What is a Soft Starter?
A Soft Starter is an electronic device used to gradually start and stop electric motors by temporarily limiting the supply voltage. This significantly reduces mechanical and electrical peaks during motor start-up.
Soft starters provide a controlled motor start, which reduces wear, prevents energy peaks, and extends the service life of machines.
🧠 How does a Soft Starter work?
- Voltage control via thyristors (SCRs)
- The supply voltage to the motor is gradually increased during start-up
- Reduction of inrush current and starting torque
- Start and stop profiles
- Configurable parameters for ramp-up time, stop delay, current limit
- Controlled acceleration and deceleration
- Bypass contactor
- Once start-up is complete, the soft starter often switches itself over to a contactor to limit energy losses
🏭 Applications in industrial networks
- Driving pumps, fans, compressors, and conveyor belts
- Systems with high inrush currents or sensitive mechanics
- Used in MCCs and control cabinets
- Can communicate with PLC or SCADA via IO modules, Profibus, ProfiNET, or Modbus
Soft starters are especially suitable where variable speed is not required but controlled starting is essential.
🔍 Soft Starter vs. DOL vs. Variable-frequency drive
| Starting method | Starting torque | Inrush current | Speed control | Application |
|---|---|---|---|---|
| DOL | 100% | 6–8× rated | ❌ | Small motors, simple tasks |
| Soft starter | Adjustable | 2–3× rated | ❌ | Pumps, fans, compressors |
| Variable-frequency drive | Fully controllable | Low to rated | ✅ | When variable speed is required |
🔐 Security aspects
- Overcurrent protection, thermal protection, and sometimes phase monitoring
- Support for fault detection: phase loss, ramp-up too long, undervoltage
- Access control via Access Control
- Coupling with PLC and alarm reporting to SCADA possible
- Soft starters are more sensitive to harmonics → proper network segmentation and earthing are crucial
For critical installations, soft starters can be combined with redundancy or bypass circuits.
📌 In summary
A Soft Starter provides controlled motor starting and stopping without the complexity and cost of a variable-frequency drive. Ideal for applications where the load is stable but inrush current and mechanical stress must be minimised.