220V Cooling Fan tachometer signal

"220V Cooling Fan" tachometer signal is generally output from the three-wire plug, three leads, yellow and black, respectively + 12V power and ground, another color of the line is the line speed signal output. It should be noted that some three-lead of a third wire DC fans are not speed signal output line, but the speed control signal line speed control signal input through it to "220V Cooling Fan" motor. So far, the fan has their own way of monitoring alarm and the speed sensor are two types of alarm sensors can use the fan speed is lower than an alarm signal when a certain threshold, while the speed signal output can be realized "220V Cooling Fan"Real-time monitoring of speeds.a