ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 Soft Starter

What separates the ABB PSTX, Schneider ATS480 and Siemens 3RW55 soft starters? All three are the flagship, closed-loop torque-control lines from their respective brands, built to IEC 60947-4-2 with a factory-fitted bypass contactor and current ratings extending to roughly 1200-1250 A. Buy the wrong one and a panel builder ends up paying for a fieldbus module or a keypad the plant never wires up, or specifying a starter that cannot hold a linear torque ramp on a load that needed it. This comparison covers torque-control architecture, bypass and thermal design, communication and HMI options, current range, and protection depth, so a specifying engineer can match hardware to the job rather than to the catalog cover.

Why compare these three and not the entry-level models

ABB also sells the PSR and PSE, Schneider the ATS01 and ATS22, Siemens the 3RW50 and 3RW52. Those cover basic voltage-ramp or current-limit duty on simple pump and fan loads. PSTX, ATS480 and 3RW55 sit above them because each adds closed-loop torque control, not just current limiting — the starter measures developed torque continuously and adjusts SCR firing angle to keep the ramp linear, rather than only tracking a voltage curve set at commissioning. That distinction matters most on high-inertia loads and pumps, where a voltage-ramp starter can still produce a torque dip or a pressure spike partway through the start.

All three also replace an older generation: PSTX supersedes PST/PSTB, ATS480 replaces ATS48, and 3RW55 sits above the discontinued 3RW44. Cross-referencing an old tag against one of these three is usually straightforward; wiring terminals and mounting footprints are close enough that most retrofits do not need panel modification.

Torque control: three closed loops aimed at the same problem

Starting torque falls with the square of applied voltage. A starter running at 50% voltage delivers roughly 25% of the locked-rotor torque the motor produces at full line voltage — not half. That is why a plain voltage ramp can still slam a check valve or stall on a loaded conveyor belt even though the current looks controlled.

ABB's PSTX runs this as its core control mode across the whole range, paired with a detachable keypad HMI for local setup. Schneider brands its version TCS, Torque Control System, on the ATS480 — same closed-loop principle, tuned with pump- and conveyor-specific profiles built into the parameter set. Siemens implements closed-loop torque control on the 3RW55 with adjustable pump ramps aimed at the same soft-stop use case: killing check-valve slam on shutdown, not just cutting inrush on start. None of the three is meaningfully "better" at the torque math — the differentiation is in how each exposes the tuning parameters and which application profiles come pre-loaded.

Bypass contactor and thermal design

SCRs dissipate roughly 1-1.5 W per amp per phase while conducting. Left running through SCRs continuously, a starter needs a bigger enclosure and forced ventilation to shed that heat. All three flagships close a built-in bypass contactor once the motor reaches full speed, shorting the SCRs so they run cool and the enclosure only has to handle bypass-contactor losses, not semiconductor losses.

The practical effect is the same across brands: enclosure sizing should use the bypassed run current, not the continuous SCR-conducting rating, because in normal operation the SCRs are only in circuit during the start and stop ramps. Where the three differ slightly is duty documentation — check the AC-53a or AC-53b table for the specific frame size before assuming bypass is automatic below a certain amperage; on some smaller frames within each range, bypass is optional rather than standard.

What we see in the field: panel builders sometimes spec forced ventilation anyway, out of habit from older non-bypass units. On a current-generation PSTX, ATS480 or 3RW55 with the bypass engaged, that airflow is often unnecessary once actual heat rise is measured — worth checking before adding a fan that just adds noise and a failure point.

Communication and HMI

This is where the three differ most, and it usually decides the winner faster than any torque-control benchmark. ABB PSTX ships with Modbus RTU built in and an Anybus communication module as an add-on for Profibus, Profinet or EtherCAT — flexible, but it is a separate part number and a separate purchase decision. Schneider's ATS480 has Modbus embedded and a graphic display for local parameter access. Siemens 3RW55 carries PROFINET and PROFIBUS onboard without an add-on module, which matters directly in a plant already standardized on Siemens automation.

HMI philosophy also differs: PSTX uses a detachable keypad that can be dismounted and read from outside the enclosure door on some installs; ATS480 and 3RW55 both use a fixed or door-mounted graphic display. None of the three requires the display for basic operation — most sites 4-20 mA or digital I/O to a PLC and rarely touch the local keypad after commissioning.

Current range and which frame covers more motors

PSTX extends to roughly 1250 A, with the related PSTB range pushing higher still for very large motors. ATS480 and 3RW55 both top out around the 1200 A class in their standard catalog. For the large majority of industrial motors — pumps, fans, compressors, conveyors in the sub-500 kW range — all three ranges overlap heavily, and the deciding factor is rarely raw amperage. It becomes relevant mainly at the top of each range, where PSTX's extension into PSTB territory gives ABB a slight edge for the largest single-starter applications before a design has to move to series-parallel SCR arrangements or a different starting method entirely.

Protection functions side by side

All three cover the same core protection set at this tier: thermal overload (I²t, selectable class), phase loss and phase imbalance, phase sequence, locked-rotor/stall detection, shorted-SCR detection, and a starts-per-hour limiter built from the SCR thermal model. Differences show up in extras — some frame sizes add undercurrent detection for dry-run pump protection, and the specific fault codes and reset behavior differ between the three HMIs. None of that changes the sizing math; a starter still has to be selected against the motor's locked-rotor current and the duty cycle, not against a feature checklist.

For the sizing exercise itself, see the soft starter sizing guide, and for the AC-53a/AC-53b duty math specifically, the AC-53a and AC-53b duty sizing article walks through the calculation in full.

Comparing the three at a glance

None of the three is a wrong choice on paper — they sit within the same tier on torque control, bypass, and protection depth. This is largely a decision driven by installed automation standard, existing spares on the shelf, and which local distributor stocks the frame size needed on short lead time. For a broader look at where PSTX, ATS480 and 3RW55 and their entry-level siblings sit relative to each other, the soft starter selection guide covers the full family tree across all three brands.

Sometimes the real question is not which soft starter to buy, but whether a VFD would serve the application better — variable speed operation, not just controlled starting, changes the economics on pumps and fans with a wide flow range. That trade-off is covered in detail in the soft starter vs VFD comparison rather than repeated here. All the underlying mechanisms — SCR conduction, bypass switching, in-line versus inside-delta wiring — apply the same way across PSTX, ATS480 and 3RW55; none of the three changes the physics. For the firing-angle fundamentals, see how SCR phase-angle control works, and for why the bypass contactor exists, see the bypass contactor explainer. Anyone still deciding between a voltage-ramp, current-ramp or torque-control mode before narrowing to these three brands should read voltage ramp vs current ramp vs torque control first.

On pump duty specifically, the soft-stop ramp matters as much as the start ramp — a linear torque-controlled stop avoids the check-valve slam that a simple voltage-ramp stop can still produce. See soft starter sizing and settings for pumps for the settings that apply across all three brands here. Stocked units from all three lines, including PSTX, ATS480 and 3RW55 frame sizes, are listed in the soft starters collection.

Conclusion

ABB PSTX, Schneider ATS480 and Siemens 3RW55 sit in the same tier: closed-loop torque control, built-in bypass, current ranges overlapping up to roughly 1200-1250 A, and comparable protection depth. The differences that actually change a purchase decision are fieldbus (PROFINET/PROFIBUS onboard on 3RW55 versus a module on PSTX, embedded Modbus on ATS480), HMI style (detachable keypad versus fixed graphic display), and which legacy line a retrofit needs to match. Start the selection from the plant's existing automation standard and spares shelf, then confirm current rating and duty against the motor's actual starting profile — not the other way around.