Soft Starter
A soft starter is a motor starting device that limits inrush current and starting torque by gradually increasing the voltage applied to an AC induction motor. It uses thyristors (SCRs) in two or three phases to control the voltage ramp, and most units close an internal or external bypass contactor once the motor reaches full speed to reduce heat losses. Soft starters reduce mechanical stress on couplings, belts and gearboxes, limit voltage dips on the supply, and can provide soft stopping to reduce water hammer in pumping systems. Unlike a VFD, a soft starter does not control running speed. Selection parameters include motor rated current, supply voltage, starts per hour, start duration, load type, ambient temperature and bypass arrangement. Low-voltage soft starters are covered by IEC 60947-4-2.
Examples
- Centrifugal pump starting with soft stop to limit water hammer
- Compressor starting on a weak supply
- Conveyor starting to reduce belt and gearbox stress