Simple way to detect if an A/c motor is on

I'd like to detect if a 120 VAC motor is on, nothing fancy, no voltage or current measurement needed, I just want to detect if the motor is on.

I was thinking of wrapping some wire around one of the power wires and feeding that wire to say A0 on my Arduino uno. Any voltage reading on A0 would be considered that the motor is on, maybe a high resistance pulldown resistor is needed or i can experiment with where the "on" threshold is.

I'm not sure if enough current would be induced into the wire such that the Arduino could detect anything

Anyone tried think this may work?

How about a clamp-on current sensor, and just look for a non-zero value on it?

That's probably the cleanest.

Thought about that, but i don't have one and my project is super temporary--hence 0 cost is ideal.

Basically my fridge is running odd and i want to monitor temperature and when the compressor is on. I've got a thermistor measuring temp and plotting temp every min on an LCD display (and saving the data to and SD card so I can plot temp vs time over say 3 days), but i'd like to plot compressor on/off with temp as well--hence why i need to determine when it's powered up.

I'll just give it a shot.

Thanks for responding.

While it isn't 0 cost, you might consider something like this load sensing dust collector switch - it is designed to turn on the dust collection system when whatever power tool plugged into it is turned on (handy for things like table saws etc.) Other versions exist out there - a bit of Googling around may discover cheaper ones, but at least they do exist.

Maybe mount an old speaker close to the compressor to act as a microphone.

Put another thermistor on the compressor or the cooling coils. They get hot when it's running.

MikeLittle

Now that’s not a bad idea, thanks.

Something else you could do is use a relay with a 110v (or 240 on the other side of the pond) coil and simply wire the coil across the motor terminals - when the pressure switch starts the compressor, it closes the relay and you could watch the relay contacts from the Arduino (the relay would provide isolation from the mains).

Basically my fridge is running odd and i want to monitor temperature and when the compressor is on.

Get a schematic of your fridge electronics and check each of the components in the circuit. Not sure what is odd, but get an understanding of how the components work. I had to repair my fridge some time back. The defrost thermostat in the freezer was not closing at low temperatures allowing the defrost heat strip to energize during the defrost cycle.

MikeLittle: Put another thermistor on the compressor or the cooling coils. They get hot when it's running.

Problem with that is the compressor casing stays hot when it shuts off. Can even get hotter due to the suction side gas flow stopping.

On the condensor would be ok but these days with some systems, the condensor is burried deep inside the casing, so unless you can detect the area in question...... One that has a fan cooled condensor coil underneath would work ok.

Choice of relay would be important if connecting across any compressor winding as in some systems, mullion heaters use the motor winding as a return path during the off cycle. i.e. they are wired across the thermostat. Point being, a high resistance coil may just remain energised even at the lower voltage.

No contact sensor sticks can be useful in these situations also but the thermistor idea on the condensor would be my pick.