Rotation speed control systems for an Electronically Commutated (EC) motor and method for controlling rotation speed of an EC motor are described. An rotation speed control system includes a phase detector for detecting a period, an ON time interval, an OFF time interval, and zero crossing time points of the phase cut AC signal; a voltage regulator for converting the phase cut an AC signal into a DC signal; a controller for generating a compatible waveform to drive the EC motor with a rotation speed instructed by the controller, in accordance with an ON/OFF time ratio determined based on the ON time interval and OFF time interval of the phase cut AC signal; and an EC motor interface, powered by the DC signal, for driving each of a plurality of windings of the EC motor with the compatible waveform.