testing kwh meters

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Post by antiscab »

hi guys,

the subject of measuring how much our EVs pull from a powerpoint to go a certain distance has come up again (though not on this forum)

i remember the last time i was playing around with kwh meters that i tried a few of these:
MS6115

i was using a cycle analyst at the time, and the readings were a little confusing.
for one my cycle analyst was saying 420W (8A @ 53v) was going into the batteries, but the ms6115 was saying only 90W was going into the charger.

the MS6115 didn't sample the current fast enough to measure the harmonic component properly (of which there is alot in just about every switch mode based powersupply).
That makes them largely useless

fast forward a year or two and there is all these on the market:
MS6160
MS6116
MS6118
Powermate

and im sure theres a few others.

does anyone have any experience with any of those?

i plan on testing a few of these newer ones, but don't want to repeat work someone else has done.

im now using these industrial kwh meters for measuring my EV's energy use.

a few others have used old household mains energy meters aswell.

Matt
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Post by Electron »

If you only want to know how much electricity you use from the mains to travel so far. Would you not just measure the distance travelled and then measure the KWH to recharge the batteries and then do some maths to get a result like Kwh per Km or $0.00 per Km or Kms per Kwh.
FYI I use $0.02 per km, when recharging on off peak.
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Post by Nevilleh »

Yes, you would. The problem occurs in that the current drawn from the mains by some chargers is not a nice sine wave and some "watt-hour" meters do not like the spiky waveform and so they don't read correctly.
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Post by bga »

The MS6115 will die after about 3 months of continuous use.
[add: I was going to hold a wake for mine, but threw it in the bin instead]

What about a nice cheap single phase KWH meter?
Like this

It's easier to read than the jaycar meters.
Last edited by bga on Thu, 08 Apr 2010, 15:02, edited 1 time in total.
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Post by antiscab »

my MS6115 (actually 4 of them) would all give different values between 10 and 30% of the actual load amount on all my SMPS backed loads.

for instance on my Vectrix, the charger pulls 1600W and 1750VA with unity displacement powerfactor.
run it for an hour = ~1.6kwh

my industrial kwh meter told me 1.6kwh
the MS6115's gave me values between 0.132kwh and 0.351kwh.

bga - the meters on that site is good, however, they are listing by the meters maximum peak current measurement ability.
if you look at the markings on the devices, the "63A" unit is only 15A, and the "100A" units are only 20A.

otherwise they look fine (and good prices too)

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Post by a4x4kiwi »

I can confirm that the Jaycar meters are way off, at least on my switch mode chargers.
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Post by antiscab »

having more of a rummage,

looks like the Efergy energy meter has similar problems with complex wave forms (read the comments).

the kill-a-watt meters seem to be ok according to this test.

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Post by acmotor »

On one hand, the only meter that 'works' is the one that reads the same as your supply authority meter.
I suspect they don't read what you might expect with complex waveforms either, although probably better (true RMS at least ?) than cheapies. But then supply authority meters are cheapies in $ terms ! (I doubt they cost more than $50 in bulk).

On the other hand, it would be nice to know real kW if you are having to generate your own electricity via PVs or wood fired boiler.

If meter reading error is largely linked to poor power factor and line harmonic errors then these should be addressed at the charger as we don't want to end up with many EVs charging and degrading the grid power ? Try some pf correcting line filtering before the SMPS rather than questioning the power meters ?
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Post by antiscab »

i doubt the cheap consumer kwh meters cost much in quantity either (the ms6115 is $9 if you buy more than 10 at electus, the single phase 5A(32A) good ones cost ~$16 if you buy more than 15, and the 3-phase ones are $50 if you buy more than 10)

the waveforms from SMPS aren't really all that "complex".
its just a spike as the grid voltage exceeds DC bus voltage.

anything pretending to be a energy meter should be able to measure it (and all the stuff intended for industrial use does)

the grid issue is more PF than voltage peak clipping as the voltage peak clipping doesn't get reflected to the medium voltage system, it stays local (and only DOL motors are really affected?)

while i agree PF correction is good, its better to look at it from a whole system POV (IMO)

say in perth for example, for every kva you pull from the wall, theres ~75W in losses in distribution.

take Rod's bad boy or example, PF is 0.4, but local eff is ~93% (power point to battery)
so at power station thats ~78% eff
only the decent meters read accurately.

take a zivan NG3, PF is 0.8, eff is ~86%.
this has passive PFC, again, only the decent meters read accurately.
eff power station to battery ~79%

or my vectrix charger PF is 0.92, eff is ~94%.
this has active PFC, but still not a sine wave.
eff from power station ~87%.

the really good charger ive ordered to replace the Vectrix charger (and put into my EV) has PF 0.99, eff 94%.
from ~87% from power station

all this charger comparisons only really help with new conversions, but I really wanted to measure the energy use of EV's that exist *now*.

very few EV chargers atm have any more than passive PFC.

anyway, im going to test out the ms6118 (the new cheap one).

i can make a consumer friendly kwh meter out of 5(32)A single phase industrial meter for ~$70 (including labour).
if i can find a pre-made one on the market thats accurate to within 5%, for about the same price, id be happy with that.

if anyone has any testing data on any energy meters on the market (or even has one that can be tested) do let me know :)

Matt

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Post by antiscab »

an update:

my energy usage meter arrived today, so i went about testing things.

Dave was over with the UWA Getz, so i measured how much the Zivan NG3 was pulling.
In: 2232W, 2938VA, PF:0.76
Out: 13A@146v

when i used it on my laptop power-supply over a period of time, the kwh reading matched that of my industrial energy meter.
That was 66W, 100VA (wow these powersupplies are bad, no wonder the ms6115 couldn't read it).

next test was my Vectrix:
1630W, 1680VA, PF:0.98
interestingly, the MS6115 gave the same reading (well it read 1680W and PF:1.00)
That makes sense as the waveform is *really* close to a sinewave thanks to the active PF correction.

I'll go around testing other chargers i have access to in more detail, and put them in a new thread.

but in the mean time this meter is the best bang for your buck so far.

That may change when i get a chance to test out the MS6118.

Matt

EDIT: The Vectrix charger when in standby also has a complex waveform that will fool energy meters like the MS6115.

both the MS6118 and the wattsup meter i sourced from the ATA said this:
0.5A, 0.33PF, 159VA, 53W.

Last edited by antiscab on Tue, 15 Jun 2010, 13:35, edited 1 time in total.
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Post by Johny »

Thanks Matt. Very useful post.
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Post by antiscab »

No worries,

now that I have a decent mains energy meter, Ill have another crack at testing chargers again.

Just have to make a big dump load.

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Post by antiscab »

today i received my MS-6118

I happened to be charging some batteries with some ecrazyman chargers (2 x 48v 3A) so I used both the known good cheap meter from the ATA, and the new jaycar one.

here were the results:
test 1:
DC: 2.1A @ 98.3V = 206W
ATA: PF: 0.51, 520VA, 270W, 2.1A RMS
ms6118: 266W, 2.1A RMS.

from that test, it looks like the ms6118 is as accurate as the ATA meter, but doesn't give nearly as much information.

A little while later, the readings became:
DC: 1.3A @ 113V = 147W
ATA: PF = 0.32, 340VA, 110W
ms6118: 110W

Thats a shame as the readings can't be accurate (that or I've just created over unity)

Tuarn happened to be around at the time (picking up my inverter) and suggested the power was in harmonics of higher order than the meters could read (big spike at mains peak voltage).

If I understand how these meters work correctly, the figure for Watts is calculated by multiplying instantaneous volts and amps (so the multiplication happens every 100ns and is accumulated).

the VA figure is derived conventionally from Vrms and Irms.

Those chargers were fairly small, so they don't have any input filtering. so it really was a cap across a rectifier bridge.
It was being run at 1/3 of rated power, so the spike had a high di/dt, and was of very short duration.

the duration was probably close to the sampling frequency of the meters.

any filtering reduces the di/dt, and increases the width of the pulse, and the problem goes away.

testing with a CRO time.

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Post by Johny »

Interesting again Matt. What are you going to be looking for with the CRO? Input current?
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Post by bga »

There's your problem. It only works in a gassy greehouse Image
What about gas fired power stations???

You have to wonder, with it being presumably microprocessor controlled and using ADCs, how come they can't get it to calculate RMS properly?

Really fast sampling isn't needed. In each second, there are 50 cycles. I suspect that statistical sampling would work fine provided that the sample size is large enough. For a 1 second accumulation, even the most miserly microprocesor should be able to manage about 1000 samples.
It may not even be important to latch V and I simultaneously to calculate a statistically accurate VI product.

Some research would be needed to determine the relationship between sample depth and error.

I don't think that this is a cost issue because the computation and accumulations aren't complex and shouldn't affect the price.

I think that it gets down to XXXXX importer not caring about details like 'it works nicely'.
They have no incentive to improve -- you bought one.

Hmm, come to think of it, so did I.

Last edited by bga on Tue, 25 May 2010, 09:34, edited 1 time in total.
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Post by antiscab »

Johny wrote: Interesting again Matt. What are you going to be looking for with the CRO? Input current?


current waveform shape, mainly how short a duration that current spike is.

the digital osciliscopes at uni have internal calculators that will give me a reliable RMS value, but it may be some time before it get around to testing that out.
bga wrote: You have to wonder, with it being presumably microprocessor controlled and using ADCs, how come they can't get it to calculate RMS properly?
I'm not sure the measured RMS current is wrong. if it is, it would be under reading due to the short pulse length.

bga wrote: Really fast sampling isn't needed. In each second, there are 50 cycles. I suspect that statistical sampling would work fine provided that the sample size is large enough.
so thats 20mS/cycle.
if the hardware is only meant to read up to the 7th harmonic, thats ~2.8mS/cycle.
~20 samples per cycle, then samples every 100uS, or 10'000 samples per second.

but the point where the rectifier bridge starts conducting, and where it stops could be only 200uS wide.
that means only 2-3

bga wrote: For a 1 second accumulation, even the most miserly microprocesor should be able to manage about 1000 samples.
It may not even be important to latch V and I simultaneously to calculate a statistically accurate VI product.
if not to simultaneous multiplication, how would you get an accurate value for Watts?
a statistical (RMS) value for V and I multiplied only gives you VA.

bga wrote: Some research would be needed to determine the relationship between sample depth and error.


yep, thats what I would hope to see on a scope.

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Post by bga »

Hi Matt,

I might have used 'statistical' incorrectly.

I would expect that the errors associated with, sequential sampling of V and I would largely cancel out on a cyclic waveform, since the opposite error is seen half a cycle later. Even slope differences should largely cancel because, while the error is greater, the time is less.

Mostly, the waveform repeats exactly on the following cycle, so the sampling can be spread out over multiple cycles.

Spikes will eventually be sampled, biasing the result in a way consistent with the energy in the spike.

One way to determine this would be to make up a data set and compute it using a synchronous VI algorithm and a sequential VI algorithm.
Most microprocessor ADCs can do 100K SPS, so 20us would be a good offset.

I think that errors we see with these cheap power meters point to something even more sleazy in the design, peak and zero crossing detection being used to infer the power factor and real power. Using ramp generators, comparitors and timers, the ADC could be avoided**.

We should never underestimate the lengths that an electronics maker will go to to save a fraction of a cent. I'm sure they're worse than car makers.

Cheers

Bruce

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Post by Mr. Mik »

The cheap meters have big problems reading the power consumption of devices with a low PF - do I understand that correctly?

If yes, then I have a question:

Will an el-cheapo meter measure the power consumption of a given device with low PF with the same error factor each time?

Or will the results be kind of random?

I ask because I would like to be able to tell (from the record of my el-cheapo meter) how long my "Special Freddy" charger has been running before it was turned off / turned itself off for some reason. It would help to decide if an EQ charge on a 240s NiMH battery has been achieved, or not yet.

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Post by coulomb »

Mr. Mik wrote: Will an el-cheapo meter measure the power consumption of a given device with low PF with the same error factor each time?

Or will the results be kind of random?

I'd say that the results would be consistent and repeatable, but I don't really know how they work, so I can't say for sure.

Anyone else care to hazard a guess?

For Mr Mik's situation, I think you could just measure how much energy a cheapo meter thinks it takes for a full equalisation, and each full equalisation (so much DC current for so many hours) should read about the same each time. If the equalisation stops half way, I'd expect the meter to read close to half the total. There may be slight variations due to the mains voltage varying, and possibly other loads having a small effect, but I'd say it would work fine. It's just that the number on the meter doesn't reflect the actual energy used by the equalisation charger, at least in standard watt-hours.
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Post by antiscab »

Mr. Mik wrote:
Will an el-cheapo meter measure the power consumption of a given device with low PF with the same error factor each time?

Or will the results be kind of random?


the MS6115 is a random number generator when used with spiky loads like your universal freddy. No two units wil agree, and the same unit used with the same stable load gives a value that jumps around.

the ms6118 and the wattsup meter give repeatable wrong results with spiky loads like your universal freddy.

so id suggest if wanting to use a meter to work out how long the balancing charge has run for, use a ms6118.

better yet, use a timer behind a diode on the DC side of the charger.
using the ms6118 would be easier though.

both the wattsup and the ms6118 seem to be of similar design.
there is some threshold of spikyness (a value of di/dt i mean) below which they will read accurately.
I just have yet to get around to measuring it.

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Post by antiscab »

I have encountered a new energy meter:
saing energy usage monitor

I ordered a couple for testing.

I tested one against my ATA meter today.

powering my laptop:
ATA: 57W 93VA
new meter: 54W

powering my 60 year old FLA charger:
ATA: 60W 90VA
New Meter: 56W

powering my desktop and a few other accesories:
MS6118: 120W 190VA
New Meter: 115W

All looks promising, it does look to be showing real power and compensating for poor harmonics.

What originally drew me to this devices wasn't the energy meter side of things, but rather the low power shut down.

When the total load goes below a user set real power value, it disconnects the load from the grid.

A few months ago I was looking for a product that would disconnect a Vectrix charger from the mains when it stopped charging.

due to a design flaw, it would draw ~40W ~100VA from the mains continuously while off.

I tried this Jaycar device, however, the standby load was above the cut off, and it never shut down.

The new China vision meter (for lack of a better name) has a programmable cutoff, that seems to work Image

Matt

EDIT: added URL

Forgot to add, the new meter, although reasonably accurate, averages things over a 15second time frame, so doesn't show transients all that well.

for steady state (like battery charging), its entirely fine.
Last edited by antiscab on Sun, 02 Jan 2011, 10:25, edited 1 time in total.
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Post by 7circle »

What can be used on the DC side?
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Post by coulomb »

antiscab wrote: ... so I used both the known good cheap meter from the ATA,
Except that this one is shown to read low with spiky loads as well. Perhaps "known not to be hopeless" Image

[ Edit: Oops! The post that I was replying to is from May 2010. Oh well, I guess my comments are still valid. In summary, Matt shows that the new meter reads close to the same as the ATA meter, but both of them read lower real power consumed than is being output at the DC end, which of course is not possible. ]
Thats a shame as the readings can't be accurate (that or I've just created over unity)
...
any filtering reduces the di/dt, and increases the width of the pulse, and the problem goes away.

It's frustrating that these meters seem to be weakest at handling what is one of the most common cases, i.e. the simple rectifier-capacitor case.

For our purposes, I guess they should always be used with some filtering. In fact, it seems a great pity that they don't incorporate a bit of filtering into the meters themselves. I suppose then they can't readily handle loads like an electric jug, without overheating and/or losing some power across the filter, making the result less accurate.

How big is a suitable filter for a typical smallish battery charger like the ones you are using?

Can they be bought economically, or perhaps made from parts you could get at Jaycar or similar?

I wonder if a little low power filtering of the voltage and current samples (this would have to be internal to the meters) would be more effective? Surely if it was, then at least some of them would do it.
Last edited by coulomb on Mon, 03 Jan 2011, 02:56, edited 1 time in total.
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Post by coulomb »

7circle wrote: What can be used on the DC side?

I would think that any digital multimeter measuring current and voltage would be fine. A true RMS meter for measuring current is probably not required, since I assume that the output of a typical charger would be quite smooth.

Of course, you have to have two meters, and multiply the results to get power, so you're probably asking whether there is a single device you can use that does the multiplication for you. I'm sure that there would be somewhere, but I'm not aware of any. Maybe some of the fancier clamp meters that read DC (i.e. hall effect based) can read DC power for you; you'd use the probes to read the voltage, and the clamp to read current. There are meters that read AC power that way, for example this clamp meter from Element 14 (formerly Farnell). The most expensive DC clamp meter they sell (a Fluke costing almost $1400) doesn't seem to have it.
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Post by antiscab »

coulomb wrote:
It's frustrating that these meters seem to be weakest at handling what is one of the most common cases, i.e. the simple rectifier-capacitor case.

For our purposes, I guess they should always be used with some filtering. In fact, it seems a great pity that they don't incorporate a bit of filtering into the meters themselves. I suppose then they can't readily handle loads like an electric jug, without overheating and/or losing some power across the filter, making the result less accurate.


Yeh, they're not hugely great at doing household loads, or chargers from really small scooters.

for chargers for 50cc equivalent mopeds and larger its fine, as those have passive PF filtering anyway (necessary to not max out a 10A powerpoint).

This particular meter (the china vision one) I intended specifically for the Vectrix, which has an active PF corrected charger.
The only time when the PF isn't corrected, the unit is supposed to disconnect it from the mains.

for other loads, perhaps a clipsal powermate would be better (but I don't want to spend the money to buy one Image ).

Matt

Edit: speeeling
Last edited by antiscab on Mon, 03 Jan 2011, 08:44, edited 1 time in total.
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