How to Select and Size a Soft Starter for a Motor
How do you size a soft starter for a motor? Match the starter's continuous current rating to the motor's full-load current (FLC) at the duty class defined in IEC 60947-4-2 — typically AC-53a 3.0-10:50, meaning 3x FLC for 10 seconds at up to 50% duty and 10 starts per hour — not just to the motor's kW nameplate. Undersizing against the duty class lets the SCRs overheat on the second or third repeat start long before the current rating alone would suggest a problem, and skipping the torque check lets a high-inertia load stall halfway through the ramp. Six checks decide the right starter: FLC-based current rating, duty class and starts-per-hour, starting torque against the load, control method (voltage ramp, current limit, or torque control), in-line vs inside-delta wiring, and bypass or protection features.
Start With Full-Load Current, Not Motor kW
Nameplate kW tells you almost nothing about which soft starter frame to order. Two 55 kW motors from different manufacturers can carry FLC values 10-15% apart depending on efficiency class and pole count, and the starter is rated in amps, not kilowatts. Pull the FLC straight off the motor nameplate or datasheet, not from a generic kW-to-amp table.
Voltage matters as much as current. A starter built for 400 V line-to-line will not run correctly on a 690 V motor circuit without a different SCR voltage class — check the starter's rated operational voltage (Ue) against the supply, not just its current rating.
Most soft starters are rated at 40°C and derate above that — a starter in a 45°C plant room or above 1000 m altitude needs the next frame size up, even if the raw current looks fine on paper.
Match the Duty Class to the Application (AC-53a / AC-53b)
IEC 60947-4-2 rates semiconductor motor starters by duty class, written as AC-53a q-r:t, where q is the current multiple of FLC, r is the start time in seconds, and t is the duty cycle percentage. A catalog entry of AC-53a 3.0-10:50 means the starter can pull 3x FLC for 10 s at 50% duty — roughly 10 starts per hour with even spacing.
This is where sizing goes wrong most often: an engineer picks a starter rated for the right continuous current, then runs it on an application needing 20 starts per hour with a 15 s ramp. The SCR thermal model trips on starts-per-hour long before the current rating is the limiting factor.
AC-53a covers a starter used continuously in bypass, or with the SCRs left in circuit; AC-53b covers a starter with an external bypass contactor sized to the AC-53a rating during the ramp and switched out afterward. See our AC-53a and AC-53b duty sizing guide for how to read a manufacturer's duty table line by line.
Check Starting Torque Against the Load (the Sizing Trap)
Starting torque falls with the square of the applied voltage, not linearly. Halve the motor terminal voltage and you get about a quarter of the locked-rotor torque a direct-on-line start would produce. A soft starter set to ramp from 40% initial voltage is asking the motor to start a load on roughly 16% of its DOL torque — fine for a centrifugal pump, marginal for a loaded conveyor, and wrong for a positive-displacement compressor.
Formula: Torque vs. Applied Voltage — Source: IEC 60034-1, motor torque-voltage relationship
T / TDOL = (V / Vline)2
| Symbol | Description | Unit |
|---|---|---|
| T | Motor torque at reduced voltage | N·m |
| TDOL | Motor locked-rotor torque at full line voltage (direct-on-line) | N·m |
| V | Motor terminal voltage during ramp | V |
| Vline | Full line voltage | V |
Run this check against the load's breakaway torque, not just its running torque. A screw conveyor packed with material can need 150% of its running torque to break away, and a soft starter that never delivers enough torque at any point in the ramp will stall the motor, trip on locked-rotor, or simply sit there drawing current-limit amps with the shaft not turning. For loads like this, a kick-start pulse or a higher current-limit setting sometimes closes the gap; sometimes it doesn't, and the honest answer is the load needs a VFD instead. Our soft starter vs VFD comparison covers that decision in more depth.
Choose the Control Method: Voltage Ramp, Current Limit or Torque Control
Three control modes cover nearly every soft starter on the market, and the choice changes both the sizing math and the price. Voltage ramp is open loop — the starter climbs from initial voltage to full voltage over a set time regardless of what the motor is doing, cheapest to buy, least forgiving of a load that changes. Current limit holds the start current at a ceiling (typically 300-400% FLC) instead of ramping voltage on a timer, which self-corrects somewhat for load variation. Torque control is closed loop, using measured current and voltage to estimate motor torque and hold a linear speed rise — the only mode that reliably kills fluid hammer on a pump start or protects a conveyor belt from a torque spike.
| Criteria | Voltage Ramp | Current Limit | Torque Control |
|---|---|---|---|
| Loop type | Open loop | Open/semi-closed loop | Closed loop |
| Best fit | Fans, light pumps, fixed loads | Variable loads, general duty | Pumps (water hammer), conveyors |
| Typical starter tier | ABB PSR, Altistart ATS01 | ABB PSE, Altistart ATS22 | ABB PSTX, Schneider ATS480, Siemens 3RW55 |
| Handles load swings | No — fixed ramp regardless of load | Partially | Yes — corrects in real time |
| Relative cost | Lowest | Mid | Highest |
What we see in the field: plenty of pumps run for years on a plain voltage ramp without complaint, because the static head and pipe layout happen to tolerate the ramp shape. It's the pump with a long horizontal run and a check valve near the discharge that slams on every start until someone puts in a torque-control unit with a soft-stop ramp. Read the load, not just the label "pump."
See our detailed breakdown of voltage ramp vs current ramp vs torque control for the settings each mode exposes.
In-Line or Inside-Delta: Wiring Changes the Starter Size
An in-line (3-wire) connection puts the SCRs in series with the motor's line conductors, so every SCR carries full line current. An inside-delta (6-wire) connection instead wires the SCRs inside the motor's own delta winding, so each SCR only carries phase current — line current divided by √3, about 58% of the line value. That difference lets a physically smaller starter frame run a larger motor, at the cost of needing all six motor leads brought out and correct phase-to-SCR matching during commissioning.
Inside-delta only works if the motor is delta-wound at the supply voltage and all six leads reach the starter — a star-wound motor, or one where only three leads are brought to the terminal box, rules it out immediately. Check the motor nameplate and terminal box before assuming a smaller frame is an option. Full wiring detail and failure modes are in our in-line vs inside-delta connection guide.
Bypass, Protection and Enclosure Considerations
SCRs dissipate roughly 1-1.5 W per amp per phase while conducting, and that heat has to go somewhere in the panel. A built-in or external bypass contactor closes once the motor reaches full speed, shorting the SCRs so they carry no current at all during the run — this is what lets a bypassed starter avoid derating for continuous run current and fit in a smaller enclosure. Most current flagship units (ABB PSTX, Schneider ATS480, Siemens 3RW55) bring the bypass in as standard; the economy tiers (ABB PSR, Altistart ATS01, Siemens 3RW50) may need one added externally or skip it and accept the heat.
Some panel builders skip the bypass on small motors to save panel space and accept the extra heat. It works, until the starter runs near its duty ceiling on a hot day and the SCRs run hotter than the thermal model expected.
Protection functions worth checking against the application before ordering: I²t overload matched to the motor's thermal class, phase-loss and phase-imbalance detection, phase sequence check, locked-rotor/stall protection, and a too-many-starts thermal model for the SCRs themselves, separate from the motor's own thermal model. A dry-running pump needs undercurrent protection too, which not every economy-tier starter includes.
For a side-by-side on how the current flagships from the three major brands stack up on these exact points, see our ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 comparison. Browse the full range of soft starters once the current rating, duty class and control method are settled.
Frequently Asked Questions
Do I size a soft starter by motor kW or by current?
By current. Pull the full-load current (FLC) from the motor nameplate and match it to the starter's rated current at the required duty class. kW is a secondary check, not the sizing basis, because FLC varies with motor efficiency and pole count even at the same kW.
What happens if a soft starter is undersized for starts-per-hour?
The SCR thermal model trips on repeat starts, or the SCRs overheat and fail, well before the continuous current rating looks exceeded. Duty class (AC-53a q-r:t) accounts for starts-per-hour and ramp time, so check it against the actual process, not just the motor's rated current.
Can a soft starter start a high-inertia load like a centrifuge?
Only if the reduced motor torque through the whole ramp stays above the load's torque curve, including breakaway. Starting torque falls with the square of voltage, so a soft starter set to a low initial voltage can leave the motor unable to accelerate a high-inertia load. In some cases a VFD is the correct answer instead.
What's the difference between in-line and inside-delta soft starter wiring?
In-line puts the SCRs in series with the full line current; inside-delta wires them inside the motor's delta winding, where each SCR only carries phase current, about 58% of line current. Inside-delta lets a smaller starter frame run a larger motor, but needs all six motor leads and correct commissioning.
Do I need torque control or is voltage ramp enough?
Voltage ramp is enough for fixed, tolerant loads like fans and small pumps. Torque control earns its cost on loads that punish an imprecise ramp: pumps prone to water hammer, and conveyors or crushers where a torque spike damages the belt or product.
Conclusion
Sizing a soft starter correctly means working through FLC, duty class, starting torque, control method, wiring topology and protection in that order. Skipping any one of them is how an apparently correct starter still faults on the third start of the day or stalls a heavy load halfway up the ramp. Start with the nameplate current and the process starts-per-hour, run the torque check against the actual load curve, then pick the control method and wiring topology that fit. Our soft starter selection guide walks through the full decision tree end to end, including where a soft starter should not be the answer.
