Formula used
- Ramp time: t = (Vfinal% − Vinitial%) / (dV%/dt)
- Ramp rate (if time entered): dV%/dt = (Vfinal% − Vinitial%) / t
- Torque estimate: T% ≈ (V%/100)² × 100 (common motor approximation)
- Current estimate: I ≈ ILRA × (V%/100) (engineering estimate for early ramp)
- Locked-rotor current: ILRA = Irated × (LRA multiple)
How to use this calculator
- Select a mode based on your commissioning method.
- Enter supply voltage, final %, and motor rated current.
- Provide either ramp rate or desired ramp time.
- Set load start torque to check starting margin.
- Press Calculate to show results under the header.
- Export the full profile to CSV or PDF for records.
Example data table
| Scenario | Supply (V) | Initial → Final (%) | Ramp rate (%/s) | Calculated time (s) | Load torque (%) |
|---|---|---|---|---|---|
| Pump start | 400 | 40 → 100 | 10 | 6.0 | 45 |
| Fan start | 415 | 35 → 100 | 8 | 8.125 | 35 |
| Conveyor start | 480 | 55 → 100 | 6 | 7.5 | 70 |
Engineering notes
Soft-start settings affect inrush current, voltage dip, acceleration time, and thermal stress. If the load demands high starting torque, increase initial voltage or reduce current limiting. If upstream power is weak, reduce ramp rate to avoid dips.
For high-inertia loads, consider adding a longer ramp and verify motor heating limits. Always confirm with protective relay settings and manufacturer curves.
Soft-start ramp objectives
Commissioning teams typically target a ramp that limits upstream voltage dip while achieving stable acceleration. A 40→100% ramp at 10%/s yields 6.0 s, which often reduces nuisance trips compared with direct-on-line starts. For weak feeders, lowering the slope to 6%/s extends the ramp to 10.0 s, cutting peak current proportionally in this model. Record bus voltage sag during trials to confirm the supply can support the chosen slope.
Estimating current during early acceleration
This calculator uses an engineering estimate where current tracks applied voltage as a fraction of locked-rotor current. With a 32 A motor and a 6× locked-rotor multiple, ILRA ≈ 192 A. At 50% voltage, the estimated current is about 96 A. Use this to compare against breaker pickup levels and soft-starter current limit settings.
Torque margin and minimum start voltage
For many induction motors, torque scales approximately with V². If a conveyor needs 70% rated torque to break away, the suggested minimum voltage is √0.70 ≈ 0.837, or 83.7%. Starting at 55% provides only ~30.3% torque, indicating likely stall. Raising the initial voltage or reducing load at start improves margin.
Ramp profile step and export sizing
The profile step sets the resolution of the time series used for plotting and exports. A 0.5 s step over 8.125 s creates 17 rows, while a 0.1 s step creates 82 rows. Smaller steps improve trend visibility in voltage, torque, and current traces, but increase CSV size and PDF rendering time on low-power devices.
Aligning settings with protection and process
Soft-start tuning should align with motor thermal limits, overload curves, and process constraints. For pumps, lower initial voltage can reduce water hammer, but insufficient torque may cause prolonged acceleration and overheating. For fans, longer ramps can reduce belt shock. Always validate against manufacturer data and site protection coordination studies. Document start frequency, ambient temperature, and load inertia because they affect heating.
Interpreting the plotted traces
The Plotly chart overlays voltage, torque, and current versus time. Voltage rises linearly by design, torque rises quadratically, and current rises linearly in the estimate. Look for a torque curve that clears load demand early in the ramp. If current exceeds limits, decrease initial voltage or lengthen the ramp and recheck torque.
FAQs
1) What does “soft start time” mean here?
It is the calculated duration for voltage to ramp from the initial percentage to the final percentage, based on your entered slope or target time.
2) How accurate are the current and torque values?
They are practical estimates: current is assumed proportional to applied voltage, and torque is assumed proportional to voltage squared. Use vendor curves for precise design verification.
3) When should I use current-limit estimate mode?
Use it when the commissioning constraint is an allowable current multiple. The tool infers an initial voltage that approximately matches the chosen current limit.
4) Why might the torque check warn even if the ramp looks smooth?
A smooth ramp can still start below required breakaway torque. If the initial voltage is too low, acceleration may stall or extend, raising thermal stress.
5) What profile step should I choose?
Use 0.5–1.0 s for quick checks and smaller steps like 0.1–0.2 s for detailed plots and documentation. Smaller steps produce larger exports.
6) Why does the PDF include fewer rows than the CSV?
The PDF report prints the on-page preview table for readability. The CSV always contains the full calculated profile for analysis in spreadsheets or scripts.