Soft Starters

A soft starter is a solid-state motor starter that uses antiparallel thyristors to gradually apply voltage to an AC motor during startup, reducing inrush current and starting torque. In an IEC 61439 panel assembly, it is installed with coordinated upstream protection such as an MCCB or fuse-switch and often a bypass contactor. After acceleration, the bypass device carries the motor current to reduce losses and temperature rise. Soft starters are governed by IEC 60947-4-2 as devices, while the panel itself must be verified to IEC 61439-1/-2 for temperature rise, short-circuit withstand, and dielectric performance. This makes them a practical choice for pumps, fans, conveyors, and compressors where controlled starting is needed without continuous speed control.

Sizing is based primarily on motor full-load current, starting duty, ambient temperature, and load inertia rather than only kW. Panel builders should verify the soft starter’s current rating at the actual installation voltage, such as 400 V, 415 V, or 690 V, and confirm whether the duty is normal or heavy. For high-inertia loads, a unit with a higher thermal class or overload capability may be required. The bypass contactor, cable cross-section, upstream protective device, and enclosure ventilation must also be checked. In IEC 61439 assemblies, thermal balance and short-circuit coordination are essential, especially where the prospective fault current is high. Manufacturer families such as ABB PSTX or Schneider ATS480 provide detailed sizing curves for industrial duty.

A soft starter is preferred when the application only needs reduced-voltage starting and stopping, not continuous speed regulation. It is typically selected for pumps, fans, conveyors, and compressors where the process runs at fixed speed but mechanical stress and electrical inrush must be limited. Compared with a VFD, a soft starter is usually simpler, smaller, and more cost-effective, with lower harmonic impact because it is bypassed after start-up. If process control requires speed variation, torque regulation over a wide range, or energy optimization from reduced speed, a VFD is the better choice. In IEC 61439 panels, soft starters also simplify heat management and may reduce EMC design complexity compared with VFD sections.

Bypass contactors are highly recommended in most industrial soft starter installations, especially in MCCs and high-duty panel applications. During ramp-up, the thyristors carry current and generate heat; once the motor reaches rated speed, the bypass contactor transfers the load current away from the semiconductor path. This reduces power loss, lowers enclosure temperature, and improves long-term reliability. Some soft starters include an internal bypass, while others require an external bypass contactor and associated interlocking. Selection must be coordinated with the soft starter’s current rating and the short-circuit rating of the assembly. In IEC 61439-compliant panels, the bypass arrangement must be included in thermal and short-circuit verification, particularly for continuous-duty pump and fan systems.

The main product standard for soft starters is IEC 60947-4-2, which covers semiconductor motor controllers and starters. When installed inside a panel, the complete assembly must comply with IEC 61439-1 and the relevant part of IEC 61439-2 for power switchgear and controlgear assemblies. If the panel is intended for final distribution or feeder applications, IEC 61439-3 or IEC 61439-6 may also be relevant depending on the architecture. In hazardous locations, IEC 60079 requirements may apply, and arc-related mitigation may involve IEC 61641 considerations. For engineers, this means the device selection alone is not enough; the enclosure, busbars, protective devices, wiring, and ventilation must all be verified as a system.

Installation should prioritize thermal management, cable sizing, protective coordination, and service access. Soft starters dissipate heat during starting and, if bypass is absent, continuously during operation, so enclosure ventilation or air conditioning must be calculated carefully under IEC 61439 temperature-rise rules. The upstream protective device, such as an MCCB or fuse-switch, must coordinate with the soft starter’s semiconductor protection limits and short-circuit rating. Cable terminations should follow the manufacturer’s terminal torque and conductor size recommendations. Control wiring should be separated from power cabling to reduce noise, and motor leads should be routed to maintain EMC performance. In industrial panels, many engineers also add phase-loss, overload, and motor protection relays to increase diagnostics and resilience.

Soft starters are most commonly used in motor-control-center sections, dedicated soft-starter panels, and custom-engineered control panels for fixed-speed process loads. They are also common in HVAC pump and fan panels, water and wastewater treatment skids, fire pump auxiliary systems where allowed by the specification, conveyor starters in material handling, and compressor starters in industrial plants. In each case, the panel must be designed to suit the duty cycle, start frequency, and environmental conditions. For large facilities, soft starters may be integrated into MCC sections with forms of separation such as Form 2, Form 3, or Form 4 to improve maintainability and limit downtime. ABB, Siemens, Schneider Electric, Eaton, and Danfoss product families are frequently specified in these applications.

Engineers should compare current rating, voltage range, duty class, starting torque control, bypass configuration, communication options, and diagnostic capabilities. Key differences include overload handling, pump-fan specific algorithms, deceleration control, motor thermal modeling, and the availability of Modbus, Profinet, Ethernet/IP, or Profibus communication. It is also important to review semiconductor protection, internal bypass design, and the permitted number of starts per hour. For panel integration, assess heat losses, enclosure size, and the manufacturer’s short-circuit coordination data with specific MCCBs or fuses. Popular product families such as ABB PSTX, Siemens 3RW5, Schneider ATS480, Eaton DS7, and Lovato ADXL differ in feature depth and application tuning, so selection should align with the process and IEC 61439 assembly requirements.