A soft starter limits the start, not the running speed
A three-phase electronic soft starter limits the voltage or current while the motor starts (and often provides a controlled stop). It is mainly meant to reduce electrical and mechanical starting stress. It does not control the running speed the way a VFD does: after the start, many models switch to an internal or external bypass, which changes which components carry the current and how the heat is dissipated.
1. Where it fits
A soft starter sits between the supply and the motor and controls how the motor accelerates. Once the motor is at speed it runs at line frequency. For continuous speed control you need a drive; see the VFD Sizing guide. For the motor currents behind these numbers see Motor Current & Starting.
2. Why motor kW is not enough
A 15 kW motor's real current depends on its efficiency, power factor and construction: two real IE3 examples are 28.5 A (ABB) and 28.3 A (WEG). Manufacturers tell you to select a starter from the motor nameplate full-load amperes, with kW or HP only as an approximate catalog pointer. A reference current is an estimate and is labelled as one when no nameplate is entered.
3. What the calculator gives
For the 15 kW, 400 V, IE3 four-pole reference motor:
- Full-load motor current In ≈ 28.46 A.
- Continuous in-line starter current to seek: Istarter,continuous ≥ 28.46 A, before any ambient or altitude derating.
- Assumed current-limit setting: Ilimit = 3.5 × In ≈ 99.62 A.
The actual start time, whether the motor accelerates, the current waveform and the thermal ability for repeated starts are NOT ASSESSED from motor power alone. The current limit is a requested control setting: not a breaker short-circuit current and not the motor feeder's continuous current.
The delta winding current is not the starter rating. The motor page may show a conditional delta winding current (16.43 A for the example) if the motor is delta-connected. A standard in-line soft starter still carries the line current, 28.46 A, subject to the manufacturer's derating and duty. An inside-delta starter needs a specifically approved topology and its own documented ratings; a 25 A in-line starter fails the 28.46 A check even though 25 A is more than 16.43 A.
4. What decides suitability
A suitable starter has to meet the voltage, the continuous current, the current-limit adjustment range, the starting time and thermal duty and the number of starts. A device can pass the continuous-current check and still be undersized for repeated heavy starts. Ambient temperature, panel ventilation, bypass design and the manufacturer's control algorithm matter, and the starting torque depends on the motor voltage and speed. In the optional candidate check of the calculator each of these is compared on its own, and the result is never an overall PASS.
5. Manufacturer-specific limits
Schneider's ATS22 documentation gives an adjustable current limit (ILt) of 200 % to 700 % of the motor nominal-current parameter, with a separate ceiling of 350 % of the soft starter's own rated current. These limits belong to that product series and must not be transferred to another brand. For a 32 A in-line candidate the 28.46 A requirement is below 32 A, and the 99.62 A limit is below 3.5 × 32 = 112 A under that ATS22-specific rule; that still does not prove the thermal duty or the torque. For a 25 A candidate the same rule gives 87.5 A, below the requested 99.62 A, so the limit check fails as well as the continuous current. If the manufacturer documents a derating factor (for example 0.85 on a 32 A unit, giving 27.2 A), that effective current is what is compared.
6. What does starts per hour mean?
Every start produces significant heat in the motor and in the electronic starter. A manufacturer can specify an allowed number of starts per hour at a given current, duration, ambient temperature and initial temperature. A motor and starter that start once may still overheat under repeated starting. The calculator can compare your requirement with the documented duty envelope if you enter it; without it the thermal duty is NOT ASSESSED. It is a comparison with documented limits, not a thermal model: I²t alone does not describe silicon junction heating or bypass dynamics.
7. Why there is no acceleration time here
Acceleration needs the torque-versus-speed curves of the motor and the load and the combined rotating inertia. Neither the motor kW nor a current limit supplies them. A current-limited soft starter can fail to accelerate a motor even when the current setting is inside its range.
8. Common mistakes and questions
- Matching only the motor kW and ignoring the actual rated amps.
- Calling 350 % a predicted motor starting current instead of a requested current-limit setting.
- Ignoring the allowed starting time and the rest periods.
- Ignoring environmental derating and enclosure conditions.
- Assuming every soft starter supports inside-delta wiring and applying a 58 % shortcut without the manufacturer's wiring rules.
- Assuming a starter controls the running speed or removes the need for short-circuit coordination and motor thermal protection.
Do I need to select the application? Not for the basic current requirements; whether a particular motor and load can accelerate is a separate engineering question that this tool does not assess. Why NOT ASSESSED when the current is enough? Thermal duty and starting torque cannot be known from the running-current capacity. Can I use the reference current on a quote? For budgeting, if it is identified as an estimate; for purchase and commissioning use the nameplate and the manufacturer's selection data. Sources: Schneider Electric ATS22 ILt FAQ and the Altistart 22 selection guide, IEC 60947-4-2:2020 with AMD1:2024. This tool is not a compliance certification.