We've covered BLDC motors with built-in inverters in the past -- they're the small kind of BLDC motor that you find used most often as fans in refrigerators. But I thought it might be useful to cover how you can you tell what's on which wire if you have incomplete specs.

Without further ado, we're using a GE refrigerator evaporator fan motor for our case study. Let's say we look at the schematic, and we see that there are four wires going to the fan, but we get no info there on what the function of each wire is. This is a real situation you run into sometimes!

We look elsewhere in the manual, and here's what we can find for specs:

Here's our challenge: from just these specs, can we tell what the function of each wire is?

Let's start with identifying our four wires: we have red, white, yellow, and blue. We know from our previous studies that these four wires will each be one of the following: a PWM speed signal, an RPM feedback signal, a DC voltage supply, and a DC ground.

If we read through these specs, it actually becomes clear pretty quickly which wire is for what just based off the amount of voltage they give for it.

Red is obviously the DC voltage supply. Not only is that the conventional wire color for DC voltage in these small BLDC motors, but that's the wire where the specs give the highest voltage value. And 13 volts is right about what you expect for the voltage supply to one of these fans.

White is also pretty obvious -- the big giveaway is that it's used as the reference for all of these voltage tests. That means it must be DC ground.

Yellow has to be either our RPM feedback or our PWM speed signal. The key to determining which is that they give different specs for high and low speed -- that has to be a PWM speed signal -- the signal from the control board telling the built-in inverter how fast to run the fan.

Blue is of course, by elimination, the RPM feedback signal. It's just a small DC voltage generated by a Hall sensor on the motor which tells the main control how quickly the fan is running.

So there you have it -- even though they didn't explicitly tell us the purpose of each wire, you can tell just from your prior knowledge of how these BLDC motors work just by looking at the given voltage specs. It's important to be able to work with incomplete information, because you'll run into that in the field.

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