Everything you need to know about BLDC motor configurations and FG and PWM signals
By Sam Brown, Master Samurai Tech/Appliantology.org
BLDC motors aren’t new technology in appliances anymore. In fact, they’ve become the norm.
As such, it’s important to be aware of the different configurations in which you’ll see these motors across appliances. These configurations fall into three categories: 2-wire, 3-wire and 4-wire.
An important thing to note before we continue: all BLDC motors in appliances are run by inverters. The inverter may be a separate board, like you’ll see in washers with BLDC motors, or it may be built into the motor itself, which is what you see with smaller ones like fan motors.
See: Drop-in vs. PWM Inverters
Our first configuration is 2-wires. This is as simple as it gets; you just have DC voltage on one line and DC ground on the other. These motors will run at one speed and one speed only when it receives power. Nothing fancy here.
Now 3-wires is where it gets interesting. Here’s an example of some 3-wire BLDC fan motors in a refrigerator schematic:
Naturally, two of those wires are still your DC voltage and DC ground. But what’s that third one doing, the one marked as FG in this diagram? That line carries the FG (frequency generator) signal. That’s a feedback signal consisting of a series of voltage pulses at regular intervals that the motor sends to the control board.
This isn’t something that the motor just does by itself; it has to be fitted with a sensor (usually a hall sensor) that can generate this signal every time the motor completes a revolution. The control counts these pulses, and then based on that calculates how quickly the motor is turning.
That’s probably the most common 3-wire configuration you’ll run into, but it’s not the only possible one. Instead of being a feedback line, that third wire could be for carrying a PWM (pulse width modulated) signal from the main control to the inverter. See here:
This signal is the control telling the inverter how quickly to run the motor. With a PWM signal, variable speeds become possible. However, in a 3-wire configuration with a PWM signal, the control wouldn’t be able to detect how quickly the motor is running — just as the control couldn’t vary the speed of a 3-wire motor that has an FG signal wire.
To determine which variant of the 3-wire configuration you have, the tech doc usually has to tell you. For example, we know that we have an FG signal setup on that first diagram because the third wire is labelled as such.
Last But Not Least
Finally, we have the 4-wire configuration. This has the best of both 3-wire variants. Two of the wires are, again, your DC voltage and DC ground. Of the other two, one will be your PWM signal from the control to the inverter, and the other will be your FG signal from the motor to the control. This way, the control can both vary the speed of the component and monitor its rotation.
Here’s an example of what one of these low-voltage, 4-wire BLDC motors looks like:
Now, you may very well ask: “How can I tell which wire is which? Do I just have to rely on the tech sheet to tell me?” Not necessarily! There is actually a wire color convention for these kinds of motors. It’s not universal, but if you see the specific combination of red, white, yellow and blue wires, then you know what’s on each wire.
Red = DC voltage
White = DC Ground
Yellow = FG signal
Blue = PWM signal
This wire color convention can sometimes help you with 3-wire configurations, too:
In this one, we can tell from the wire colors that the yellow wire is an FG signal.
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Sam Brown is “Professor of Appliance Repair Mastery” at BrandSource service partner MasterSamuraiTech.com (MST), an online training academy for appliance repair personnel, and is the administrator of its sister tech support site, Appliantology.org. For more information, email Sam at samuraitechhelp@gmail.com or call (603) 290-5516.
