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.

The firing pulses can vary between 0deg to 180deg. The decrease in the angle of the firing pulse increases the conduction period of the thyristor, thus allowing high voltage through it.

Once the motor attains its full rated speed (at o deg firing angle), the thyristors are completely bypassed using a bypass contractor under normal operation. It increases the efficiency of the soft starter since the SCR stops firing. During motor stops, the SCR takes control and starts firing in an orderly fashion to reduce supply voltage.

Advantages and disadvantages of soft starter:

Advantages:

• The soft starter starts the motor by gradually increasing the voltage, avoiding the high current shock during direct starting and reducing the impact on the power grid.
• The soft starter can reduce mechanical stress and extend the service life of the motor and related mechanical equipment.
• The soft starter has a simple structure, high reliability, and is easy to install and maintain.
• Compared with the frequency converter, the soft starter has a lower cost and is particularly suitable for projects with limited budgets.

Disadvantages:

• The soft starter can only control the start and stop process, and cannot adjust the speed of the motor during operation.
• Although the starting current can be reduced, it cannot accurately control the various parameters during the starting process like the frequency converter.
• After the motor starts, the soft starter basically no longer works and cannot improve the operating efficiency.