ABB PSE vs PSTX vs PSR: Which ABB Soft Starter

28 Jul, 2026
Posted by: Muhammet KUL

What separates the ABB PSR, PSE, and PSTX soft starter families? All three ramp motor terminal voltage through back-to-back SCRs per IEC 60947-4-2, but they split hard on control method, bypass, and rating: PSR tops out near 30 A / 15 kW at 400 V with voltage ramp only and no built-in bypass, PSE adds current limit plus a built-in bypass contactor up to a few hundred amps, and PSTX runs closed-loop torque control to roughly 1250 A with fieldbus and a full motor protection suite. Get the choice wrong and you either pay for headroom you never use or under-protect a motor that needed torque control from day one. This article covers control method, rating overlap, bypass and heat, protection and comms, and where each family earns its price.

Control Method: Voltage Ramp, Current Limit, Torque Control

PSR runs open loop. It ramps output voltage from an initial pedestal to full line voltage over a set time, set by an analog trimmer, with no feedback on current or torque during the ramp. Fine for a fan or a lightly loaded pump where the load torque curve is predictable and low.

PSE adds current limit. The unit still ramps voltage, but it clamps the ramp if start current would exceed a set ceiling, protecting the supply and the motor windings on loads with some inrush variability. It sits between PSR's open-loop simplicity and PSTX's closed-loop control.

PSTX is the only one of the three with true torque control, a closed loop that targets motor torque directly rather than voltage or current, producing a near-linear speed rise. That matters most on pumps: torque control avoids the pressure spike a voltage ramp leaves at the end of acceleration. What we see in the field: panel builders spec PSE on general-purpose motors and reach for PSTX only once a pump, conveyor, or crusher is on the load list.

Formula: Torque vs. Voltage Ratio — Source: IEC 60947-4-2, motor torque-speed theory

T_x = T_DOL × (V_x / V_line)²

Symbol	Description	Unit
T_x	Motor torque at reduced voltage	N·m
T_DOL	Locked-rotor torque at full line voltage (DOL start)	N·m
V_x	Applied starter output voltage	V
V_line	Full line voltage	V

At 50% voltage a motor produces roughly 25% of its DOL locked-rotor torque, the square-law relationship that catches out an under-specified PSR or PSE on a stiff load. PSTX doesn't escape this physics; it just manages the ramp against actual torque instead of a fixed voltage curve, so the starter can hold closer to the load's real breakaway point without stalling.

Key takeaway: if the load has variable or high breakaway torque (crusher, conveyor under load, positive-displacement pump), PSTX's closed loop is not a luxury upgrade from PSE — it is the difference between a start and a stall.

ABB PSR, PSE, PSTX Ratings: Where the Ranges Overlap

PSR covers small motors only, roughly to 30 A / 15 kW at 400 V. Beyond that it isn't offered. PSE picks up from there through a few hundred amps. PSTX (with the PSTB extension) runs from small frames up to roughly 1250 A, covering the same mid-range PSE occupies and then extending well past it. PSTX also supersedes the older PST/PSTB generation, so current ABB spec sheets and spare-parts programs point to PSTX for any new design in that upper range.

The overlap band — roughly 100 to 300 A — is where the decision stops being about amps and starts being about control method and protection. Two starters can carry the same nameplate current; only one of them controls torque.

Key takeaway: don't size by amps alone in the 100-300 A band. Check the load's start profile first, then pick PSE or PSTX; sizing by current rating alone in that band is a coin flip.

Bypass Contactor and Heat Dissipation

PSR has no built-in bypass. The SCRs stay in the current path for the entire run, dissipating roughly 1-1.5 W per amp per phase continuously. That heat has to go somewhere — a bigger enclosure, forced ventilation, or a derated continuous rating. Not always a problem on a small fan motor. It becomes one on anything running near-continuous duty at the top of PSR's range.

PSE and PSTX both carry a built-in bypass contactor that closes once the motor reaches full speed, shorting the SCRs out of the circuit. Bypassed operation means the SCR heat problem disappears for the rest of the run — smaller enclosure, no continuous-duty derating, and a cooler panel overall.

Bypass contactor is an electromechanical contactor wired in parallel with the SCR bank that closes after the ramp completes, carrying full run current so the semiconductors stop conducting and stop generating heat (per IEC 60947-4-2 AC-53b duty definitions).

Motor Protection and Communications

PSR keeps to the basics: ramp-up, ramp-down, and whatever protection the trimmer-set analog control offers. No display, no fieldbus, no motor thermal model beyond simple overload.

PSE narrows that gap with current limit and bypass but stops short of a full protection suite or fieldbus connectivity — it fills the space between PSR's simplicity and PSTX's instrumentation without trying to match PSTX feature for feature.

PSTX is the only family with a detachable keypad HMI, full motor protection (phase loss, phase imbalance, phase sequence, locked rotor, shorted-SCR detection, undercurrent for dry-run pumps), and standard Modbus RTU with optional Anybus modules for Profibus, Profinet, or EtherCAT. On a plant floor running SCADA or a PLC-supervised motor control center, that communications layer is often the deciding factor over PSE, independent of amps.

AC-53a / AC-53b duty rating is the IEC 60947-4-2 code describing a soft starter's start current multiple, start time, duty cycle, and starts per hour (e.g., 3.0-10:50 = 3× FLC, 10 s start, 50% duty). AC-53a covers integral or continuous bypass; AC-53b covers external bypass.

PSR vs PSE vs PSTX: Which One Fits

Criteria	PSR	PSE	PSTX
Control method	Voltage ramp only	Voltage ramp + current limit	Closed-loop torque control
Built-in bypass	No	Yes	Yes
Rating ceiling	~30 A / 15 kW @ 400 V	Few hundred amps	~1250 A (PSTX/PSTB)
HMI	Analog trimmer	LCD-less	Detachable keypad display
Protection depth	Basic overload	Current limit, bypass	Full motor protection suite
Fieldbus	None	None	Modbus RTU standard, Anybus optional
Typical duty	Small fans, light pumps	General-purpose motors, mid-range	Pumps, conveyors, crushers, SCADA-integrated lines

Use PSR where the motor is small, the load is light, and nobody needs a display or a fieldbus tag. Use PSE where the motor is mid-size, general-purpose, and current-limit control plus a bypass is enough — most standard fan and pump duty falls here. Reach for PSTX when the load has real breakaway torque, when starts-per-hour and thermal duty need tracking against AC-53a limits, or when the starter has to report into a PLC. For the full sizing and selection process, work through locked-rotor torque, starts per hour, and duty class before locking in a family — see our soft starter selection guide for the full method. On pump duty specifically, torque control's value shows up directly in soft starter sizing for pumps, where fluid hammer at the end of a voltage ramp is the failure mode PSTX exists to remove.

Key takeaway: PSE and PSTX overlap in amps but not in what they protect against. If the spec sheet stops at current and voltage, you're still exposed to a torque-related stall or nuisance trip that only a closed-loop starter catches.

All three families sit in ABB's soft starters range alongside comparable lines from other manufacturers; for a look at how PSTX stacks up against competing flagships, see ABB PSTX vs Schneider ATS480 vs Siemens 3RW55.

Frequently Asked Questions

Can a PSE replace a PSTX on the same motor?

Only if the load doesn't need torque control or fieldbus. On general-purpose fan or pump duty within PSE's amp range, yes. On a high-inertia or pressure-sensitive load, PSE's current-limit-only control can still let a pressure spike or a stall through that PSTX's torque loop would have caught.

Does PSR need a separate bypass contactor?

PSR has no built-in bypass, so if the application runs long or continuous duty at the top of PSR's rating, an external bypass contactor and matching control logic can be added to cut SCR heat. Most PSR applications are light enough that this isn't necessary.

What is the maximum motor size for ABB PSTX?

PSTX and its PSTB extension cover the family's full range, up to roughly 1250 A, well past where PSE's range ends. Exact frame and current combinations depend on the specific catalog reference.

Is PSTX overkill for a simple fan motor?

Often, yes. A fan with a predictable, low breakaway torque curve rarely needs torque control or fieldbus reporting. PSR or PSE covers that duty at lower cost; PSTX's value is in loads where the extra control loop and protection depth actually get used.

Do PSE and PSTX use the same wiring, in-line or inside-delta?

Both support in-line and inside-delta connection depending on the reference selected; the connection choice is independent of which family you pick and comes down to available motor leads and current per SCR leg.

Conclusion

PSR, PSE, and PSTX are not three sizes of the same product — they're three different control philosophies wearing the same brand. PSR is open-loop and bypass-free, sized for small, forgiving loads. PSE adds current limit and a bypass for mid-range, general-purpose duty. PSTX adds closed-loop torque control, full protection, and fieldbus for loads and plants where that instrumentation earns its cost. Match the family to the load's torque profile and the plant's reporting needs, not just to the motor's amp draw.