The function of LD is to output high level alarm signal when the fan speed is below the limit speed, such as 5000RPM of the fan speed standard of a certain specification, and to start alarm when the fan speed is below 70% (that is, below 3500RPM) . The limit speed value can be determined by the customer, generally default to less than 70% rated speed to start the alarm. The difference between this function and the Rd alarm signal is that the RD function does not alarm as long as the fan is running, no matter how much the speed is, as long as it does not stop running, while the LD function starts to alarm when the speed is lower than the set value. When the fan is used for a long time, there will be dust between the bearing assembly, or between the stator and the rotor, which may lead to a reduction in the rotating speed of the cooling fan, and of course, a bad component may also lead to a reduction in the rotating speed If it is used, RD function will not alarm prompt, but using LD function, it will alarm prompt replacement fan, so as not to fan too low speed unable to heat dissipation equipment temperature is too high, and can not work properly. 

The Working Diagram of Rd and LD functions is shown above

http://www.dxymotors.com