Tag Archives: alternative current

Why does electricity hum — and why is it a B flat in the US, and a G in Europe?

The electricity hum (also called the “mains hum”) emerges because electricity runs on alternating current (AC), which transposes voltage in the pattern of a sine wave. In the US, the frequency of this current is 60 Hz, which creates a tone almost exactly halfway between A♯ and B. In Europe, it’s 50 Hz tone, which is closer to a G. If that doesn’t mean much, let’s look at it in a bit more detail.

Alternative current

There are two types of current: direct and alternative. Direct current (DC) is a unidirectional flow of electric charge; think of current flowing from a battery, that’s DC. Alternating current, on the other hand, is less intuitive. As the name implies it, it alternates — it periodically reverses direction. Alternating current is how electric power is delivered to businesses and residences, where its voltage can be increased or decreased with a transformer.

As a result, while the voltage of direct current is a straight line, the voltage of alternating current is a wave, most commonly as a sine wave, as can be seen below.

Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage. Typically, it oscillates tens of times a second. Image via Wikipedia.

Our modern society is pretty much built on alternating current. All the devices in our day to day life need current, and that almost ubiquitously comes as alternating current. But as James Clerk Maxwell showed in the 19th century, electrical current and magnetism go hand in hand. Sometimes, stray magnetic fields cause the enclose and accessories to vibrate, generating an electric hum. Secondary sources are magnetostriction and corona discharge around high voltage power lines.

The “hum” intensity is a function of the applied voltage, which brings us to the next point.

Current and sound

In music, a note is governed by its pitch, which is based on the frequency of the sound. The higher the frequency, the higher the pitch. Generally speaking, a scale starts with A, which is 440 Hz. If you go an octave below, you’d get another A at 220 Hz. The next (lower) A is at 110 Hz, then at 55 Hz, and so on. Since the electrical hum depends on the frequency of the current, so the sound will pretty much have the same frequency as the current. In the US, the current frequency is 60 Hz tone. The 60 Hz tone is almost exactly halfway between A♯ (58.24 Hz) and B (61.68 Hz). However, the 60 Hz frequency is pretty much only used in the Americas (mostly), Saudi Arabia, South Korea, the Philippines and about half of Japan. The rest of the world uses 50 Hz current frequency, which means that the note resulting from the electrical hum is closer to a G (a bit sharp).

Hums can also appear at the frequency harmonics, though with a much lower intensity. So in the case of a 60Hz current, you could have some humming at 120 Hz, 240 Hz, and so on, up until very high frequencies. There’s an entire spectrum of electrical hum. In terms of sound, that means you not only output the original note, between A♯ and B, but also the same note one octave higher, and another one, and so on.

The spectrum of an example of mains hum at 60 Hz.

Getting rid of and using electrical hum

Generally speaking, the hum is an annoyance, especially in musical instruments that involve electricity. At a venue, this electrical hum is often picked up via a ground loop. In order to fix this, stage equipment often has a “ground lift” switch which breaks the loop. An alternative way to fix this is the audio humbucker. Electric guitars especially (and sometimes microphones) use one or several humbuckers, which are basically two coils instead of one. The two coils are arranged in opposed polarity so that the AC hum is cancelled, while still producing the intended signal for the sound.

However, electrical hum can sometimes be important — especially in forensic analysis. Forensics use a technique called Electrical Network Frequency which allows them to validate audio recordings. They compare how the frequency changes in the background mains hum to a pre-existing database. Basically, they use the mains hum signal as a digital watermark which can identify when the recording was created and help detect any edits in the recording. In the German federal state of Bavaria for instance, this technique has been used by authorities since 2010.

Electronic energy meters might be off by almost 600%, Dutch study finds

While electricity meters can vary widely from country to country, a new study has revealed a worrying fact about them — they can be widely off.

Test situation for this study. Credit: Image courtesy of University of Twente

Energy meters are a funny thing. Almost everyone has them, but we almost never think about them unless something goes wrong — case in which we hope it gets fixed as soon as possible so we can go back to not thinking about them. Still, at the end of the day, we all pay the electricity bill, and that meter decides how big the bill is. They also come in a great variety, and they are different from country to country. A few decades ago, they traditional energy meters (kWh) were all over the place, but now, electronic devices are being used in their stead more and more. A study carried out by researchers from the University of Twente and The Amsterdam University of Applied science wanted to see just how accurate these devices are. Their findings are truly worrying.

I mean, we all know that someone who thinks “the corporations are ripping him off,” but this time, he may be right. Kind of. The Dutch scientists analyzed 9 meters, finding that in some setups, these were up to 582 percent higher. Two of them also outputted 30% lower figures and overall, there was a disturbingly high inaccuracy.

The experiment design

Using a switchboard, researchers connected the meters to a range of power-consuming appliances, such as saving light bulbs, heaters, LED bulbs and dimmers. They purposely avoided using complex or unusual equipment or appliances, replicating conditions you’d find in an average house. Cees Keyer, one of the researchers involved in the study, explained:

“OK, these were laboratory tests, but we deliberately avoided using exceptional conditions. For example, a dimmer and 50 bulbs, while an average household has 47 bulbs.”

After a while, they compared the actual power consumption to what the meter showed. Although it’s quite a small sample size, if this sample size is relevant for the entire country, then at least 750,000 Dutch households have faulty energy meters which show either more or less than what they should. Since there’s no reason why Dutch meters are inherently worse than those in other parts of the world, the findings might be applicable to many other countries.

Their experiment is easily replicable, and they encourage others to do so, as this might be a massive problem in many parts of the world. The Van Swinden Laboratory (the Dutch Metrology Institute) has already conducted a countercheck and confirmed the findings.

Why this happens

The meters in question were manufactured between 2004 and 2014, so they were not old, rugged models, but rather modern ones. Frank Leferink (Professor of Electromagnetic Compatibility at the UT) points out that:

“The energy meters we tested meet all the legal requirements and are certified. These requirements, however, have not made sufficient allowance for modern switching devices”.

But it’s not like the electricity company is trying to rob you. Rather, it’s a case of technology evolving too fast for us to adapt to it. The biggest problems were encountered when they tested energy-saving appliances.

Alternative current is called this way… well because it alternates. Think of it as a continuous sine wave, going up and down, up and down all the time. Energy meters use this sinewave to detect the current, but when you have energy-efficient switching devices, they alter this waveform and create an erratic pattern, which confuses the meters.

Researchers associated found that the meter design is also very important: those with excessively high readings contained a ‘Rogowski Coil’ while those associated with excessively low readings contained a ‘Hall Sensor’.

Testing your meter

If I’m a consumer, however, I care less about Rogowski Coils and Hall Sensors and more about how I can test my meter and make sure it works properly. I want to test it. Well, this can greatly vary from country to country. In the US, most states have a Bureau of Weights and Measures which you can contact for testing your meter. In other parts of the world, there are other accredited inspection companies or bureaus which can test your meter. The problem is, researchers say, that standardized tests don’t make allowance for waveform-contaminating power-consuming appliances. So the test could fall short in exactly the same way your meter does. Therefor, Leferink recommends any consumers who doubt their meter readings to contact their supplier, who will hopefully pass the concerns on to the power grid operator.

Journal Reference: Frank Leferink, Cees Keyer, Anton Melentjev. Static energy meter errors caused by conducted electromagnetic interference. IEEE Electromagnetic Compatibility Magazine, 2016; 5 (4): 49 DOI: 10.1109/MEMC.2016.7866234