One of the current cutoff feedback circuits

In order to prevent the thyristor from being damaged due to excessive current during motor
starting, overload, forward and reverse rotation, etc. in the speed control system , a current cutoff feedback circuit needs to be introduced. The principle is: when the armature current exceeds a certain value, the feedback voltage is added to
the input end of the transistor amplifier, so that the conduction angle of the thyristor decreases rapidly, thereby reducing the current to protect the tube.
    The circuit is shown in Figure 16-37. It is suitable for speed regulating circuits of small capacity motors below 5kW . Adjust the potentiometer RPi so that when the armature current exceeds the limiting current value, the voltage regulator tube VS breaks down.