[Ben Krasnow] Explains Kilowatt Hour Meters


[Ben Krasnow] is back, and this time he’s tearing down a kilowatt hour meter (kWh). While not as exciting as making aerogel at home, or a DIY scanning electron microscope, [Ben’s] usual understated style of explaining things makes a complex topic simple to digest.

These old mechanical meters have been a staple on the sides of houses and businesses since the dawn of commercial power. We always thought the meters were a basic electric motor. Based upon [Ben’s] explanation though, these meters are a complex dance of electromagnetic fields. Three coils create magnetic fields near an aluminum disk. This creates eddy currents in the disk resulting in a net torque. The disk spins, turning a clockwork and advancing the dials.

Why three coils? One is a high turn high gauge voltage coil, and the other two are low turn low gauge current coils. The voltage coil has to be phase shifted 90 degrees to create the proper torque on the disk. Confused yet? Watch the video! [Ben] does a much better job explaining the field interactions than we could ever do in text.

One interesting point is the fact that the disk is so tightly balanced that a small “anti-creep hole” is used to disrupt the eddy currents ever so slightly. This hole keeps the disk from slowly turning when no load is being placed on the meter.

[Ben] doesn’t mention the old tales of stopping electric meters. We all remember those ads in the back of PopSci and other magazines. While impractical, apparently some of the meter stopping techniques do work. It should be noted though that power companies will notice changes in a given customer’s electric consumption, so we can’t recommend trying that one at home.

27 thoughts on “[Ben Krasnow] Explains Kilowatt Hour Meters

  1. These old meter is also easily hacked or manipulated. People have tried placing strong magnet near the meter, which affected the field and made it read at the wrong rate. Electric company usually catch these people. The newer digital meter can’t be influenced by magnet.

  2. the digital meters are easier to hack and maybe even get away with it because of lax security in smart meters someone can hack into it and change the amount before it phones home.

    the newest meters only phone home once a month and can be hacked.

    1. and when they come around to inspect the meters, you are screwed

      In my town they physically inspect the meters randomly.

      Feel free to advocate breaking the law, all this stuff is recorded by the NSA and WILL be used against you someday.

      1. and let me guess, you will be sitting on your high-horse because you stuck up for the government when that day ‘supposedly’ comes? Chah-right bro! you’ll be down in the dirt with the rest of us and will be forced to OBEY because you won’t know how to fight back. I’m willing to bet you’re such a nice guy you never even illegally downloaded anything. Congratulations, your reward will make the nihilist in you gleam without pleasure or disposition because your reward is absolutely n-o-t-h-i-n-g.

      2. I see no advocating of breaking the law. only mentioning the fact that it can and is being done. No different from you warning that there are physical check-ups (Now all the criminals know that thanks to you!!!)

    2. The newest meters also have dozens of different fraud detection sensors (cover, m and will raise an alert if you attempt any kind of tampering. Also any kind of “hacking” attempts via the opto port will also be logged and will raise an alert flag.

      All this depends on the brand of meter and the security options enabled by your provider though. YMMV.

      Source: I do this stuff for a living.

      1. Just like you continue to improve security (hopefully) the others are continually improving their hacking techniques. That is pretty much how the world works – what is perfectly safe today is horribly unsafe tomorrow.

  3. The design of this meter is to compensate the metric with “power factor” and that wasn’t mentioned.

    Take for instance a large solenoid. It may draw a massive current and convert that to kinetic energy. It would then convert that kinetic energy back to current that is returned to the power source (grid). You don’t want to be charged for all this current that in reality is being returned. You just want to be charged for the actual power used for the load and friction. This sort of thing happens with inductive loads such as motors.

    The copper introduces a phase shift of 90 degrees by conversion from magnetic energy to eddy current, this eddy current then causes magnetic energy 90 degrees later. This is because magnetism is not caused by voltage, it is caused by change in voltage or cosine when compared to sine.

    Cosine between the current phase and the voltage phase represents power factor.

    All of this occurs because at the end this meter is an accumulator so instantaneous power is not useful metric and hence the need for power factor compensation.

    Household power represents a complex combination of in phase resistive loads like heating elements and many different inductive loads all of varying phase relationships. The double magnetic to eddy current conversion allows the spinning disk to resolve these complex loads to a total that includes power factor compensation for all the various phase relationships.

    The shape of the disk is in itself the mathematical formula for RMS (root mean squared) or true power.

    Eddy currents induced into the disk have a linear relationship. However when those eddy currents collapse into a magnetic field the lines of flux concentrate to the outer extremes due to the nature of magnetism providing the squared function of RMS. ie the relationship is Pi * Radius Squared where all of this occurs 90 degreed out of phase to provide the function cosine.

    So this is a great article and I learned something new about the hole in the disk for a fixed load.

    Now that I see all of this it’s amazing that someone designed such a complex calculator by using the laws of physics with not one transistor or micro controller in sight.

    1. I have an electronic meter now but I used to have one of these electromechanical ones and I vaguely knew that they were based on phase difference so I wondered what effect having a large inductive load would have on the rate of spinning of the disc? To power my 12v lights I have an massive, ex-military 240 -> 12volt transformer (I have to mount this on rubber standoffs to reduce hum through the ceiling) and was worried that the phase shif it induced might cause my meter to run faster or slower. Can you tell me if it has an effect or not please?

    2. Thanks Rob, that’s an excellent explanation. Ben Krasnow does mention the two current coils at the end of the video and asks for an explanation, could these also be for power factor compensation? Similar to RCD devices where the two coils produce a net force when the current leaving doesn’t equal the current returning.

    1. Quite the opposite. The mechanical power meter is a brilliant piece of design. Read “Rob”s explanation in his post above. New “smart” meters are primarily electronic so they can report your power usage vs. time back to the power company (and the Government), who will then use that information to control your life by extracting more money from you with high “usage fees”. Imagine a greedy ISP or Cellular Operator with data-plan caps and over-use fees running your power system – that’s what they’re aiming for.

    2. I must be living in third world relics (and USA isn’t helping by being shut down). I have 2 of the old analog meter on my house. One that uses incorruptible power (cheaper, runs risk of being cut off during unusually heavy electric usage) for the hot water heater and other on regular rate for the rest of the house.

  4. So how easy to recycle are modern smart meters compared to classic technology like this?

    Also, how long will the modern smart meters last (given that cryptography loses effectiveness against brute force attacks as computers get ever faster).
    The old style meters will last for decades, how well will smart meters hold up?

    1. It’s been pretty easy for a while now to use encryption powerful enough, that if you built a computer out of all the atoms in the Universe, it’d still take longer than the projected lifespan of the Universe to brute-force it. I can’t remember the source of this claim but it’s out there somewhere.

      I can’t remember exactly how powerful, something like 512-bit keys. Something certainly usable with modern PCs. After all, for each bit, you have to brute force twice as many combinations.

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