DC motor automatic commutation circuit

For DC motors that require frequent commutation operation, the direction of rotation of the motor is usually changed by changing the direction of the armature current. The automatic control commutation circuit is as shown in the figure. When the forward start button STF is pressed, the forward DC motor The contactor KMF is powered and closed, and its auxiliary contact KMF moves. On the one hand, the normally open contact KMF closes to achieve self-locking. At this time, even if the STF is released, the coil KMF still maintains the closed state; on the other hand, the normally closed contact KMF is released, cutting off the reverse coil KMR circuit, ensuring that even if someone presses the reverse start button STR by mistake, the KMR will not be activated, thus avoiding accidents caused by misoperation. After KMF pulls in, its main contact KMF moves, causing the power current to pass through the armature from left to right, and the motor rotates forward. In the same way, when the reverse button STR is pressed, KMR operates, the power current passes through the armature from right to left, and the direction of the current through the excitation coil L1 remains unchanged, so the motor rotates in the opposite direction. In order to avoid overvoltage damage to the motor, a current-limiting resistor R1 is placed in series with the armature; a discharge resistor R2 is placed in series with the excitation circuit, and its resistance is generally several times the resistance of the L1 coil.