slow starter circuit

In the thyristor rectifier device (DC motor speed regulating device), in order to prevent the impact when the motor is started and meet the needs of the production process , a slow start circuit
needs to be installed in the rectifier circuit , as shown in Figure 16-70. The master voltage taken out     from the potentiometer RPi charges the capacitor C through the potentiometer RP3, so that the signal voltage of the transistor VT gradually builds up, so the voltage on the VT emitter output resistor Rz gradually increases, thus controlling the acceleration time . The charging time constant can be adjusted by RPa.