ABB PSTX Soft Starter: Full Range Review

What is the ABB PSTX soft starter range? PSTX is ABB's flagship soft starter, a reduced-voltage motor starter that adds closed-loop torque control and a built-in bypass contactor across ratings from small pump duty up into the 1000+ A class. Torque control tracks the load through the whole ramp instead of following a fixed voltage curve, so it starts high-inertia pumps and conveyors that a plain voltage-ramp unit stalls partway through. This review covers PSTX frame sizes and current ratings, the three control modes, the bypass and protection package, fieldbus options, sizing math, and how PSTX compares to ABB's own PSE and PSR ranges.

PSTX Frame Sizes and Current Ratings

PSTX covers a wide current span in one family, from small frames in the tens of amps up to roughly 1250 A at 400-690 V. The PSTB extension pushes further for the largest process motors, using the same torque-control platform and keypad HMI. One part family across that range means a panel builder standardizes on one firmware and one commissioning procedure whether the motor is a 30 kW pump or a 500+ kW compressor. Browse the full range in our soft starters collection for exact SKUs and current stock.

PSTX supports both in-line and inside-delta connection, so the same frame size can run at full line current or be wired for a lower per-leg current on a bigger motor. What we see in the field: the inside-delta option gets skipped more often than it should, because it needs all six motor leads pulled to the starter — worth checking before ruling out a smaller frame on cost.

Control Modes: Voltage Ramp, Current Limit and Torque Control

PSTX runs all three control strategies from one platform. Voltage ramp increases output voltage over a set time, open loop, simplest to commission and adequate for lightly loaded fans. Current limit holds start current to a ceiling — typically 300-400% FLC — regardless of how the load behaves. Torque control closes the loop on estimated motor torque, delivering a close-to-linear speed rise that voltage ramp cannot guarantee once the load's torque curve stops being linear.

The distinction matters most on pumps: a voltage-ramp start can still slam a check valve or cavitate an impeller partway through the ramp, because voltage and torque rise linearly but the load's torque demand does not. Torque control adjusts firing angle in real time to hold the torque profile the operator set. See our breakdown of voltage ramp vs current ramp vs torque control for the full comparison across brands.

Built-In Bypass Contactor and Thermal Behavior

SCRs dissipate roughly 1-1.5 W per amp per phase while conducting. Run continuously through the SCRs, a starter this size needs a derated current rating and a larger enclosure just to shed that heat. PSTX closes an internal bypass contactor once the motor reaches full speed, shorting the SCRs so line current flows through metal contacts instead of silicon.

The practical effect: no continuous-duty derating on the SCRs, and a smaller enclosure footprint per amp than an SCR-only design on loads with long run times. Duty is rated per IEC 60947-4-2 AC-53b (with bypass) rather than only AC-53a — check the datasheet against actual starts-per-hour and start time, not just motor kW.

Motor Protection and Diagnostics

PSTX carries the protection set expected of a flagship unit: I²t overload with selectable trip class (10/20/30), phase loss and phase imbalance, phase sequence, locked-rotor/stall, too-many-starts thermal modeling, shorted-SCR detection, and undercurrent for dry-run pump protection. None of this is unique to PSTX — PSE covers a subset — but the detachable keypad HMI puts fault history and live current/torque readings in front of the technician without a laptop.

Some panel builders still wire a separate overload relay downstream "just in case." Unnecessary on PSTX. The thermal model already accounts for start frequency and SCR temperature, and a second relay only adds another trip point to coordinate during commissioning.

Communication and HMI Options

PSTX ships with Modbus RTU built in. An Anybus module adds Profibus, Profinet, or EtherCAT for plants standardized on those buses, a straightforward retrofit since the fieldbus card is modular rather than baked into the base unit. Analog and digital I/O handle simple start/stop and speed-reference interfacing where a fieldbus is unnecessary.

The detachable keypad is worth calling out specifically: it unplugs from the starter and can be panel-mounted on the door, so an operator reads status and resets faults without opening the enclosure. Neither PSE nor PSR offers this — PSR has no display at all, set entirely by analog trimmer.

Sizing PSTX: Torque vs Voltage

Starting torque falls with the square of the ramped voltage, not linearly. At 50% voltage a motor delivers about 25% of its direct-on-line locked-rotor torque, the single most common sizing mistake with any soft starter, PSTX included. The load's breakaway and accelerating torque must stay below the motor's reduced torque at every point through the ramp, not just at full speed, or the motor stalls before it reaches run speed.

PSTX's torque-control mode uses this relationship in reverse: the operator sets a target torque ramp, and the unit adjusts firing angle to hold it instead of tracking a fixed voltage curve and hoping the resulting torque suffices. For a full walkthrough on matching a specific motor and load, see our soft starter sizing guide.

PSTX vs PSE vs PSR: Where It Sits in ABB's Range

ABB sells three soft starter families, and PSTX is not the right choice for every job. An 11 kW fan on a fixed schedule does not need torque control or a fieldbus card.

Where PSTX earns its price over PSE: torque control on high-inertia loads, fieldbus integration into a plant SCADA, and a keypad HMI for troubleshooting from the panel door. Where it does not: a fixed-speed exhaust fan running a handful of starts a day gets no benefit from torque control it never uses.

PSTX also competes outside ABB, against Schneider's Altistart ATS480 and Siemens' SIRIUS 3RW55 — all three offer closed-loop torque control, built-in bypass, and fieldbus comms in the same current class. See the full ABB PSTX vs Schneider ATS480 vs Siemens 3RW55 comparison for the criteria that actually separate them on a given job. Pumps are where the torque-control tier earns its keep across all three brands; our soft starters for pumps guide covers ramp and stop settings specific to centrifugal and positive-displacement loads.

Conclusion

PSTX is ABB's answer for soft-starter jobs that need more than a voltage ramp: torque control for high-inertia and pump loads, a built-in bypass for thermal headroom, fieldbus integration, and a keypad HMI for panel-side diagnostics. For simpler duty, PSE or PSR cover the job at a lower cost. Start sizing from the load's torque and duty cycle, not just motor kW — our soft starter selection guide walks through choosing between PSTX and the rest of the field for a specific motor.