Measuring current in independient circuits

Question

I'm doing a project to monitor and control 5 separate air conditioner units in my house.

The AC Units run in 240V power (two 120V phases), each unit has its own breaker in the box.

I'm using five current transformers, as outlined here: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface to detect the current flowing to the AC (<0.01 = off, between 0.03 and 0.8A just the fan, and 2A and up means full compressor running).

Everything mostly work, the only issue I have is that when I turn on a unit I get a current spike in another unit for a while, and that throw off my logs. I can confirm this reading with my clamp shell ammeter

I think this is because the cables are inducing current on each other (they all go through the same conduit).

Is there anything that I can do on the electrical side to get rid of this effect?

Answer 1

That spike is likely the compressor/fan/whatever turning on. Most HVAC units (and anything with a motor in it, really) will draw an immense amount of current when it turns on. That's why your house light dim when your heat pump kicks on.

I would do this: place a zener diode that's rated at whatever voltage you would expect to be at max current, and this will clamp the voltage at that. That way you will never see that spike, and you should just get a flat line on a graph if it tries to stay high current long enough (which would blow the AC breaker).

If 2.5A is the max current you expect, and at that current 5V are visible, then put a 5V zener (or maybe slightly higher) across the contacts like this:

^{simulate this circuit – Schematic created using CircuitLab}

For AC sensing:

^{simulate this circuit}

That should set you up so that you don't see that massive spike (which is perfectly normal).