Exam Question

(a) The soft starter is a type of motor starter that uses the voltage reduction technique to reduce the voltage during the starting of the motor. The soft starter offers a gradual increase in the voltage during the motor startup. This will allow the motor to slowly accelerate and gain speed in a smooth fashion. It prevents any mechanical wear and tear due to the sudden supply of full voltage.
The torque of an induction motor is directly proportional to the square of the current, and the current depends on the supply voltage. So, the supply voltage can be used to control the starting torque. In a normal motor starter, applying full voltage to the motor generates maximum starting torque, which poses a mechanical hazard to the motor.
The main component used for controlling the voltage in a soft starter is a thyristor. It is a controlled rectifier that starts the conduction of the current flow in only one direction when a gate pulse is applied, called the firing pulse. In a three-phase induction motor, two SCRs are connected in an anti-parallel configuration along each phase of the motor, making it a total of 6 SCRs. These are controlled using a separate circuitry that can be a PID controller or a microcontroller. The logic circuitry is powered from the mains using a rectifier, as shown in the figure.
The angle of firing pulse determined how much of the input voltage cycle should be allowed through it. Since AC swings between maximum and minimum peak, forming a complete 360-degree cycle, we can use the angle of the firing pulse to switch the thyristor for a specific duration and control the supplied voltage.