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Author Topic: How does an electricity meter work?  (Read 26232 times)

chris

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How does an electricity meter work?
« on: 08/01/2012 22:04:19 »
Dear electricity meter-ologists (he says, doing his best NeilEp impression)

I was asked by someone the other day "how does an electricity meter work?"

Can anyone help with a simple explanation?

Chris

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Re: How does an electricity meter work?
« Reply #1 on: 09/01/2012 00:37:03 »
The traditional meter usually has a metal disc that rotates as power is consumed. The disc is actually part of an electric motor that rotates at a speed that is proportional to the product of the voltage and the current being supplied. If there is voltage but no current flow, it does not rotate, and if there happened to be a current but no voltage (very unlikely), it would not rotate either.

The motor drives a train of gears that count the number of revolutions of the disc.

The only slightly tricky bit is that the disc reacts to the components of voltage and current that have the same phase relationship, and therefore it is measuring the actual power delivered. It really is recording energy delivered, usually in kilowatt-hours.

If, for some reason, the voltage drops and the lights go dim, the motor does slow down, so you are not paying for power that was not delivered.

RD

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Re: How does an electricity meter work?
« Reply #2 on: 09/01/2012 03:39:25 »
I was asked by someone the other day "how does an electricity meter work?"

i.e. "how can I stop my electricity meter from working?, (or run backwards)".   :)
« Last Edit: 09/01/2012 04:59:12 by RD »

CliffordK

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Re: How does an electricity meter work?
« Reply #3 on: 09/01/2012 04:03:31 »
I was asked by someone the other day "how does an electricity meter work?"
i.e. "how can I stop my electricity meter from working?, (or run backwards)".   :)
Oh, that is easy enough to do.
And, perfectly legal too.

Just attach a bunch of solar panels, and a "grid attached inverter".  There are specific code requirements that you will have to pay attention to.  Some people target running the meter backwards in the summer, and forwards in the summer...  to equal a net of zero.

There is a law in the USA requiring power companies to buy power from small power generators.  Unfortunately, it doesn't specify how much they must pay.

I thought the wheel thingy looked like an electric motor.

One can, of course, also run the system digitally by calculating the Volts, Amps, and time.  A bypass resistor is often used in circuits, so the majority of the current bypasses the sensitive electronics.  Or, I suppose, amps can be measured by inductance.

I actually have two meters.  The interesting one is on the house.  The main incoming power comes through some kind of bypass resistor or inductor.  And, only little wires go up to the meter.  Disconnecting the meter, and I still get power. 

Unfortunately the power company uses these funky seals...  And, eventually they will put in a meter that will not only say how much power is being used, but also when it is being used, so then they might look for suspicious usage patterns.

Geezer

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Re: How does an electricity meter work?
« Reply #4 on: 09/01/2012 07:01:44 »
I was asked by someone the other day "how does an electricity meter work?"

i.e. "how can I stop my electricity meter from working?, (or run backwards)".   :)


There may be an even simpler method. In the US the meters plug into a socket that is symmetrical. All you need to do is plug it in upside down and it will run backwards.

However, the utility companies go a bit fed up with this, so the newer meters have means to prevent this.

Geezer

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Re: How does an electricity meter work?
« Reply #5 on: 09/01/2012 08:42:35 »

i.e. "how can I stop my electricity meter from working?, (or run backwards)".   :)


I remember hearing a story about someone (no doubt a very close friend of a neighbor's brother-in-law) who concealed a powerful magnet in the door of the cupboard where the meter was. According to the story, when the door was closed the meter ran slow, but when the meter reader opened the door to read the meter, everything appeared perfectly normal.

Unfortunately, this is a load of baloney. The magnetic field from a permanent magnet won't do a dang thing to the induction motor in a meter.

However, if one was to arrange a for strong altern

Geezer

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Re: How does an electricity meter work?
« Reply #6 on: 09/01/2012 08:46:59 »
Sorry about that. My power was suddenly cut off.

syhprum

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Re: How does an electricity meter work?
« Reply #7 on: 09/01/2012 22:03:32 »
I have a small power checking device which I can plug into a socket and then plug the device which I wish to check into it and read the power consumption.
It has no rotating wheels just a small electronics board with a series resitor to meausure the current (about 0.01) ohm and seems pretty accurate and take power factor into account.
PS
If you buy one do not power it via a dimmer device as this causes its destruction                       

CliffordK

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Re: How does an electricity meter work?
« Reply #8 on: 09/01/2012 22:48:00 »
That is a point.  A lot of the Chinese stuff isn't made to last.
But, your power meter has to be pretty "bomb-proof".
It has to remember what the power usage was before every power outage.
It has to withstand ordinary power failures and power spikes.
It has to last for years with little or no maintenance.
And, they are often located outside...  Hot, Cold, Humidity.

Geezer

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Re: How does an electricity meter work?
« Reply #9 on: 09/01/2012 23:08:47 »
It has to remember what the power usage was before every power outage.

Yes - that's what makes the electro-mechanical variety so hard to beat. It's quite tricky to do it electronically.

syhprum

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Re: How does an electricity meter work?
« Reply #10 on: 10/01/2012 01:21:59 »
I own two properties both of which have had electronic meters for ten years with no problems, there is a simple explanation for the faliure caused by powering the device from a dimmer.
The low voltage for the electronics is derived from a bridge rectifier fed from the mains via a capacitor then on to a voltage regulator chip.
when you power the device from a dimmer the capacitor presents a much lower impedence giving a greater output from the bridge rectifier overwhelming the voltage regulator, a more expencive version could easily incorperate a more sophisticated PSU. 
« Last Edit: 22/01/2012 17:11:29 by syhprum »

Geezer

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Re: How does an electricity meter work?
« Reply #11 on: 10/01/2012 01:26:03 »
I own two properties both of which have had eletronic meters for ten years with no problems,

I wonder if they were made by the outfit I used to work for  :D

kowalskil

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Re: How does an electricity meter work?
« Reply #12 on: 10/01/2012 14:38:27 »
This thread reminded another problem with kWh meters. Most of them are designed under the assumption that the electric current changes sinusoidally, for example, only at 50 or 60 Hz. But this is not when the load is a ark lamp, or a welding device.
[irrelevant link removed. NB. Please keep personal info limited to your profile only]
« Last Edit: 11/01/2012 11:32:29 by peppercorn »

wolfekeeper

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Re: How does an electricity meter work?
« Reply #13 on: 20/12/2013 21:10:39 »

i.e. "how can I stop my electricity meter from working?, (or run backwards)".   :)


I remember hearing a story about someone (no doubt a very close friend of a neighbor's brother-in-law) who concealed a powerful magnet in the door of the cupboard where the meter was. According to the story, when the door was closed the meter ran slow, but when the meter reader opened the door to read the meter, everything appeared perfectly normal.

Unfortunately, this is a load of baloney. The magnetic field from a permanent magnet won't do a dang thing to the induction motor in a meter.
Actually, there is a permanent magnet in the meter to control the rate the disc spins at. It acts as an eddy current brake; otherwise the disc would run at close to the synchronous speed all the time.

If you strategically added a second magnet it would increase the drag on the rotor and the rotor would run slow.

Whether that's practical to do, I have my doubts. Also the electric company doubtless have meters that add up a district's electricity usage, and if the results don't tally, they would investigate.
« Last Edit: 20/12/2013 21:13:27 by wolfekeeper »

evan_au

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Re: How does an electricity meter work?
« Reply #14 on: 21/12/2013 21:24:04 »
In an old-style hand-held multimeter, which measures voltage, current and resistance, the mechanical movement of the needle is opposed by the mechanical force of a spring, producing a deflection proportional to voltage or current, or 1/Resistance, respectively. A rectifier can change AC into pulsed DC, which is averaged by the inertia of the needle.

In an old-style AC electricity meter, the mechanical torque on the aluminium disk is generated by magnetic fields which are the product of the current and voltage, which represents instantaneous Power. If both are +, the disk rotates in one direction; if both are -, the disk rotates in the same direction; but if one is + and the other - (such as happens part of the time when you connect an old-style fluorescent lamp with its inductive ballast), the torque tends to rotate the disk in the opposite direction, and opposes the motion. The inertia of the disk averages these torques, so that if you apply a current and voltage which are exactly 90 degrees out of phase, the disk just sits there and hums.

However, the electricity company doesn't charge you for Power, but for Energy consumption. Energy is the integral of Power, so the mechanical dials count the rotations of the disk to accumulate Energy. 

The rotating disk meters are very sensitive - when you work out the torque rotating them, it is miniscule. Their accuracy was improved many years ago when the ball-bearing holding up the disk was exchanged for a magnetic bearing, with no mechanical contact at all. However, there is still a requirement for a certain minimum power level below which the meter does not run at all, otherwise a completely idle circuit could run up a large consumption, given enough time.

The rotating disks are also very rugged - they withstand short-term lightning strikes reaching thousands of Amps. However, I saw one that was installed on a pump by a river - it stopped working after a flood which left a muddy brown line across the middle of the glass.

They are also very cheap, and the technology is very stable - when I was working on electronic power meters (many years ago), this new technology was regarded with considerable suspicion by the power industry - they wanted to be sure that the electronics would last 50 years, that the purchase price would be similar, and the idle power consumption would be no higher than the electromechanical meters. These basic requirements were not offset by a long list of new features - like the ability to measure power consumption & generation separately, the ability monitor supply voltage stability, or provide power consumption divided into 15-minute intervals, or remote meter reading. 

Electronic power meters calculate power by multiplying the instantaneous current by the instantaneous voltage (both of which are sine waves at 50 or 60Hz), producing the instantaneous power (which is a sine wave at 100 or 120Hz). The instantaneous power is then added up (integrated) to calculate energy consumed. Software can track the energy consumption in different counters based on the time of day.

If the only load is a triac-controlled incandescent lamp, the voltage from the street transformer is still a sine wave, but the voltage towards the load is zero for part of the sine wave. For this part of he waveform, Non-zero voltage x Zero current = Zero power.

CliffordK

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Re: How does an electricity meter work?
« Reply #15 on: 21/12/2013 22:30:57 »
I did have one of those dial type meters go bad last summer. 
I don't know exactly what happened, but for whatever reason my house has separate meters for the house and pumphouse/shop. 

I started an effort to decrease the power consumed by the pumphouse which would then leave the meter idle for days at a time (except perhaps a very small amount of power consumed by an electronic VFD on standby).

Anyway, eventually the meter just stopped, and failed to resume when I'd turn on the lights, or other power usage.

So, the next month the power company came out to replace it with a fancy new digital meter, and managed to bill me for power that I didn't use.

alancalverd

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Re: How does an electricity meter work?
« Reply #16 on: 26/12/2013 02:09:27 »
Quote
I have a small power checking device which I can plug into a socket and then plug the device which I wish to check into it and read the power consumption.                     

Can anyone enlighten me as to why one should buy such a device? The power consumption of every electrical machine I have ever bought, from a lightbulb to an MRI scanner, is written on the machine by its manufacturer.

wolfekeeper

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Re: How does an electricity meter work?
« Reply #17 on: 26/12/2013 02:30:00 »
There is no 'should', but the device keeps track of the actual power and energy used, rather than the worse-case peak power that is usually printed on the back. If something is being switched on and off a lot then you wouldn't normally know how much energy it's used.

For example, a fan heater is normally on a thermostat. How many kilowatt hours have you actually used if it's been on for 4 hours and it's a 3 kilowatt device. The answer is virtually never 12 kWh.

evan_au

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Re: How does an electricity meter work?
« Reply #18 on: 26/12/2013 04:09:47 »
There are a surprising number of people who can't pay their power bills on time.

With a gadget like this, people could work out how much it costs to run the air conditioner for an hour, compared to the lights, TV or the internet. They would then find out how incredibly cheap electricity is.

They might then be able to make sensible decisions about whether it is more economical to drive for an hour, take several people to the movies, or to stay home and read a book, watch TV, or visit this website.

They might - if they were sufficiently motivated by having their electricity rationed for non-payment, or if it were a homework assignment for the kids, or if floods caused loss of 10% of generating capacity and there was the threat of rolling blackouts.

syhprum

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Re: How does an electricity meter work?
« Reply #19 on: 26/12/2013 08:22:59 »
Alancalverd

The power monitoring device cost very little and as other correspondents have pointed out will show integrated power consumption rather than peak
« Last Edit: 26/12/2013 08:30:35 by syhprum »

alancalverd

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Re: How does an electricity meter work?
« Reply #20 on: 26/12/2013 08:48:09 »
There's a lot of nonsequitur flying around here!

If I can't pay my electricity bill, paying another £30 for a power meter isn't going to improve my ability to do so.

I know how cheap electricity is because I get nicely printed bills from the supplier.

If I have a thermostatically controlled fridge or fan heater, the mean power consumption over time depends on the ambient temperature (and indeed the contents of the fridge) so I'd have to monitor the device for a year in order to find out how much the electricity company is going to charge me for using it for a year, by which time they will have sent me a bill that conveniently includes all the other things I haven't monitored.

And if it turns out that my thermostatically controlled fan heater is running at, say, 30% mark/space ratio, how will that affect my behaviour?

The largest consumers of electricity in UK homes are cookers, immersion heaters, and storage or underfloor heaters, all of which are permanently wired and thus can't be monitored with a plug-in gadget. The few domestic airconditioners also tend to be hardwired. I've just installed an airsource heat pump: such modern contrivances display their (hardwired) electricity consumption anyway, and it turns out that the most expensive aspect of heating the 1500 sq ft new building will be the annual maintenance charge on the heat pump!

Most UK houses are now heated by gas or oil, so your largest energy consumption can't be measured with an electrical energy meter. 

My knowing that my computer is consuming 40W isn't going to alter the power company's decision to black out the district, or God's decision to strike the street transformer with lightning.

If a school wants kids to measure the power consumed by a light bulb, they can damn well supply the equipment. And what would a child learn from the assignment? At best, that a properly calibrated power meter tells you what is written on the light bulb. But how do they know it is properly calibrated? And of course it only applies to mains-voltage incandescent bulbs running at constant maximum temperature, so either the class teacher has to answer 30 different questions about power factors, duty cycles and starting currents, or the entire exercise is a waste of time.   

syhprum

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Re: How does an electricity meter work?
« Reply #21 on: 26/12/2013 10:22:43 »
My device cost £5.00 !

alancalverd

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Re: How does an electricity meter work?
« Reply #22 on: 27/12/2013 00:50:14 »
Fair enough. Do you cost your time involved in using it? Being selfemployed, I am very conscious of that parameter. My annual electricity bill costs me 12 hours' work, so it's not worth spending more than 72 minutes per year to reduce it by 10%, and I can't see how a power meter could achieve any reduction at all.

MrVat7

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Re: How does an electricity meter work?
« Reply #23 on: 21/01/2014 22:54:42 »
They work on principle of electromagnetic induction by eddy currents.

 

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