A soft starter controls output voltage by controlling the thyristor conduction angle. It is therefore essentially an automatically controlled reduced-voltage starter — and since it can regulate output voltage freely and run current closed-loop control, it offers more advantages than traditional reduced-voltage starting. Let’s look at its main functions.
1. Overload protection: the soft starter introduces a current control loop that tracks motor-current changes at all times. With an overload-current setting and inverse-time control, it provides overload protection — switching off the thyristors and issuing an alarm when the motor overloads.
2. Phase-loss protection: during operation it continuously monitors the three-phase line currents and responds with phase-loss protection as soon as an interruption occurs.
3. Overheat protection: an internal thermal relay monitors the thyristor heatsink temperature and automatically switches off the thyristors with an alarm once the temperature exceeds the allowed value.
4. Circuit-parameter measurement: while the motor runs, detectors inside the soft starter constantly supervise the motor’s operating state and send the monitored parameters to the CPU, which analyzes, stores, and displays them — so the soft starter also measures circuit parameters.
5. Other functions: through combinations of electronic circuits, various additional interlock protections can be implemented in the system.
In the coming years, the soft-starter market will keep growing steadily, though far slower than the VFD market; as competition intensifies and smaller, weaker players exit, market concentration will increase. Soft-starter applications already touch many sectors of the economy — power, metallurgy, building materials, machine tools, petrochemicals and chemicals, municipal services, and coal being the seven main industries.
