Electronic – What does FG stand for

I would try to get the technical information for the equipment it was salvaged from (start with Google). If you find that, it should tell you everything you need to know.

I would not assume that the color coding taken in isolation means anything. Lacking the service info, I would look at what I knew about the original equipment - was it run from household AC? Was the motor for a variable speed fan? A refrigeration compressor? Or...? Then I would try to determine type of motor it appeared to be: split phase? capacitor start, run or...? Next, get out the ohmmeter and trace out the connections. How many actual separate windings are there? Are some tapped?

Assuming this is from consumer grade equipment, it "sounds like" a fan motor with several run windings, each for a different speed, but that's very much a W.A.G., and doesn't tell you how to connect it. However, judging only from the size, it is probably far too low powered for a lathe.

EDIT: Thanks to fuggetaboudit, I did some more research, and the pinout, I'd guess, is:

Black = GND

Red = +12V

Yellow = Trip-Point Alarm (see option -04 in the document)

Green = PWM (option -06 in the document)

The document I'm referring to is here. In it, it says a suffix of -09 means:

PWM speed control circuit (option -06 or -56) terminated in a third lead wire (standard = blue) and an open-collector, non-latching, low-pass/high-fail, trip-point alarm circuit (option -04 or -54) terminated in a fourth lead wire (standard = yellow).

Given that your yellow wire acts like an open-collector output and your green wire controls the fan speed depending on an applied voltage, I'd bet this is the correct pinout.

Somewhat relevant: http://www.nidecamerica.com/apps_pwm.htm

I would be very certain that Mark is correct:

Black = GND

Red = +12V

Yellow = Tach Trip-Point Alarm (After checking the Nidec Datasheet)

Green = PWM

Every Nidec fan I've encountered has followed this color scheme, and most common 4-wire PWM fans in desktop computers also use the same color scheme. Empirical evidence, but given your measurements, I'd be willing to bet they're correct.

If you have access to a signal generator, try generating a 25kHz, square-wave, 0-5V signal to the green wire, and seeing how the duty cycle of that signal affects the fan speed.