PSTX720 — what fits on the line side
The ABB PSTX720 (1SFA898218R7000) is a 720 A soft starter for three-phase induction motors on 208–690 V AC supplies, 50/60 Hz. Auto phase sequence detection is baked in — it checks rotation before the ramp starts, saving a contactor and a callout on pump or conveyor direction. Ramp time is adjustable from 1 to 120 seconds, with initial voltage from 10 to 99 % and a step-down special ramp from 100 to 10 % for soft stop. Current limit function spans 1.5 to 7.5 x Ie. Fieldbus coverage is wide: Modbus-RTU, Modbus-TCP, Ethernet/IP, EtherCAT, DeviceNet, CANopen, Profibus, Profinet, BACnet-IP, and BACnet-MSTP. That list covers most PLC backbones without an extra gateway. Analog outputs are configurable as 0–10 V, 0–20 mA, or 4–20 mA for drive feedback to a VFD or SCADA input card.
Protection and signalling — what trips and what tells you
Built-in electronic overload protection with separate overload curves for start and run (dual overload). Protection functions include ground fault, earth fault, phase reversal, locked rotor, under/over voltage, current imbalance/underload, PTC connection, too-long start time, and too-long current limit. Warning outputs mirror most of these — current imbalance, underload, electronic overload time-to-trip, EOL, faulty fan, locked rotor, motor runtime limit, over/under voltage, phase loss (standby), power factor underload, short circuit (limp mode), THD(U), thyristor overload, and voltage imbalance. Four signal relays are standard — run, fault, bypass, and overload — each with a dedicated LED on the front panel (green for run, red for fault, yellow for protection). The control circuit runs on 24 V DC; the control supply accepts 100–250 V AC. Main circuit terminals are bars with a tightening torque of 45 N·m; control circuit connections accept rigid 1 x 2.5 mm² wire.
Ambient operating temperature is -25 to +60 °C. Main terminals are IP00 per IEC 60529 and IEC 60947-1 — open bus bars, so the panel enclosure provides the ingress protection. The unit supports inside-delta connection for reduced cable sizing on the motor side. Service factor is rated at 100 %.