The voltage at both ends of the motor is maintained between 0 and Vs (without considering the switching voltage of the diode), to prevent the voltage at both ends of the motor from being too high or too low, the two diodes at the bottom of the line voltage is less than 0, and the two diodes at the top are switched on with VS when the voltage is greater than VS, so that the voltage is maintained between 0 and Vs. Prevent motor ends with too high upkick or too low downkick.