The invention relates to a system and method for using and controlling a permanent-magnet alternating-current motor, particularly to a system and method for controlling the speed of a permanent-magnetalternating-current motor based on a delay angle of an alternating-current voltage signal of three-end bidirectional silicon-controlled rectifier switch element control of a three-end bidirectional silicon-controlled rectifier switch element. A simulation load connected to a three-end bidirectional silicon-controlled rectifier switch element enables a load current to flow and generates a signal.A first detector detects a zero-crossing point of an alternating-current voltage signal and a second detector detects a follow-up connection time of the three-end bidirectional silicon-controlled rectifier switch element. A speed command generator measures an interval between the zero-crossing point and the follow-up connection time and converts the delay angle into a speed command for controllingthe speed of the motor. The simulation load includes resistors enabling the load current to be lower than a maintaining current rated value of the three-end bidirectional silicon-controlled rectifierswitch element, so that the three-end bidirectional silicon-controlled rectifier switch element is turned off after measurement of the interval.