Soft Starters in Motor Control Center (MCC)

Select the soft starter based on the motor full-load current, starting frequency, load inertia, and duty class, then verify the assembly thermal and short-circuit limits under IEC 61439-1/2. For MCC feeders, the device current rating should be checked against enclosure ambient temperature, grouping, and ventilation. Also confirm coordination with the upstream MCCB or fuse using the manufacturer’s type-tested tables under IEC 60947-4-2. In practice, the selected soft starter must not only start the motor reliably, but also fit the panel’s busbar rating, temperature-rise envelope, and declared short-circuit withstand level. Bypass versions are often preferred in MCCs to reduce losses after ramp-up and improve long-term thermal performance.

The required short-circuit rating depends on the available fault current at the MCC incomer and the coordination scheme chosen by the designer. Common project requirements are 25 kA, 36 kA, 50 kA, or 65 kA at 400/415 V, but higher values may be specified for large industrial plants. The soft starter itself is usually protected by an upstream MCCB or fuse combination, and the complete feeder must be validated as an assembly under IEC 61439-1/2. Use the soft starter manufacturer’s approved combination tables to ensure the contactor, overload relay, and protective device remain compliant with IEC 60947 coordination requirements and do not exceed the starter’s conditional short-circuit rating.

Yes, bypass contactors are strongly recommended in most MCC applications. During acceleration, the soft starter controls voltage and current using thyristors, but after the motor reaches nominal speed, the bypass contactor carries the steady-state current. This reduces heat dissipation inside the panel, improves efficiency, and lowers stress on semiconductor components. In an IEC 61439 MCC, bypass selection is important because thermal management is a major design criterion, especially in grouped motor sections. The bypass contactor must be coordinated with the soft starter and overload protection per the manufacturer’s data and IEC 60947-4-2 guidance.

Yes. Most modern soft starters support communication modules for Modbus RTU, Modbus TCP, Profibus, Profinet, Ethernet/IP, or similar industrial protocols. This makes them suitable for SCADA and BMS integration in process plants, utilities, and building services MCCs. Typical data points include motor current, start/stop status, fault codes, thermal capacity, and event logs. When integrating into an MCC, ensure wiring segregation, EMC practice, and terminal accessibility are compatible with IEC 61439 assembly design. For larger plants, communication-enabled soft starters simplify diagnostics, remote resets, and condition monitoring, which reduces downtime and supports predictive maintenance.

Soft starters contribute heat during starting and especially during prolonged ramp periods because of thyristor conduction losses. If the MCC section uses a bypass contactor, steady-state heat is much lower after the start sequence completes. Under IEC 61439-1/2, the panel designer must account for the soft starter’s power dissipation, the number of starts per hour, cabinet ventilation, internal spacing, and grouping of other heat-producing devices such as VFDs, relays, and power supplies. Thermal verification may be based on design rules, comparison with tested assemblies, or calculation/measurement methods accepted by the standard. Proper thermal design is essential to maintain reliability and avoid nuisance trips.

Soft starter cubicles in MCCs are commonly designed with Forms 2, 3b, or 4 separation depending on service continuity and maintenance requirements. Form 3b or Form 4 is often used where the incoming busbar, functional units, and terminals must be segregated for safer maintenance and better fault containment. The choice must align with the project’s operational philosophy and the assembly’s verified layout under IEC 61439-2. In higher-risk environments, greater segregation can also improve arc-flash risk management, especially when combined with robust busbar systems and tested internal barriers. The final arrangement should be validated against the manufacturer’s assembly documentation and internal design rules.

A soft starter is preferred when the objective is reduced-voltage starting and stopping without variable speed control. It is typically more economical, compact, and simpler than a VFD for pumps, fans, compressors, and conveyors that run at fixed speed. In an MCC, soft starters are a good fit when mechanical stress reduction, lower inrush current, and bypass operation are the main requirements. By contrast, a VFD is better when speed control, energy savings across variable load profiles, or process regulation is needed. The decision should consider motor duty, harmonic impact, thermal performance, and the panel’s IEC 61439 design limits.

The main standards are IEC 61439-1 and IEC 61439-2 for low-voltage switchgear assemblies, and IEC 60947-4-2 for AC semiconductor motor controllers and soft starters. Depending on the site, additional standards may apply: IEC 61641 for internal arc fault testing, IEC 60079 for hazardous locations, and IEC 60947-4-1 for contactors and overload coordination. For the complete MCC, the manufacturer must document rated current, busbar withstand, temperature-rise behavior, and protection coordination. In projects built by Patrion, these standards are used together to ensure the soft starter feeder is safe, maintainable, and suitable for real industrial operating conditions.