Soft Starter Explained
A soft starter is a solid-state motor starter that uses thyristors (SCRs) to gradually ramp up voltage to a 3-phase AC motor during start-up, controlling inrush current and starting torque. Unlike a VFD, a soft starter does not provide speed control during running — once at full speed, the motor runs at line frequency. Soft starters are cheaper, smaller, and produce less heat than VFDs, making them the right choice when only the start matters.
How a soft starter works
A soft starter has SCRs (thyristors) in each of the three phase wires. By controlling the firing angle of the SCRs, the device varies the effective voltage delivered to the motor — from near zero at start to full line voltage at the end of the ramp.
- Start ramp — voltage ramps from initial torque setting (typically 30%) to 100% over 1-30 seconds (configurable).
- Bypass contactor — once at full speed, an internal contactor bypasses the SCRs to eliminate ongoing power loss.
- Stop ramp — optionally ramp down voltage at stop to prevent water-hammer in pumps or to control belt deceleration.
- Current-limit start — alternative to fixed-time ramp; soft starter increases voltage just enough to keep current at a defined limit.
When to use a soft starter
Soft starters are the right choice when:
- Motor runs at constant speed once started — no need for speed control during operation.
- Inrush current matters — undersized utility supply, soft transformers, weak grids that struggle with across-the-line starts.
- Mechanical stress matters — large fans, conveyors with belt slipping, pumps with water hammer issues.
- Cost ceiling is tight — soft starters are 30-50% cheaper than VFDs of equivalent rating.
- Cooling/space is constrained — soft starters dissipate near-zero heat at full speed (bypassed); VFDs continuously dissipate switching losses.
Soft starters are NOT the right choice when you need variable speed during operation. For pumps that need throttling, fans that need staged operation, or any process tuning, use a VFD.
Common applications
- Air compressors (constant-speed running, soft start matters most)
- Crushers and mills in mining (massive inertia at start)
- Large constant-speed pumps in water utilities
- Fans where soft start prevents belt damage but variable speed isn't needed
- Conveyors with abrasive products that suffer from sudden starts
- Motors above 50 HP where across-the-line starts trip utility breakers
Major vendors
- Allen-Bradley SMC — SMC-Flex, SMC-50. ControlLogix EtherNet/IP integration.
- Siemens SIRIUS 3RW — strong PROFINET / TIA Portal integration.
- ABB PSE / PSTX — full feature set, communication options.
- Schneider Altistart — ATS22, ATS480.
- Eaton S801 — common in North American MCC applications.
- WEG SSW — cost-effective, Brazilian-made, strong in mining and pulp/paper.
Frequently asked questions
What is a soft starter?
A soft starter is a solid-state motor starter that uses SCR thyristors to gradually ramp up voltage to a 3-phase AC motor during start-up, controlling inrush current and starting torque. Once at full speed, the soft starter typically bypasses itself with an internal contactor and the motor runs at line frequency, just like a direct-on-line starter.
What is the difference between a soft starter and a VFD?
Both ramp up motor voltage at start. But a VFD continues to control voltage and frequency during running, providing speed control and energy savings; a soft starter only controls the start. Once at full speed, a soft starter bypasses itself and the motor runs at line frequency. VFDs cost 30-50% more and dissipate more heat but offer variable speed; soft starters are cheaper and simpler when speed control isn't needed.
When should I use a soft starter instead of a VFD?
Use a soft starter when the motor runs at constant speed and you only need to control the start (constant-speed compressors, large constant-speed pumps, crushers, mills). Use a VFD when you need variable speed during running (centrifugal pumps with PID flow control, HVAC fans with thermostatic control, conveyors with recipe-driven speed).
Do soft starters save energy?
Soft starters do not save running energy — once at full speed, they bypass themselves and the motor runs as if connected directly to the line. The savings are in starting current (lower inrush) and reduced mechanical stress (longer motor and drivetrain life). For energy savings during running on centrifugal loads, you need a VFD.
What size motor needs a soft starter?
Generally motors 25-30 HP and above benefit from soft starting. Below 25 HP, across-the-line starting is usually fine unless the utility supply is weak or the load has unusual mechanical sensitivity. Above 100 HP, soft starting (or VFD) is almost mandatory to avoid utility breaker trips and mechanical damage.