Pehmeäkäynnistin

Pehmeäkäynnistin, known in English as a soft starter, is a solid-state control device used for controlling the electric motor's starting current and torque. It is not a variable frequency drive (VFD) and does not provide continuous speed control. Instead, it is designed to gradually increase the voltage supplied to the motor during startup, thereby reducing the mechanical stress on the motor and connected equipment.

The primary function of a soft starter is to limit the inrush current that occurs when an electric motor is switched on. This inrush current can be several times higher than the motor's full load current, potentially causing damage to the motor, power supply, and driven machinery, as well as creating voltage sags in the electrical system. By ramping up the voltage, the soft starter allows the motor to accelerate smoothly, reducing wear and tear.

Soft starters typically use semiconductor devices like thyristors or silicon-controlled rectifiers (SCRs) to control the voltage applied to the motor. The acceleration time and initial torque can often be adjusted to suit specific application requirements. Beyond just starting, some soft starters also offer features like soft stopping, which applies a controlled deceleration to the motor, and overload protection.

Applications for soft starters are widespread and include pumps, fans, conveyors, compressors, and other industrial equipment where smooth acceleration and reduced electrical stress are beneficial. They offer a cost-effective alternative to direct-on-line starters when variable speed operation is not required.