I've heard stories about people having old kWh meters and have eiter lower/higher bills than they should. I can understand how an old meter will show up short on kWh, my question is, how can the meter "overcharge" you.... with the parts inside moving faster
Josh, A kWh meter is a pretty simple sort of a thing, using only a Current (Series) coil and a Voltage (Parallel) coil to excite an Aluminium disc, in the crude form of a motor. The only real thing that will cause an upset is a shorted winding or two, in the Voltage coil. It can happen, but very rarely. What's the problem you have?.
A meter has two coils: the current and the voltage coil. Each of these produces magnetic flux proportional to the respective component, and the composite flux produces _torque_ on the meter disk, proportional to the power flow. This torque would tend to accelerate the meter disk. In order to make the _speed_ of the meter disk proportional to power, some drag is added. The torque reacts against the drag, and you get speed, which you then measure.
If something happened to the drag magnets, the meter would run much more quickly than it should.
It is important to note that meters are not "true RMS" meters. What this means is that anything that affects the waveform can result in the meter reading incorrectly- usually "fast." An example of this is "power factor." If the power factor of a load is low, then the meter will register more watts than are actually used. Correct with capacitors, and the power bill goes down!
Most things that can go wrong with a meter will cause them to read "low." Some customers have been known to do all sorts of things in the area around the meter to effect a lower reading. (I won't give examples, as I'm not running a class for cheats).
I have worked 12+ years in on site meter testing, calibration of electromechanical kWh meters and electronic ones.
Running fast, not often but is due to the weakening of the brake magnet. Around 1998 we still took out in Devonport, Auckland NZ, Ferranti FDb meters and Chamberlain & Hookham 10 Amps LR meters which were last tested in 1926. These had an average error of around 7 % at 10 Amps Inom. Considering the normal peak house loads of 40 to 60 Amps + @ 230 Volts the meter would be on overload anyway and the current coil in saturation with a meter error of - 20%. I have proven that on a test bench.
A lot of current coils had shorted turns where the varnish and insulation had burnt of and the metal cased meter was live at mains potential.
On older meters the weakening of the brake magnet and the wearing out of the rotor bearings tend to cancel out each other a little.
The Floton bearing meters 1970's ( Sangamo series ) did not have any major problems at all except Jamming jumping dials.
Most meters up to 50 years old are in general very accurate and overall errors are well wirhin the ±2½ % margin.
Some electronic meters do run fast, we had certain batches of meters around 1994 with capacitors on the circuitboards going faulty and the meter taking off. errors of 600% were found. This affected 1Ø and 3Ø meters. It was quite visible on the meterdial that the meter had a problem with hardly any load the led was blinking like crazy and the numbers of the dial rolling over. :
The product of rotation, excitation and flux produces electricty.
There was one that made the paper a few years back here in Windsor Ontario Canada.Apparently a family purchased an abandoned house that had outrages electrical bills.They complained to the poco for a few months but like everyone already said it's very rare for a meter to fail like this so they didn't believe him.Before they could come test the meter the special tactical unit showed up and kicked his door in looking for a grow op.Turns out a contract meter reader had turned them in.The write up in the paper said he forgot to lock the door before going to bed so he woke up staring up the barrels of quite a few automatic weapons and a bedroom full of cops in full assault gear.Can you imagine!They also said the poco was inundated with calls to check meters and that for a meter to fail like this is extremely rare.LOL
As noted, meters can run fast, although it is rare compared to how many run slow.
The braking magnet can be weakened by lightning hits. The worst I saw was 30% fast...but the lightning also fried everything in the house as well. The newer (>40 years old) alnico magnets are hard to mess up and the electromechanical meter is one of the most dependable, accurate mass produced products there is.
Considering bearing failure, register drag, debris on the disk, fire ants, fire ants, FIRE ANTS and (did I mention??) fire ants, there are lots more reasons for it to run slow. So look for a reason for the high bills BEFORE you call the POCO, because if the meter turns out to be slow, they WILL speed it up.
Can't comment yet on the electronic ones. They don't have that much of a track record yet in my neck of the woods.
Qoute: "If the power factor of a load is low, then the meter will register more watts than are actually used. Correct with capacitors, and the power bill goes down!"
I'll have to contest that. The single phase house meter is a Kwh meter. It will read watts accurately regardless of powerfactor. I think you are confusing Kva with Kw. There are meters that log Kvah and, in that case, you are correct that the Kva could be lowered by powerfactor correction. But not watts.
Quote: "Around 1998 we still took out in Devonport, Auckland NZ, Ferranti FDb meters and Chamberlain & Hookham 10 Amps LR meters which were last tested in 1926. These had an average error of around 7 % at 10 Amps Inom. "
Curious as to whether they averaged fast or slow.
[This message has been edited by WFO (edited 12-25-2005).]
In our area, the meters are all being converted to the digital read out type. Its no fun anymore, I used to love watching the disc spin in the meter on heavy user apps. I take it these are more accurate or harder to tamper with?
[This message has been edited by Sandro (edited 12-26-2005).]
We are going to smart meters next year i guess we will have to either literally swipe the meter with our debit card or go to the local convince store(7-11) and buy a card like the like the pre pay phones.oh joy...