DC motor starting circuit controlled by back electromotive force

Close the circuit breaker QF, press the start button ST, the DC contactor KM is closed, and the motor M is connected in series to R1, R2, and R3 to start. As the speed increases, the counter electromotive force increases, and the voltage at both ends of the motor M gradually increases, causing 1KM, 2KM, and 3KM to operate in sequence. Their main contacts short-circuit R1, R2, and R3 one by one, and finally the motor M is put into full operation. Pressure operation. 1KM, 2KM, and 3KM are all DC contactors, and the rated voltage of their pickup coils must satisfy U1KM