Spotting a Microwave Oven in the Wild

Microwave ovens are an almost universal feature of modern kitchens.

How this came to be is an interesting story in itself. Here I want to share some of the clues appliances offer in their electricity consumption patterns, and how Sense uses these clues to detect devices.

As background to my explanation, let me first provide a little information about the operating principles of a standard commercial microwave oven. The core of such an oven is the cavity magnetron, which generates a stream of high-frequency electromagnetic waves (microwaves) from an electric power source. The magnetron is an extremely ingenious device: it consists of a cathode which produces a steady stream of electrons, surrounded by an anode. When suitable high voltages are applied to both anode and cathode, the cathode electrons induce currents in the anode, which in turn lead to microwave-frequency resonances inside of the anode. These oscillations are tapped with a strategically-placed antenna, and directed to the interior of the oven where they do their magic with our popcorn, pizza or pork chops.

The wattage-against-time signature for one run of a typical microwave oven is shown below.

(The signature is a snapshot lasting 18 seconds, of which the highlighted section spans about 3 seconds, and the steady-state power with the oven in operation is about 1700 W above the baseline wattage). So, can you explain the main features of this signature after reading the relevant Wikipedia section?

Let’s work backwards from the major power sink that is consuming around 1700W for the last 13 seconds or so of the signature. Since the magnetron is the main source of microwaves, this must be the signature of the magnetron as it is drawing current to produce those heating excitations. The slight waviness of the power consumption confirms that this is the case: the resonance currents are the result of a complicated dynamic system, and is known to result in some variability in the microwave power produced. This, in turn, means that the electricity consumed by the magnetron is somewhat variable, as we see in the signature.

The next most noticeable item in our signature is the sharp peak near its beginning. Remember, this is wattage against time, so something is drawing a large amount of current for only a brief amount of time. The peak actually results from two different components being charged up - in some microwave ovens, they in fact show up as two separate peaks, separated by about 100-500 milliseconds. These components are a high voltage transformer and a capacitor used for voltage doubling; both produce a strong surge current when voltage is first applied.

And what about the region between these two events? There is clearly some non-negligible energy consumption during that period (around 230W over about 1.5 seconds for this sample), and this has to be the current drawn by the cathode as it warms up and spews forth the stream of electrons that will drive the magnetron during steady state-operation of the oven.

Is that all? Well, not quite … If you look at the section before the initial transformer-capacitor surge, you see that the power consumed drops by a few watts for a brief period. Negative wattages can only mean that something has turned off - in this case, probably the oven light which turned off automatically when the oven door was closed.

These four elements are typical of many microwave ovens, but there are many variations - so many, in fact, that I hope to write a blog on some of the typical variations in the near future.

Now that you have read about how the Sense home energy monitor detects microwaves, are there specific devices you are curious about? To learn more about how Sense conducts device detection to identify specific appliances drop us a note at feedback@sense.com or tweet at us @tellsense and let us know which appliances you would like to read about next.