Use this soft starter sizing calculator to quickly estimate the required current rating based on motor power, voltage, and load type.
Motor Soft Starter Calculator
Quickly estimate the right soft starter size for your motor.
Choosing the right soft starter for a motor is not just about matching voltage or kW. In real projects, the wrong selection often leads to startup failure, overheating, or unnecessary overspending.
This guide breaks down how to choose, size, and apply a motor soft starter correctly, based on real industrial scenarios.
What Is a Soft Starter (And When Should You Use It)?
A soft starter is a solid-state motor control device that gradually increases voltage during startup and decreases it during stopping.
In practical terms, it helps you:
- Reduce inrush current
- Minimize mechanical shock
- Prevent voltage dips in the grid
👉 Unlike a VFD, a soft starter is designed for start/stop control only, not continuous speed regulation.
Step 1: Collect Motor Nameplate Data
Before sizing a soft starter, confirm all key motor and application details.
- Rated power (kW / HP)
- Rated voltage (400V / 690V)
- Full Load Current (FLC)
- Frequency (50Hz / 60Hz)
- Load type (pump, fan, conveyor, etc.)
- Starting torque requirement
- Load inertia (light / medium / heavy)
Step 2: Size the Soft Starter by Current
Soft starter rated current ≥ Motor FLC
| Load Type | Typical Applications | Recommended Size |
|---|---|---|
| Light duty | Fans, pumps | 1.0 × FLC |
| Standard duty | Conveyors, compressors | 1.2 – 1.5 × FLC |
| Heavy duty | Crushers, mixers | 1.5 – 2.5 × FLC |
Step 3: Understand Starting Torque
Torque ∝ Voltage²
- 70% voltage → ~49% torque
- Lower voltage = lower torque
Practical impact:
- Light loads → OK
- Heavy loads → may fail to start
Step 4: Select the Right Features
- Ramp time control
- Initial voltage
- Current limit
- Soft stop
- Bypass contactor
- Motor protection
Step 5: Match the Application Scenario
- Light load
- Soft stop required
- 1.0–1.2 × FLC
- Medium load
- Smooth start
- 1.2–1.5 × FLC
- High torque
- ≥ 2.0 × FLC
- Consider VFD
Step 6: Check Power Supply
- Voltage (380V / 400V / 690V)
- Frequency (50 / 60 Hz)
- Control voltage
Step 7: Start Frequency & Thermal Limits
- Standard: 10–20 starts/hour
- Heavy-duty: fewer starts
Step 8: Environmental Factors
- Temperature
- Altitude
- Dust / humidity
- Cooling method
Soft Starter vs VFD (Which One Should You Choose?)
| Feature | Soft Starter | VFD |
|---|---|---|
| Cost | Lower | Higher |
| Speed control | No | Yes |
| Starting torque | Limited | High |
| Energy saving | Low | High |
| Complexity | Simple | Complex |
👉 If your process requires speed control or constant torque, a VFD is the better choice.
Real Selection Example
FLC ≈ 100A
Medium duty
Soft Starter
✔ Built-in bypass preferred
Common Mistakes to Avoid
- Selecting based on kW only
- Ignoring load type
- Undersizing for heavy loads
- No bypass contactor
- Overlooking start frequency limits
FAQ
Not significantly during normal operation. It mainly reduces starting current and mechanical stress, unlike VFDs which optimize running energy.
Avoid using a soft starter if:
Variable speed control is required
High starting torque is critical
Load fluctuates frequently
Yes, in most industrial applications. It:
Reduces heat inside the soft starter
Improves system efficiency after startup
Final Advice (From Field Experience)
In real-world projects, the safest approach is:
- Start with motor FLC
- Adjust based on load type
- Add 20–50% margin
- Never ignore duty cycle and environment
Need Help Selecting the Right Soft Starter?
Choosing the right model can be challenging—especially for heavy-duty or non-standard applications.
👉 Share your basic requirements:
Our engineers will help you select the exact model and configuration, and provide a cost-effective solution directly from the manufacturer.
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