra current, so this will cause extra I^R heating, in fact (1.25)^2 times as much, or about 1.56 times the copper losses. This extra loss has to be supplied by the battery, so this is the reason that in practice, the battery current will be higher than if we had a pure resistive (100% power factor) load of 4.8 A. It won't cost us 56% extra battery current, of course, since the losses are small compared with the total power. If the losses were 10%, then we'd have about 5.6% extra current from the battery.

Now this is all for inductive or capacitive loads, which operate linearly. For a non-linear load, such as when a diode rectifier is involved, none of the power ever comes back to the mains, so it gets more complicated. I've seen texts where they introduce a "distortion factor" D, with S^2 = P^2 + Q^2 + D^2. But it's by no means universally accepted. Most texts say that the reactive power component, Q, is simply undefined when the current is not sinusoidal. Unfortunately, I don't know enough about this to say what happens to the battery current if you have say a 60% power factor load due to a full wave rectifier with no smoothing on the mains side of the diodes. Perhaps others can weigh in here.
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Power and Energy meters

Post by offgridQLD » Fri, 26 Jun 2015, 21:35

Thanks Coulomb & Weber for the detailed replies on PF.

Kurt





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