[hinkley]

I don’t know about economical but there are devices that sense current flow in a line without a tap, just from the EM fields. People with wood shops use these to automatically turn on the dust collection system when any power tool is turned on.

Between small production sizes and the relay there are probably cheaper DIY solutions. OP is just using a smart plug, so no real electronics involved.

[gambiting]

The problem with those is that they only work if they are wrapped around a live wire, not live and neutral. So unless you have exposed L/N wires in your cable, it's not going to work at all.

[Scoundreller]

These clamp on systems may not work too well on North American 240V circuits like a dryer since the emf will cancel, no? You have two 120V feeds exactly 180 degrees out of phase with eachother. Unless you unravel the wires a bit and just clamp a single conductor.

Or maybe I have this all wrong.

[jtriangle]

You are correct, current clamps can't read accurately in this situation. All you'll get is any disparity between the phases, which, will be small.

That said, it's probably worth a try, because it's likely that internally the drier's control circuits are only tapping one leg, and you might be able to read that, and it might change reliably enough to trigger an alert.

Usually, you'd use a vibration sensor for this job however. You can also use a thermal sensor on the exhaust air, combined with a time delay (because they run cooldown/dewrinkle cycles), and you're good to go.

[pavel_lishin]

Huh, I thought 240V circuits used some sort of three-phase system.