MCC Panels

Contactors & Motor Starters

DOL/star-delta/reversing starters, overload relays, Type 2 coordination

Contactors & Motor Starters

Contactors and motor starters are core control components in IEC 61439 low-voltage switchgear and controlgear assemblies, providing reliable switching, isolation-by-function, and overload protection for three-phase motors and auxiliary loads. In practical panel applications, these devices are selected by utilization category under IEC 60947-4-1: AC-3 for squirrel-cage motor starting and stopping, AC-4 for inching, plugging, and reversing duties, and AC-1 for resistive or lightly inductive loads. Typical assemblies range from compact 9 A to 95 A starters for small pumps and fans up to 250 A or higher in feeder-controlled motor control centers, with coordination to upstream MCCBs or fuses to achieve Type 1 or Type 2 coordination per IEC 60947-4-1. Type 2 coordination is preferred in critical processes because the contactor and overload relay must remain suitable for continued service after a short-circuit event, subject to manufacturer test data. A complete motor starter typically includes a power contactor, a thermal bimetal or electronic overload relay, auxiliary contacts, terminal blocks, and a short-circuit protective device. Common product families include Schneider Electric TeSys D and TeSys F, Siemens SIRIUS 3RT and 3RU/3RB overload relays, ABB AF contactors with electronic coil technology, Eaton DILM contactors and ZB overload relays, and Lovato Electric BF and RF series. For demanding applications, electronic overload relays offer phase-loss sensitivity, adjustable trip classes such as Class 10, 20, or 30, thermal memory, and communication options for PLC integration. Starter topologies are chosen according to motor size and process requirements. Direct-on-line (DOL) starters are used where the supply network can tolerate inrush current, commonly on small conveyors, exhaust fans, and packaged pumps. Star-delta starters reduce starting current and mechanical stress, but require motors with six accessible terminals and are best suited to moderate inertia loads. Reversing starters are used on hoists, gates, and directional conveyors and require electrical and mechanical interlocking. Soft-starter panels use controlled voltage ramping and current limiting, often with bypass contactors, while variable-frequency-drive panels may retain contactors only for isolation or bypass duty. In power-factor-correction panels, contactors must be capacitor-duty types with inrush-limiting contact design, while harmonic-filter or capacitor-bank panels often use heavy-duty switching contactors rated for high repetitive capacitor currents. In IEC 61439 assemblies, the thermal and dielectric performance of starters must be verified within the panel’s design envelope, including spacing, busbar loading, wiring cross-section, enclosure temperature rise, and short-circuit withstand ratings. Typical panel ratings may include 400 A, 630 A, 800 A, 1600 A, or higher main busbars, with starter feeders coordinated to prospective short-circuit currents such as 25 kA, 36 kA, 50 kA, or 65 kA for 1 second or the declared peak withstand value. For hazardous areas or dust-exposed installations, the overall system may also need evaluation against IEC 60079 requirements, while arc containment and internal fault behavior may be addressed under IEC 61641 for certain low-voltage assemblies. Contactors and motor starters are widely used in motor control centers, soft-starter panels, PLC automation panels, custom engineered panels, automatic transfer-related auxiliaries, and process skids across water treatment, HVAC, mining, food processing, material handling, and utilities. Correct selection depends on motor FLC, duty cycle, ambient temperature, coordination class, enclosure ventilation, and the available fault level at the installation point. For EPC contractors and panel builders, the most reliable designs pair a manufacturer-verified starter combination with documented SCCR, coordinated protection settings, and clear wiring diagrams that comply with IEC 61439 and IEC 60947 requirements.

Panels Using This Component

Motor Control Center (MCC)

Centralized motor control with starters, contactors, overloads, and VFDs in standardized withdrawable/fixed functional units.

Power Factor Correction Panel (APFC)

Automatic capacitor switching for reactive power compensation. Thyristor or contactor-switched, detuned or standard configurations.

Automatic Transfer Switch (ATS) Panel

Automatic changeover between mains and generator/UPS. Open or closed transition, with or without bypass.

Variable Frequency Drive (VFD) Panel

Enclosed VFD assemblies with input protection, line reactors, EMC filters, output reactors, and bypass options.

PLC & Automation Control Panel

Process and machine control panels housing PLCs, I/O modules, relays, HMIs, and communication infrastructure.

Custom Engineered Panel

Bespoke panel assemblies for non-standard requirements — special ratings, unusual form factors, multi-function combinations.

Soft Starter Panel

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

Harmonic Filter Panel

Active or passive harmonic filtering to mitigate THD from non-linear loads. Tuned LC filters, active filters, or hybrid configurations.

Capacitor Bank Panel

Fixed or automatic capacitor bank assemblies for bulk reactive power compensation in industrial and utility applications.

Related Knowledge Articles

Type 2 Coordination for Motor Starters

Selecting coordinated MCCB-contactor-overload combinations.

Motor Control Center (MCC) Design Guide

Comprehensive guide to MCC design and engineering.

ACB vs MCCB: Selection Criteria for Panel Design

Choosing between air circuit breakers and moulded case circuit breakers.

Frequently Asked Questions

A contactor is the switching device; a motor starter is the complete control/protection package. In IEC 60947-4-1 terms, the contactor performs frequent make-and-break operations under AC-3 or AC-4 duty, while the starter combines the contactor with an overload relay and often a short-circuit protective device such as an MCCB or fuse. For panel builders, this distinction matters because starter selection must match motor full-load current, utilization category, and coordination class. Product families such as Schneider TeSys D, Siemens SIRIUS 3RT, ABB AF, and Eaton DILM are widely used in IEC 61439 assemblies for this purpose.
Use a DOL starter when the supply network and mechanical load can tolerate the motor’s inrush current and starting torque is needed immediately. DOL is common for smaller pumps, fans, compressors, and conveyors. Star-delta is preferred where reduced starting current is necessary, but it requires a motor with six terminals and is better suited to medium-inertia loads. IEC 60947-4-1 does not mandate one topology over the other; the decision depends on motor size, load torque curve, and permissible voltage dip. In practice, DOL often uses compact contactors and overload relays, while star-delta requires three contactors plus timers and interlocking logic.
Type 2 coordination means that after a short-circuit fault, the starter components may be suitable for further use without replacement, provided they have not suffered welds or significant damage beyond manufacturer-defined limits. This is defined in IEC 60947-4-1 and is a preferred design goal for critical industrial loads. Achieving Type 2 coordination requires pairing the contactor, overload relay, and upstream SCPD exactly as tested by the manufacturer. Examples include Schneider TeSys with specified Acti9 or ComPacT protective devices, Siemens SIRIUS coordinated combinations, and ABB AF starter combinations with tested short-circuit ratings.
Thermal overload relays are cost-effective and widely used for standard applications, while electronic overload relays are better for demanding or digitally integrated systems. Electronic relays offer adjustable trip class settings, phase-loss detection, ground-fault options on some models, alarm contacts, and communication with PLCs or SCADA. They are often preferred on larger motors, frequent-start applications, and critical pumps where diagnostic visibility matters. IEC 60947-4-1 governs overload relay performance, and selection should consider motor FLA, service factor, ambient temperature, and the required trip class, commonly Class 10, 20, or 30.
Yes, but usually for isolation, bypass, or network switching rather than for direct motor starting under variable-frequency operation. In VFD panels, the drive handles acceleration, deceleration, and speed control, so the contactor is typically placed on the input side, output side only when the manufacturer permits it, or in a bypass circuit for emergency operation. Incorrect switching on the VFD output can damage the drive. Panel designers should follow the drive OEM’s instructions and coordinate the contactor’s utilization category and the panel’s short-circuit rating under IEC 61439 and IEC 60947.
Capacitor-duty contactors are required because capacitor banks draw very high inrush currents at energization. Standard motor contactors are not ideal for repeated capacitor switching. In power factor correction and harmonic filter panels, manufacturers such as ABB, Schneider Electric, Siemens, and Eaton offer dedicated capacitor-switching contactors with pre-charge resistors or damping resistors to limit peak current. Selection must consider kvar rating, line voltage, switching frequency, ambient temperature, and reactor presence. These systems are typically designed in accordance with IEC 60947 and, where integrated into the assembly, within the verified enclosure and thermal limits of IEC 61439.
Reversing starters require two contactors with mechanical and electrical interlocking to prevent simultaneous forward and reverse energization. The contactors must be rated for the motor’s AC-3 current, and in some applications AC-4 if plugging or frequent direction changes occur. The interlock logic, auxiliary contact arrangement, and wiring segregation are important in panel design to avoid unsafe cross-connection. In IEC 61439 assemblies, the builder must also verify clearance, creepage, thermal rise, and fault withstand for the starter compartment. Common reversing solutions are built from Schneider TeSys D, Siemens SIRIUS, ABB AF, or Lovato modular systems.
Contactors and motor starters are most commonly found in motor control centers, soft-starter panels, PLC automation panels, variable-frequency-drive panels, custom engineered panels, capacitor-bank panels, harmonic-filter panels, and power-factor-correction panels. They are also used in transfer-related auxiliary circuits and process equipment panels for pumps, fans, conveyors, mixers, and compressors. In IEC 61439 assemblies, the choice depends on feeder current, fault level, enclosure ventilation, and coordination with upstream MCCBs or fuses. For EPC contractors and facility managers, the best practice is to specify a manufacturer-tested starter combination with documented coordination and a verified short-circuit rating.

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