Soft Starters Explained
Soft starters reduce the inrush current and mechanical stress of motor startup. Learn how they work, when to use them instead of VFDs, and their limitations.
What is a Soft Starter?
A soft starter (also called a reduced-voltage starter) gradually increases the voltage to a motor during startup, reducing the inrush current and starting torque. This reduces mechanical stress on the motor, coupling, and driven equipment, and reduces the electrical demand on the power supply.
Once the motor reaches full speed, the soft starter is bypassed by a contactor, and the motor runs directly from the line power. The soft starter does not control speed during operation, only during startup.
Soft Starter vs VFD
| Feature | Soft Starter | VFD |
|---|---|---|
| Speed control | Startup only | Continuous, variable |
| Energy saving | Minimal (startup only) | Significant (variable speed operation) |
| Cost | Lower | Higher |
| Size | Compact | Larger |
| Complexity | Simple | Complex |
| Harmonics | Minimal | Significant (requires filtering) |
| Motor requirements | Standard motor OK | Inverter-duty motor recommended |
When to use each
Use a soft starter when you only need to reduce startup stress and inrush current, but the motor runs at full speed. Use a VFD when you need variable speed during operation or want to save energy on variable-load centrifugal equipment. If the application needs both soft starting and variable speed, a VFD does both.
Benefits of Soft Starters
- Reduced inrush current: Standard DOL starting draws 6 to 8 times full-load current. Soft starters reduce this to 2 to 4 times, reducing electrical demand charges and voltage dips on the supply.
- Reduced mechanical stress: Gradual acceleration reduces shock loading on couplings, belts, gears, and bearings. Extends equipment life.
- Reduced water hammer: For pump applications, gradual startup prevents the sudden pressure surge that causes water hammer.
- Lower cost than VFD: Soft starters are significantly cheaper than VFDs for applications that only need soft starting.
- Simpler than VFD: Fewer settings, less complexity, easier to commission and maintain.
Limitations
- No speed control: Soft starters do not control motor speed during operation. If variable speed is needed, use a VFD.
- No energy saving: Because the motor runs at full speed, there are no affinity-law energy savings.
- Reduced starting torque: Soft starters reduce starting torque. High-inertia loads (large fans, flywheels) may not start with a soft starter.
Tip: If you have a motor that starts and stops frequently, a soft starter reduces the wear and stress of frequent starts. But first investigate why the motor is short-cycling. An oversized receiver tank or a sequencing control change may be a better solution.