Sanftanlauf

Sanftanlauf, or soft start, denotes techniques used to start electric motors and related loads with a controlled, gradual increase of voltage and/or frequency. The goal is to limit peak inrush current and torque surges, reduce voltage dips in the electrical system, and minimize mechanical stress, thereby extending motor life and improving electrical reliability.

Typical implementations include reduced-voltage starters and electronic soft starters. Reduced-voltage schemes use automatic transformers or reactances to limit initial voltage, while electronic soft starters employ solid-state devices such as SCRs or TRIACs to ramp the voltage according to a programmable profile. Modern systems often pair soft-start with acceleration control or incorporate a subsequent speed ramp via a variable frequency drive (VFD), which can regulate both voltage and frequency for smoother, energy-efficient operation.

Applications are widespread in industry and cover pumps, fans, conveyors, compressors, and other machines where high inrush currents or sudden torque are undesirable. Soft start is commonly used in HVAC, water treatment, and manufacturing plants to avoid tripping breakers, reduce electrical interference, and lessen mechanical wear.

Advantages include reduced inrush current, smoother torque, lower electrical stresses, and diminished mechanical shock. Limitations include that some schemes dissipate energy in resistive elements, that not all loads benefit equally, and that VFD-based solutions entail higher cost and potential acoustic or harmonic considerations. The choice depends on starting torque requirements, duty cycle, and economic trade-offs.

Historically, soft starting began with auto-transformer and reactor starters and evolved to electronic soft starters with SCRs, progressing further with VFDs that provide comprehensive speed control.