PlantFlow 3D › Guides › Soft starters: gentle starting for big motors
Soft starters: gentle starting for big motors
How a soft starter limits starting current and mechanical shock by ramping the motor voltage, and how it differs from a VFD.
Why start softly
Starting a large motor direct-on-line draws several times its rated current and gives the machine a sudden jolt. Belts slip, couplings wear, water hammer appears in pump pipes and the supply voltage dips. A soft starter reduces all of this.
How it works
A soft starter has pairs of thyristors in the phases. By switching them on later or earlier in each half-cycle, it controls the voltage applied to the motor. It starts at an initial voltage, for example 40 %, and raises it to 100 % over the ramp time. Starting torque falls roughly with the square of the voltage and the starting current falls with it.
When full voltage is reached, an internal bypass contactor closes so that the thyristors do not stay hot during normal running.
Soft starter or VFD?
A soft starter only controls starting and, optionally, stopping. The motor then runs at its normal fixed speed. A VFD controls the speed all the time and starts with even lower current, but it costs more and needs more attention to cables, harmonics and cooling. For a fan or pump that always runs at full speed, a soft starter is often enough.
Wiring and protection
Like a drive, many soft starters have their own 24 V inputs for start and stop. The overload protection, either built in or a separate relay, must open the stop circuit so that an overload actually stops the motor.
Try it yourself
Choose “Soft starter” in the simulator and compare its starting current in the status bar with the DOL starter. Then make the ramp time longer.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.