During the operation of synchronous brushless DC motor, it is usually necessary to sample the phase current of the three-phase coil of the motor to realize the closed-loop control of the motor. In the traditional three-phase current sampling method, three-resistance or double-resistance is often used to sample the motor current. However, the double-resistance and three-resistance sampling control circuit is complex and the manufacturing cost is high. Therefore, the method of using single resistance to sample the three-phase current of the motor gradually replaces the traditional sampling method.

 
At present, when single resistance current sampling method is used to sample phase current of motor, the phase in series with sampling resistance can be determined by controlling the turn-on or turn-off of insulated gate bipolar transistor . The IGBT is turned on or off to determine the phase in series with the sampling resistance in the three phases of the motor, and then the sampling current on the sampling resistance can be obtained by the micro control unit. The sampling current is the phase current in series with the sampling resistance. Therefore, each corresponding current in the three-phase of the motor can be obtained by sampling the current through the sampling resistance. However, in practical operation, after the IGBT is turned on or off in the control sampling circuit, the current needs a certain time to adapt to the change of the circuit, which may result in the delay of the current on the sampling resistance. As a result, the sampling current is inaccurate, and even the current can not be collected.
 

In the process of modulating the motor with modulated wave, the duration of the three phase currents of the motor lies in the high-level duration of each phase current in the PWM wave corresponding to the modulated wave. Thus, according to the duration of the high level and the delay time of the current on the IGBT, the corresponding first sampling point and the second sampling point can be set. Because the delay time of the current on the IGBT is taken into account when setting the sampling point. Therefore, the time period of current delay can be avoided when the sampling points are set, and the values of the three phase currents of the motor obtained from the single resistance current sampling through the set sampling points are more accurate.