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Topic: How, specifically, does an electric meter measure wattage? (Read 11757 times)previous topic - next topic

draythomp

Good one Lefty!

I can think of a couple of other possibilities off hand.  If you only need a little power, just wrap some wires around one leg of the incoming before the meter and suck some power out inductively.  The other one is to use something that has a very low power factor and returns everything back to the power company as reflected power.  I used to do this with a fluorescent tube, just connect the ends without turning on the filaments then strike a spark with something else to get it to fire up and away you go, free light.  You can also heat the filaments with a battery and get it to fire and disconnect the battery; big battery though.

A long, long time ago with completely different meters than we have today, we would use a really powerful magnet and retard the aluminum disk by putting the magnet right up against the meter's glass and holding it on with duct tape.  This wouldn't stop the meter, but it would slow it down noticeably.  Doesn't work with any of the new meters though.

You could also slip a bribe to the guy that reads the meter.

strykeroz

Did he specify what meter he'll be using?
"There is no problem so bad you can't make it worse" - retired astronaut Chris Hadfield

mauried

About the only way I can think of doing this is to try and exploit the accuracy limitations that all
electronic electricity meters have.
Most modern electronic electricity meters are Class 1 meters which have a variety of accuracy specs
depending on how much energy they have measured.
A modern utility meter is Class 1 with a very tight requirement to be accurate to +/- 1% down to 5% of load.
If the Meter has a maximum current rating of 100 A, then the meter will be accurate within 1% down to
50 ma , so anything below this means that the meters accuracy is not defined.
If the supply voltage is 240 V , then a load of less than 12 watts will be below the meters minimum accuracy standard
and whilst the meter may measure this amount of power, you could most likley argue that the reading is not accurate.
Running a 10 mw led of such a meter would most likley not be recorded at all.

PeterH

#18
Jan 09, 2013, 02:18 pmLast Edit: Jan 09, 2013, 02:28 pm by PeterH Reason: 1
I've no practical experience of how electricity meters work, but I seem to remember when I was taught about power factors being told that these meters typically measure current, and that the mean square adjustment and integration over time was then used to calculate the transferred charge and hence energy. This was to explain why industrial consumers were concerned about power factor - because they are charged for current but get value from energy so they want the power factor to be as high as possible so they don't pay for energy they didn't use.

Given that these devices are specifically designed to measure current accurately over a wide range of conditions, I doubt that it will be easy to cheat them. You could demonstrate reducing the power factor so that the meter shows power that you didn't get, but there's no way (that I know of) to get a power factor greater than 1.

What scope do you have to interfere with the power supply coming in to the meter? Is it entirely under your control, are you allowed to use inductive coupling etc, or is it completely inaccessible to you? If you only need to demonstrate power going through the meter without being measured and the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

David82

....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

what do you mean by this?

David82

Would inductive coupling be detected by the meter?

Quote
We have to power something, like a light bulb without the meter registering it.

That seems a very strange task, what exactly are you supposed to learn from it?

If you don't wire round the meter as suggested then the power will flow through the meter which will register it unless you tamper or mess with the meter in some way.

retrolefty

How about you try powering a light bulb with a series capacitor, that may throw off the phase angle enough to cheat the meter. Just be sure you use a non-polarized cap rated for well above the line voltage you are using. Look for 'motor starter capacitors'.

Lefty

PeterH

....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of a very high frequency AC well outside the range the meter is designed to pick up.

what do you mean by this?

Well, if the meter is designed and optimised to detect AC current then it may not detect DC current - or may not detect it well. If you have complete control over the circuit and can arrange for your supply to be DC rather than AC (or to have a significant DC component in addition to the AC) and IF the meter does not measure DC current then you'd be able to get power through the meter without it registering.

Alternatively, if the meter has been designed to selectively pick up mains frequency signals then it may not be able to pick if signals at a much higher frequency. For example, it's possible that if you send an RF signal down the line, the meter would ignore it (it would be filtered out). I'm speculating, because I have no idea what range of frequencies the meter deals with, but if you have full control over the circuit then it would be worth a try.

I don't feel optimistic that any of these approaches will work, but if you have a meter and time to play with it, what do you have to lose? Probably the most promising way to obtain power without it registering at the meter is to cheat. But that's hardly going to get you any credit when it's found out.

David82

How about you try powering a light bulb with a series capacitor, that may throw off the phase angle enough to cheat the meter. Just be sure you use a non-polarized cap rated for well above the line voltage you are using. Look for 'motor starter capacitors'.

Lefty

yea, AC compressors use them right? So digital utility meters measure the phase angle to measure current?

HazardsMind

Are you allowed to make a Tesla coil? You wirelessly power something if it was close enough. I'm not sure if you will be able to measure the current or not. If you think about it, the Tesla coil would be the power source and what your powering would be the load, and since they are not directly connected, you might not be able to measure any current.

It's just an idea.
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https://github.com/AndrewMascolo?tab=repositories

David82

....the whole circuit is under your control, then you could try either using DC (I have no idea whether meters would detect that) of
Well, if the meter is designed and optimised to detect AC current then it may not detect DC current - or may not detect it well. If you have complete control over the circuit and can arrange for your supply to be DC rather than AC (or to have a significant DC component in addition to the AC) and IF the meter does not measure DC current then you'd be able to get power through the meter without it registering.

Alternatively, if the meter has been designed to selectively pick up mains frequency signals then it may not be able to pick if signals at a much higher frequency. For example, it's possible that if you send an RF signal down the line, the meter would ignore it (it would be filtered out). I'm speculating, because I have no idea what range of frequencies the meter deals with, but if you have full control over the circuit then it would be worth a try.

I don't feel optimistic that any of these approaches will work, but if you have a meter and time to play with it, what do you have to lose? Probably the most promising way to obtain power without it registering at the meter is to cheat. But that's hardly going to get you any credit when it's found out.

We can't control the supply. It's just 240v. 60Hz. We can only create circuits after the meter. Does this rule out your ideas?

PeterH

We can't control the supply. It's just 240v. 60Hz. We can only create circuits after the meter. Does this rule out your ideas?

Yes, it does. I don't think there is any practical way to get current through the meter without being registered by the meter, without tampering with the meter or upstream wiring.

draythomp

OK, here's what you do.  Get about a meter of EL wire.  Get a lightbulb, dismantle it, and remove the filament and glass support.  Shove the EL wire into the bulb and solder the end of it  to the base such that you can plug it into a light socket.

When you screw the bulb in, it will light up.  This will use....maybe 15ma...  You'll have to wait a few days before the meter would record anything, .....if ever.

The color and intensity won't be up to what EL wire is capable of since you're only running it at 60hz, but it's a light that can be switched on and off.

David82

Is it possible to draw a lot of current faster than the meter can detect? If so, how would you do that? By charging a bank of capacitors??

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