Soft Starter Start Current, Starts Per Hour and Duty Cycle

What sets a soft starter's start current, and how does duty cycle change the sizing math? The current-limit setting on the starter, typically 300-400% of motor full-load current (FLC), caps how hard the SCRs let the motor pull during ramp, well below the 6-8x FLC a direct-on-line (DOL) start draws. That number alone does not size the unit. IEC 60947-4-2 duty codes (AC-53a, AC-53b) combine the current multiple with start time and duty cycle into one thermal rating, because a starter that survives one 10-second start at 350% FLC can still overheat on the fifteenth start inside an hour. This article covers: how the current-limit setting fixes start current, reading an AC-53a duty code, why starts-per-hour is a separate limit from current, what the bypass contactor does to duty-cycle heating, sizing for high-cycling loads, and the mistakes that show up as nuisance trips six months after commissioning.

How the Current-Limit Setting Fixes Start Current

A soft starter's control loop measures line current and adjusts SCR firing angle to hold it at or below the current-limit setting, expressed as a percentage of FLC. Set it to 350%, and the starter fires the SCRs harder until current reaches that ceiling, then holds there for the rest of the ramp. This is different from a plain voltage ramp, which increases voltage on a timer regardless of what current actually results — on a stiff load, a voltage ramp with no current limit can still pull DOL-level current because the motor slip stays high.

The relationship is direct enough to write as a formula, and it is the number a panel builder checks first against the starter's rated current, not against motor kW.

What we see in the field: a current limit set too low for the load's breakaway torque does not protect anything — it just stalls the motor partway through the ramp, and the starter sits at current limit indefinitely until it trips on stall or too-long-start protection. The setting has to clear breakaway torque first, current draw second.

Reading an AC-53a Duty Code

IEC 60947-4-2 rates soft starters with a compact code: a current multiple, a start time, and a duty cycle. A rating of 3.0-10:50 means the starter can run 3x FLC for 10 seconds, at a 50% duty cycle. AC-53a covers a starter used with an integral or bypass contactor (or continuous run through the SCRs on some ratings); AC-53b covers a starter used with an external bypass contactor wired separately. The two are not interchangeable when reading a datasheet — a unit's AC-53a figure can look conservative next to its AC-53b figure because the bypass path changes the thermal picture entirely.

Duty cycle in this notation is not "starts per hour" directly — it is the fraction of a reference period the starter spends conducting start current, and the standard's test cycles derive an equivalent starts-per-hour figure from it. Two starters with the same current multiple and start time can carry different starts-per-hour numbers if their duty percentage differs, which is exactly why the full code, not just the current multiple, belongs on a spec sheet. For a full breakdown of how AC-53a and AC-53b figures translate into an actual starts-per-hour allowance, see the AC-53a and AC-53b duty rating breakdown.

Why Starts-Per-Hour Is a Separate Limit from Current

Every start pushes current through the SCRs while they are still in phase-angle control, and semiconductor heating during that window scales with current squared, not current itself. A start at 4x FLC does roughly twice the SCR heating of a start at 2.8x FLC for the same duration — this is why the current-limit setting and the start-time figure both drive the thermal model, and why raising current limit to "start faster" can quietly eat into the starts-per-hour budget the standard's duty rating assumes.

The starter's own protection tracks this: a thermal model estimates junction temperature from the accumulated I²t of recent starts and blocks a new start if the model is not clear, tripping "too many starts" or "start inhibit" rather than letting the SCRs overheat. Two starts back-to-back can exceed the thermal limit even if each one individually falls inside the duty rating, because the interval between them, not just the count, decides whether the SCRs have cooled enough. This matters on machines that cycle in short bursts — compressors with tight discharge-pressure bands, or conveyors that start and stop with product flow — more than on a single pump that starts once per shift.

Feature-level units — ABB PSTX, Schneider Altistart ATS480, Siemens SIRIUS 3RW55 — carry a documented thermal model and a "starts remaining" or restart-inhibit function on their HMI, so the panel builder does not have to derive the interval by hand. Economy units (ABB PSR, Altistart ATS01, SIRIUS 3RW50) typically only give a duty-rating table and leave interval tracking to the operator. On a duty-cycle-heavy machine, that difference is worth checking against the soft starters range before specifying the economy line.

What the Bypass Contactor Changes About Duty-Cycle Sizing

SCRs dissipate roughly 1-1.5 W per amp per phase while conducting in phase-angle mode. On a machine that starts often but runs long between starts, that loss is a rounding error next to the heat from the starts themselves. On a machine that runs close to continuously with only occasional restarts, that steady-state SCR loss becomes the bigger design driver — which is exactly what the bypass contactor removes. Once the motor reaches full speed, the bypass closes, shorts the SCRs, and the run current no longer passes through the semiconductors at all.

This decouples the duty-cycle question in two directions: without bypass, the SCR thermal budget has to cover both starts and the full run current, and an AC-53a rating without bypass credit is noticeably lower than the same frame with bypass. With bypass, the duty rating is really a start-cycle rating; running current is a contactor-sizing question, not a semiconductor one. For the mechanics of why this split exists, see bypass contactor heat reduction.

Sizing for High-Cycling Applications

A pump that starts twice a shift barely touches its duty rating. A batching conveyor that starts and stops every few minutes, or a reciprocating compressor cycling on pressure switches, can run through a nominal duty allowance inside an hour. Sizing for these loads means checking three numbers against the actual duty cycle, not just against motor kW: the current-limit setting needed for breakaway torque, the AC-53a start time at that current multiple, and the starts-per-hour the starter's thermal model actually permits at that combination — which is frequently lower than the frame's continuous current rating would suggest.

This depends on the load's inertia and how close together the starts land: a fan with a light rotor and 20 starts per hour spaced evenly is a different sizing problem than a crusher with a heavy flywheel restarting twice in the same two minutes after a jam clears. The general method for matching frame size to load and duty is covered in the soft starter sizing guide; the pump-specific version, where soft-stop settings interact with start frequency, is in the guide on soft starters for pumps.

Common Duty-Cycle Sizing Mistakes

The most frequent one: sizing purely on motor kW and picking the next frame up, without pulling the duty-rating table at all. A frame rated for the motor's FLC can still be undersized on duty if the application cycles more often than the standard test conditions assume. Second: reading only the current multiple off a datasheet header and ignoring the start-time and duty-percentage figures that go with it — a 3.0-10:50 rating and a 3.0-30:25 rating are not interchangeable even though both list "3.0x". Third: not re-checking the duty rating after a commissioning change that stretches ramp time to smooth out a mechanical issue — a longer start time at the same current multiple consumes more of the thermal budget per start, which can turn a marginal starts-per-hour figure into a fault-prone one. For the standards context behind these ratings, see the IEC 60947-4-2 compliance guide; for how the three lead brands compare on thermal protection depth, see ABB PSTX, Schneider ATS480 and Siemens 3RW55 compared.

Conclusion

Start current on a soft starter is set directly by the current-limit percentage against FLC, but sizing a unit for real duty means reading the full AC-53a code, current multiple, start time, and duty cycle together, and checking it against how often and how close together the machine actually starts. Bypass changes which part of that budget matters day to day; high-cycling loads make the starts-per-hour figure the binding constraint more often than the current figure itself. Confirm the duty rating against the actual start pattern before locking in a frame size, not just against motor kW.