MCC Panels

Soft Starter Panel

Enclosed soft starter assemblies for reduced voltage motor starting with torque control, ramp-up/down profiles, and bypass contactor options.

Soft Starter Panel

A Soft Starter Panel is a purpose-built low-voltage motor control assembly designed for controlled acceleration and deceleration of squirrel-cage induction motors using solid-state reduced-voltage starting. Built in accordance with IEC 61439-1 and IEC 61439-2, these assemblies integrate a soft starter, incoming protection, motor protection, bypass switching, control power, and optional communication modules into a single tested panel architecture. Typical implementations range from 7.5 kW to 800 kW at 400 V, with higher-voltage variants for 690 V systems and current ratings commonly from 16 A up to 1600 A depending on duty and enclosure size. The main objective is to limit starting current, reduce mechanical shock on pumps, fans, compressors, crushers, conveyors, and mixers, and avoid supply dips that can affect other plant loads. The power section of a soft starter panel usually includes an MCCB or ACB as the incomer, line and bypass contactors, semiconductor thyristor/SCR modules inside the soft starter, motor overload or electronic protection relay functions, and a busbar system sized and verified for the declared short-circuit withstand current. Depending on the application, the panel may be configured with Forms 1, 2, 3, or 4 internal separation to improve maintainability and fault containment. For large process drives, engineers often select form of separation 3b or 4b to isolate outgoing feeders and simplify service while maintaining operational continuity in multi-motor systems. Soft starter technology is distinct from variable frequency drives. A soft starter controls motor voltage during start and stop through phase-angle control, typically limiting starting current to about 2 to 4 times full-load current, rather than the 6 to 8 times associated with direct-on-line starting. Many panels include a bypass contactor that closes once full speed is reached, removing heat dissipation from the semiconductor stack and improving efficiency during continuous running. This is especially valuable in high-duty applications such as water and wastewater pumping stations, HVAC systems, cement plants, and material handling lines. Design and verification must consider IEC 61439 temperature rise limits, dielectric properties, clearances and creepage, short-circuit performance, and circuit robustness. Component selection should align with IEC 60947-2 for circuit breakers, IEC 60947-4-1 for contactors and motor-starters, and IEC 60947-4-2 for soft starters and semiconductor controllers where applicable. Panels installed in hazardous areas or adjacent to explosive atmospheres may require additional evaluation against IEC 60079 and, where used as an enclosure-related safeguard against ignition of dust, IEC 61641 arc-resistant considerations are sometimes specified by project standards. EMC performance is addressed through IEC 61000 measures such as proper segregation of control wiring, shield termination, surge protection devices, and filtering for PLC, HMI, and fieldbus compatibility. For industrial-manufacturing, oil-and-gas, mining-metals, pharmaceuticals, and food-and-beverage facilities, a well-engineered Soft Starter Panel offers a practical balance of reduced starting stress, compact footprint, and lower lifecycle cost than full VFD-based solutions when continuous speed control is not required. Optional features include bypass timer logic, motor thermistor inputs, earth-fault protection, power metering, Modbus or Profibus communication, and IP-rated enclosures for harsh environments. When designed and type-tested correctly, the result is a reliable IEC 61439 assembly suited to repetitive starts, critical pumping duties, and process equipment that benefits from controlled torque rather than variable speed operation.

Components for This Panel

Soft Starters

Soft Starters selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Moulded Case Circuit Breakers (MCCB)

Moulded Case Circuit Breakers (MCCB) selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Contactors & Motor Starters

Contactors & Motor Starters selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Busbar Systems

Busbar Systems selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Protection Relays

Protection Relays selection, integration, and best practices for Soft Starter Panel assemblies compliant with IEC 61439.

Applicable Standards

IEC 61439-2 (PSC)

IEC 61439-2 (PSC) compliance requirements, testing procedures, and design considerations for Soft Starter Panel assemblies.

EMC Compliance (IEC 61000)

EMC Compliance (IEC 61000) compliance requirements, testing procedures, and design considerations for Soft Starter Panel assemblies.

IP Protection Ratings

IP Protection Ratings compliance requirements, testing procedures, and design considerations for Soft Starter Panel assemblies.

ATEX / IECEx Certification

ATEX / IECEx Certification compliance requirements, testing procedures, and design considerations for Soft Starter Panel assemblies.

Industries Using This Panel

Industrial Manufacturing

Soft Starter Panel assemblies engineered for Industrial Manufacturing applications, addressing industry-specific requirements and compliance standards.

Oil & Gas

Soft Starter Panel assemblies engineered for Oil & Gas applications, addressing industry-specific requirements and compliance standards.

Water & Wastewater

Soft Starter Panel assemblies engineered for Water & Wastewater applications, addressing industry-specific requirements and compliance standards.

Mining & Metals

Soft Starter Panel assemblies engineered for Mining & Metals applications, addressing industry-specific requirements and compliance standards.

Food & Beverage

Soft Starter Panel assemblies engineered for Food & Beverage applications, addressing industry-specific requirements and compliance standards.

Pharmaceuticals

Soft Starter Panel assemblies engineered for Pharmaceuticals applications, addressing industry-specific requirements and compliance standards.

Related Knowledge Articles

Soft Starter vs VFD: When to Use Which

Comparing soft starters and VFDs for motor starting applications.

Type 2 Coordination for Motor Starters

Selecting coordinated MCCB-contactor-overload combinations.

Complete Guide to the IEC 61439 Standard Series

Comprehensive overview of all IEC 61439 parts, their scope, and how they apply to different panel types.

Frequently Asked Questions

A Soft Starter Panel is an IEC 61439-2 low-voltage assembly that uses SCR-based soft starters to reduce motor starting current and torque during acceleration and deceleration. It is ideal when the process does not need continuous speed control. A VFD panel, by contrast, varies motor frequency and voltage for both starting and running, enabling speed control but adding higher complexity, harmonics, and cost. Soft starters typically limit inrush to about 2 to 4 times full-load current, while direct-on-line starting can reach 6 to 8 times FLC. In most panels, the soft starter is bypassed after ramp-up using a bypass contactor to reduce heat losses and improve efficiency. IEC 60947-4-2 is the key product standard for semiconductor motor controllers, while IEC 61439 governs the assembly design and verification.
Typical Soft Starter Panel ratings depend on the motor duty, enclosure, ambient temperature, and starting frequency, but common ranges are 7.5 kW to 800 kW at 400 V, with 690 V options for larger industrial systems. Current ratings often span from 16 A to 1600 A, though project-specific assemblies can be higher when multiple feeders or large incomers are included. The panel’s short-circuit withstand rating must be verified as part of IEC 61439 design verification, and the chosen MCCB or ACB, busbar system, contactors, and soft starter must all be coordinated to that value. In practice, the correct rating is set by motor full-load current, starting duty, ambient derating, and the thermal behavior of the SCR modules and bypass path.
A typical Soft Starter Panel includes an incoming MCCB or ACB, a soft starter unit with thyristor/SCR power elements, line and bypass contactors, overload or electronic motor protection, control power supply, terminal blocks, and often a communication module for PLC or SCADA integration. For larger systems, panel builders may add current transformers, power meters, phase-loss and phase-sequence protection, surge protection devices, and fan control for thermal management. In IEC 61439 assemblies, these components must be arranged to satisfy temperature rise, short-circuit, creepage, and clearances. Where the motor is installed in a critical process line, designers often add protection relays and thermistor inputs to improve fault detection and reduce downtime.
A bypass contactor is not mandatory in every case, but it is strongly recommended for most industrial Soft Starter Panels. Once the motor reaches full speed, the bypass contactor closes and carries the load current directly, removing the SCR losses from the power path. This reduces heat generation, improves efficiency, and extends the life of the semiconductor stack. Bypass is particularly valuable in high-duty applications such as pumps, fans, and conveyors that run for long periods after start-up. For IEC 61439 verification, the bypass arrangement must be included in the thermal and short-circuit assessment. In addition, the contactor selection should comply with IEC 60947-4-1 and be coordinated with the soft starter’s rated current and duty class.
Soft Starter Panels may be built with Form 1, Form 2, Form 3, or Form 4 internal separation depending on the required level of segregation between busbars, functional units, and terminals. Form 1 offers minimal separation, while Form 4 provides the highest degree of compartmentalization and service continuity. In industrial systems, Forms 3b and 4b are commonly selected when outgoing feeders need isolation for maintenance without shutting down the entire assembly. The choice affects accessibility, fault containment, cable management, and maintenance practices. Under IEC 61439-2, the form of internal separation must be clearly declared and verified as part of the assembly design, especially where multiple motors, critical processes, or large short-circuit levels are involved.
Soft Starter Panels can be used in plants with hazardous areas, but the installation location and any associated equipment must be evaluated carefully against IEC 60079 requirements. The panel itself is typically installed in a safe area or in a suitably certified enclosure arrangement, while field devices and motors may require Ex-rated protection methods depending on the zone classification. For oil-and-gas, mining, and chemical applications, the panel builder must confirm enclosure IP rating, temperature rise, cable glands, grounding, and protection against arcing or hot surfaces. If dust ignition risk is relevant, IEC 61641 considerations may also be specified by the project. Always coordinate with the hazardous-area classification and the site’s Ex documentation before selecting the final panel design.
Although soft starters generally produce less harmonic distortion than VFDs, EMC still matters because thyristor switching can generate electrical noise and transients. IEC 61000 practices are used to manage electromagnetic compatibility through proper segregation of power and control wiring, shielded cables for communication lines, correct grounding and bonding, and surge protective devices on control circuits. Control relays, PLC inputs, and communication modules should be kept physically separated from the power section, especially in large panels with long cable runs. For plants with sensitive instrumentation, the panel may also include filtering, ferrites, and optimized routing of motor leads. EMC-conscious design helps prevent nuisance trips, communication errors, and failures in SCADA-connected systems.
Soft Starter Panels are widely used in applications where controlled starting is needed but continuous speed variation is unnecessary. Common sectors include water and wastewater pumping stations, industrial manufacturing, HVAC, mining and metals, food and beverage processing, pharmaceuticals, and oil-and-gas utilities. Typical loads include centrifugal pumps, fans, compressors, crushers, belt conveyors, agitators, and mixers. These loads benefit from reduced mechanical stress, lower electrical stress on the supply network, and smoother process start-up. In many facilities, the panel provides a lower-cost and lower-maintenance alternative to a VFD while still meeting operational requirements. For EPC projects, this makes soft starter assemblies a practical choice when the engineering objective is torque control and start limiting rather than variable-speed operation.

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