Measuring power network frequency using junk you have in your closet
35 points by calvin
35 points by calvin
This is a hack that’s even neater than the author gives himself credit for! He says:
The proper way to do this would probably be to step down line voltage to something more manageable and then just record that signal directly and find the dominant frequency component in that
…and while there are mains frequency meters that work like that, what the author is describing (an audio cable wrapped around the power cable a few times) is kind of like a pretty bad current transformer, and that’s exactly how a bunch of monitoring equipment (including frequency meters) work. Well… maybe not exactly, they’d use a better (edit: or, perhaps more accurately, an actual) current transformer, but the basic logic is the same.
This sort of system is useful for two reasons. First, it allows you to monitor frequency on lines whose voltage is otherwise inconvenient to step down. Think 110 kV grid lines, it would really suck if you had to step that down to 3.3 V. Second, it requires no conductive contact with the load circuit, so it’s easier to install, safer to operate, and less susceptible to damage in case of a bad network event.
Edit: it’s late and I didn’t exactly read this through, I now realise the author is wrapping the audio cable around the whole power cable, i.e. including the return wire. This isn’t quite the same as a clamp meter :-D. The author is probably picking up mains hum, which is… I think an even neater hack than he is giving himself credit for!?
Yes, picking up mains hum is what the author is trying to do.
I couldn’t replicate this at all with an 120 V system. There was no audio signal from the wire
I don’t know if the wire voltage has much to do with it. A long time ago, when I lived in an apartment made in the 1980s with a very dubious electrical installation, I could reliably pick up mains hum like that, probably because there was always some non-zero voltage on the ground wire. I just tried it where I live now and it’s either not there, properly filtered out by my sound card (!?), or drowned in noise. Both are on 230V systems.
I love this!
I wonder if this could be adapted to measure amperage, perhaps only “relatively”; e.g. “this device takes twice as much current as this one”
That’s really interesting! I wonder how this was actually done - I’m far from an EE, but (to an outsider) synchronizing two separate power grids sounds pretty hard to do. I’d love to read a writeup about this.
It also turns out that there’s a pretty interesting Wikipedia article about this: https://en.wikipedia.org/wiki/Wide_area_synchronous_grid