What is impedance protection?
This is a method for protecting fan motors from burning that is activated when a motor operating at the rated voltage locks up for some reason with the power still being supplied. It raises the impedance (AC resistance) of the motor windings to suppress increases in current and keep the motor temperature from rising above a preset level.
UL standards, which stipulate no burning and a winding temperature of 150°C maximum after the motor is operated in a locked condition for 15 continuous days, are used as safety standards.
Impedance protection is used on OMRON Axial-flow Fans that have a diameter of 120 mm or smaller.