What is a soft starter in an IEC 61439 panel assembly?

A soft starter is a solid-state motor control device that uses SCRs to reduce voltage during motor start and, in some cases, stop. In IEC 61439 assemblies it is typically mounted in an MCC feeder or dedicated motor starter section with upstream short-circuit protection and downstream overload or motor protection as required. Unlike a VFD, it does not provide continuous speed control; its purpose is controlled acceleration, reduced line disturbance, and lower mechanical stress. Common product families include ABB PSTX, Schneider Altistart 22/ATS480, Siemens 3RW series, and Eaton S811+.

How do I size a soft starter for a motor panel?

Sizing is based primarily on motor full-load current, starting duty, load type, ambient temperature, and the required starting torque. For centrifugal loads, a unit rated near motor FLA may be sufficient; for heavy-duty starts or frequent starts, choose a higher current frame and verify the manufacturer’s overload and start-duty curves. Always check IEC 60947-4-2 data, Type 1/Type 2 coordination with the upstream MCCB or fuses, and whether the panel will use internal bypass or external bypass. In IEC 61439 design, the thermal profile of the complete assembly also matters.

When should I use a soft starter instead of a VFD?

Use a soft starter when the process needs reduced inrush and smoother starting, but not continuous speed variation. Typical examples are pumps, fans, conveyors, and compressors that run at fixed speed after startup. A VFD is better when you need energy optimization, speed control, torque control across the operating range, or process feedback. Soft starters are often lower cost, simpler to engineer, and generate less harmonic distortion than VFDs, making them attractive in IEC 61439 motor-control-centre panels where the motor only needs controlled starting and stopping.

Do soft starters need a bypass contactor?

Many soft starters use an internal or external bypass contactor so the SCRs are not carrying current continuously after the motor reaches full speed. This reduces losses, cabinet heat, and component stress. Internal bypass is common in compact units, while external bypass may be preferred for high-duty or higher-current applications. The choice depends on the manufacturer’s design and coordination data. In panel assemblies, bypass selection must be reflected in thermal calculations, wiring layout, and maintenance access in accordance with IEC 61439-1 and IEC 61439-2.

What protections should be included with a soft starter panel?

A typical soft-starter panel includes upstream short-circuit protection using an MCCB or fuses, a disconnecting means, the soft starter itself, and motor overload protection or electronic motor protection depending on the model. Many modern devices add phase loss, underload, jam, thermal, and communication diagnostics. For compliance, verify coordination against IEC 60947-4-2 and the manufacturer’s Type 1 or Type 2 tables. In more demanding installations, integration with protection relays, contactor feedback, and SCADA via Modbus, PROFIBUS, PROFINET, or Ethernet/IP is common.

Where are soft starters most commonly used in MCCs and process panels?

Soft starters are most common in motor-control-centre feeders for pumps, fans, blowers, compressors, crushers, mixers, and conveyors. They are also widely used in packaged pump skids, HVAC plant rooms, water treatment stations, irrigation systems, and district cooling plants. In these applications they reduce mechanical shock and help limit voltage dips on weak networks. In IEC 61439 assemblies, they are typically grouped with other motor feeders and may share ventilation, segregation, and control wiring strategy with contactor-based starters or VFD feeders.

What standards apply to soft starters inside low-voltage panels?

The key standards are IEC 61439-1 and IEC 61439-2 for the assembly design and verification, IEC 60947-4-2 for semiconductor motor controllers and soft starters, and IEC 60947-4-1 where contactors and overload coordination are involved. If the panel is installed in potentially explosive atmospheres, IEC 60079 becomes relevant for the overall installation concept. For arc-flash resistant enclosures or specific test requirements, IEC 61641 may also apply. Panel builders should also follow the manufacturer’s application manual for clearance, cooling, and protection device selection.

Can a soft starter reduce water hammer in pumping systems?

Yes. Soft starters with soft-stop or pump-control functions can reduce sudden deceleration of the motor and limit hydraulic shock when stopping pumps. This helps mitigate water hammer, pressure surges, and pipe stress, especially in booster stations, irrigation networks, and municipal water systems. The feature is particularly useful when a full VFD is not required. However, the control settings must be tuned to the pump curve and system inertia. In an IEC 61439 panel, ensure the thermal and coordination data still align with the chosen stop profile and duty cycle.

Component

Soft Starters

Reduced voltage motor starting, torque control, bypass options

Overview

Soft starters are solid-state motor control devices used to reduce inrush current, limit torque shock, and improve starting behavior for AC induction motors in industrial and building services applications. They are typically built around antiparallel SCRs/thyristors on each phase, with advanced control algorithms such as voltage ramp, current limit, torque control, kick start, and adaptive acceleration. In IEC 61439 low-voltage assemblies, soft starters are commonly installed in motor-control-center sections, custom engineered power panels, and dedicated soft-starter panels for pumps, fans, compressors, crushers, mixers, and conveyor drives. Typical ratings range from 7.5 kW up to 800 kW and beyond at 400 V or 690 V, with current frames from about 17 A to 1200 A depending on the manufacturer and duty class. Leading product families include ABB PSTX, Schneider Electric Altistart 22 and ATS480, Siemens 3RW40/3RW44/3RW55, Eaton S811+, Danfoss MCD 600, and Rockwell Automation SMC-3/SMC Flex, each offering distinct bypass, communications, and protection options. From a panel design perspective, soft starters must be coordinated with upstream protection devices such as MCCBs or ACBs, motor overload relays where applicable, contactors, and short-circuit protection sized according to the manufacturer’s Type 1 or Type 2 coordination data per IEC 60947-4-1. In many systems, the soft starter’s internal bypass contactor is used to reduce heat dissipation after run-up, while external bypass arrangements may be preferred in high-duty or high-ambient installations. Thermal design is critical because SCR losses are present during start and, if bypass is not used, during steady-state operation. Panel builders must verify cabinet ventilation, creepage/clearance, conductor sizing, and temperature rise limits under IEC 61439-1 and IEC 61439-2, especially in dense MCC sections with VFDs, protection relays, and control power supplies. Selection is driven by motor full-load current, load inertia, start frequency, required starting torque, permissible voltage dip, and process requirements such as soft stop for water-hammer mitigation in pumping stations. For centrifugal pumps, fans, and HVAC chillers, soft starters can provide a lower-cost alternative to VFDs when continuous speed variation is not needed. For heavy-duty applications such as mills or screw compressors, models with current limiting, pump control, and adaptive acceleration are preferred. In hazardous-area related installations, panel integration must consider enclosure classification and, where relevant, equipment selection in accordance with IEC 60079. For arc-flash resistant assemblies, the panel may also be evaluated under IEC 61641. Soft starters are often incorporated in MCC panels for utility and process plants, packaged pump skids, fire pump bypass systems where permitted by local codes and standards, and infrastructure panels for water treatment, district cooling, and irrigation. A well-designed IEC 61439 assembly will clearly define functional unit segregation, form of separation where applicable, heat management, and access for maintenance. Properly selected and installed, soft starters deliver controlled acceleration, reduced mechanical wear, lower electrical stress, and improved system reliability without the complexity of full speed control.