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, thereby reducing starting current, avoiding the high current shock associated with direct starting, and minimizing voltage dips on the power system.
• It provides smooth acceleration of the motor and reduces mechanical stress on shafts, couplings, gears, belts, and other connected equipment, thereby extending the service life of the motor and machinery.
• It increases motor life by reducing both thermal stress and mechanical stress during starting.
• It eliminates switching transients that occur in conventional starters such as star-delta starters.
• It offers adjustable starting characteristics, including current limit, ramp time, and initial voltage.
• The soft starter has a simple structure, high reliability, and is easy to install and maintain.
• Compared with a frequency converter (VFD), the soft starter has a lower cost and is particularly suitable for projects with limited budgets.
• It is suitable for applications such as pumps, fans, compressors, conveyors, marine machinery, and other motor-driven equipment.

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 various motor parameters during starting like a frequency converter (VFD).
• After the motor starts, the soft starter basically no longer works and cannot improve operating efficiency during normal running conditions.
• It has a higher cost than DOL and star-delta starters.
• It produces harmonics in the supply due to phase-angle control.
• It operates with a poor power factor during starting.
• SCRs generate heat and therefore require suitable cooling arrangements.
• It provides reduced starting torque, which may be unsuitable for heavy-load starting applications.
• It requires more complex control circuitry than conventional motor starters.