Soft Starter Cross-Reference: PSTX, Altistart, SIRIUS
What is a soft starter cross-reference? A cross-reference matches a soft starter from one brand — ABB PSTX/PSE/PSR, Schneider Altistart ATS480/ATS22/ATS01, Siemens SIRIUS 3RW55/3RW52/3RW50 — to the closest equivalent in another, based on control method, current rating, connection type and duty class per IEC 60947-4-2, not part number alone. Skip the control-method step and a "matching" replacement can undersize on the actual load even though the amperage lines up on paper. This article covers: family-to-family tier mapping across the three brands, why amp rating alone is not a valid match, in-line vs inside-delta connection checks, bypass and communications differences, the duty-rating math to run before a swap, and when a cross-reference stops working and you need to resize from scratch.
Why a straight amp-for-amp swap fails
Nameplate current is the first number everyone checks, and the last number that should decide a cross-reference. The three brands split into the same three control tiers, and a tier mismatch changes what the starter can do on the load, not just how it looks on a drawing. ABB PSR, Schneider Altistart ATS01 and the base Siemens 3RW50 are voltage-ramp only — open loop, no torque feedback, no built-in bypass on the smallest frames. ABB PSE and Altistart ATS22 add current limiting and bypass across most references, closing part of the gap. PSTX, ATS480 and 3RW55 are the torque-control tier: closed-loop control that holds motor torque on a linear rise, built-in bypass, HMI, and fieldbus as standard.
Replace a PSTX with an ATS22 at the same amp rating and the amperage checks out, but the torque-control loop is gone. On a pump or a conveyor with a stiff breakaway, that difference shows up as nuisance stalls the first week, not on the commissioning sheet.
Family tier mapping across ABB, Schneider Electric and Siemens
The table below lines up the three brands by tier. It is a starting map, not a substitute for checking the actual current, voltage class and duty of the motor being started — see the ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 comparison for the flagship-tier detail.
| Criteria | ABB | Schneider Electric | Siemens SIRIUS |
|---|---|---|---|
| Economy tier | PSR — voltage ramp, no bypass | Altistart ATS01 — voltage ramp | 3RW50 (lower band) — voltage ramp + current limit |
| Mid tier | PSE — current limit, bypass | Altistart ATS22 — voltage/torque, bypass on most refs | 3RW50/3RW52 — current limit, bypass, added safety on 3RW52 |
| Flagship tier | PSTX — torque control | Altistart ATS480 — TCS closed loop | 3RW55 — closed-loop torque control |
| Built-in bypass | PSE and up | ATS22 (most refs) and up | 3RW50 and up |
| Fieldbus on flagship | Modbus RTU, optional Anybus (Profibus/Profinet/EtherCAT) | Embedded Modbus | PROFINET/PROFIBUS |
| Legacy predecessor | Older PST/PSTB | ATS48 (superseded by ATS480) | 3RW30/3RW40/3RW44 |
Current rating and connection type have to cross-check together
Two starters rated for the same line current are not interchangeable if one was sized for in-line and the other for inside-delta. In-line wiring puts the full line current through the SCRs. Inside-delta wiring splits it — each leg carries roughly 58% of line current (line/√3) — which is how a physically smaller starter covers a bigger motor, but only with all six motor leads brought out and the correct delta connection at the terminals. Cross-referencing a compact inside-delta unit into a panel wired for in-line, or the reverse, means re-terminating the motor, not just swapping the starter. See in-line vs inside-delta soft starter connection for the wiring detail.
What we see in the field: a panel builder cross-references by frame size and amp rating alone, orders the replacement, and only discovers the six-lead requirement when the motor terminal box has three leads in it.
Duty rating is where cross-references quietly break
IEC 60947-4-2 rates soft starters by duty class, not just current: AC-53a (bypassed/continuous duty, e.g. 3.0-10:50 meaning 3× FLC for 10 seconds, 50% duty, up to 10 starts per hour) and AC-53b for units used with an external bypass. Two starters with identical AC-53a nameplate ratings can still differ once the actual start time and starts-per-hour of the application are checked against the datasheet curve, because the SCR thermal model behind that number is brand-specific.
Formula: Start current from current-limit setting — Source: IEC 60947-4-2, AC-53a duty designation
Istart = current-limit × FLC
| Symbol | Description | Unit |
|---|---|---|
| Istart | Starting current drawn from the line | A |
| current-limit | Current-limit setting on the starter (typ. 300-400%) | % of FLC |
| FLC | Motor full-load current | A |
Run that number against the replacement's AC-53a rating at the actual start time and starts-per-hour, not just against its continuous current rating. A cross-reference in soft starter selection guide terms is a superset match: the replacement's duty envelope must cover the application's start time and frequency, not just meet the peak amps once. See AC-53a/AC-53b duty sizing for the full derating tables.
Bypass, HMI and communications: what actually changes on the replacement
Even inside the same tier, the three brands differ on what ships standard. ABB PSTX uses a detachable keypad HMI and Modbus RTU natively, with an Anybus module needed for Profibus, Profinet or EtherCAT. Schneider ATS480 has an embedded graphic display and native Modbus. Siemens 3RW55 ships with an HMI and native PROFINET/PROFIBUS, which fits Siemens-heavy PLC architectures without a gateway. A cross-reference that matches current and duty but changes the fieldbus protocol still means a new gateway or a PLC program change — budget for that separately from the starter itself.
Bypass contactor presence matters for heat, not just for control. SCRs dissipate roughly 1-1.5 W per amp per phase while conducting; a bypassed starter shorts them out once the motor is at speed, so the enclosure runs cooler and the starter is not derated for continuous run current. Confirm the replacement bypasses on your exact reference, not just on the family name — see soft starter bypass contactor explained.
When a cross-reference is not enough
A cross-reference assumes the original starter was sized correctly. If the failed unit was undersized for the load in the first place — a common cause of repeat SCR failures on high-inertia loads — matching it brand-for-brand just repeats the mistake in a different enclosure. At that point the job is a resize, not a cross-reference: run the starting-torque check (torque falls with the square of voltage, so 50% voltage gives roughly 25% of DOL locked-rotor torque) against the actual load curve. See how to select and size a soft starter for a motor for the full sizing procedure, and soft starter for pumps if the application is pump-specific, since torque control and soft-stop settings matter more there than raw amperage.
This depends on the load, not the brand: a stiff conveyor or a high-inertia fan can outrun what any voltage-ramp starter delivers, regardless of which nameplate is on it. In that case the cross-reference question is really a control-tier question, answered in the soft starter selection guide.
Frequently Asked Questions
Can I replace an ABB PSTX with a Schneider ATS480 directly?
Both are flagship, torque-control, built-in bypass units, so the control tier matches. Still check amp rating at the actual voltage class, connection type (in-line vs inside-delta), and duty rating at the real start time before ordering, since frame breakpoints differ between the two lines.
Does the amp rating on the nameplate need to match exactly?
No. It needs to equal or exceed the application's required current-limit setting and duty class, not the old starter's nameplate. A slightly higher-rated replacement in the same tier and connection type is normal and often preferable.
What changes if I cross-reference from a voltage-ramp starter to a torque-control unit?
You gain a closed loop that can hold a linear speed rise and cut fluid hammer on pumps, plus better protection functions. Commissioning takes longer because torque-control units have more parameters to set, and some require a load-specific autotune.
Do I need to rewire the motor if I cross-reference between brands?
Only if the connection type changes. An in-line replacement for an in-line original keeps the existing three-lead wiring; moving to or from inside-delta needs all six motor leads brought to the starter, which some existing motor terminal boxes do not support without opening the winding connection.
Is a cross-referenced starter automatically compliant with the original's certification?
Not automatically. Each brand certifies its own line to IEC 60947-4-2 and, where applicable, UL 508; confirm the specific reference you are ordering carries the certification your project requires rather than assuming it inherits the family's rating.
Where do legacy models like ATS48 or 3RW44 fit in a cross-reference?
Both are superseded lines: ATS48 by ATS480, and 3RW44 by 3RW55 as the high-end successor. Cross-referencing a legacy unit should target the current flagship, not a like-for-like reissue, since the older parts are progressively harder to source.
Conclusion
A soft starter cross-reference across ABB, Schneider Electric and Siemens works when it is done in order: control tier first, then connection type, then current rating checked against the duty curve at the real start time and starts-per-hour, then bypass and communications. Skip a step and the replacement looks correct on paper while missing what the load actually needs. When the original unit was undersized to begin with, no cross-reference fixes that — only a resize does. For SKU-level stock and lead time across all three brands, see the soft starters collection.
