Mitsubishi S-K20 Magnetic Contactor – S-K Series

Mitsubishi S-K20 — 20A Three-Pole AC Magnetic Contactor for Industrial Motor Control

The Mitsubishi Electric S-K20 is a three-pole AC magnetic contactor rated at 20A continuous current under AC-3 duty classification, designed for direct-on-line (DOL) starting and switching of three-phase squirrel-cage induction motors up to 5.5 kW at 400V AC. It belongs to the S-K Series, Mitsubishi Electric’s compact IEC-compliant contactor family engineered for high-density motor control center (MCC) assemblies, standalone motor starters, and PLC-driven automation panels. The S-K20 occupies a well-defined position in the motor control hierarchy: it handles the power-switching function between the upstream protective device and the motor terminals, while its auxiliary contact outputs feed discrete status signals back to the control logic layer.

In a typical control loop, the S-K20 coil is energized by a PLC digital output module (24V DC or 110/220V AC depending on coil variant), a relay interposing stage, or a hardwired pushbutton circuit. Upon coil energization, the three main contacts close simultaneously, connecting line voltage to the motor. The 1NO auxiliary contact mirrors the main contact state and is wired to a PLC digital input for run-confirmation feedback, enabling the controller to detect welded-contact faults or coil dropout anomalies. The 1NC auxiliary contact is used in interlocking schemes — particularly in reversing starters and star-delta transition circuits — to enforce mutual exclusion between forward/reverse contactors or between the star and delta switching stages.

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Technical Parameters

Hardware Logical Analysis

The S-K20’s electromagnetic actuator uses a laminated silicon-steel E-I core geometry. The lamination stack suppresses eddy-current losses and reduces audible hum at 50 Hz operation — a persistent issue in older solid-core designs. The shading ring embedded in the stationary core face generates a phase-displaced flux component that prevents the armature from releasing at each AC zero-crossing, eliminating the 100 Hz chatter that degrades contact surfaces over time.

Contact material in the S-K20 main poles is a silver-cadmium oxide (AgCdO) or silver-tin oxide (AgSnO₂) alloy, selected for its resistance to arc erosion and contact welding under inrush conditions. During DOL motor starting, the inrush current can reach 6–8× the rated full-load current for 50–200 ms. The contact geometry — with a defined contact gap and over-travel — ensures arc extinction within the arc chute before the contact surfaces re-engage, preventing re-strike and contact material transfer.

The auxiliary contact block is mechanically linked to the main armature via a rigid actuator bridge, not a separate solenoid. This ensures zero timing offset between main and auxiliary contact state transitions — critical for PLC feedback loops where a delayed auxiliary signal could trigger false fault detection in the control program. The bridge mechanism also provides the physical basis for the 1NC interlock: the NC contact opens before the NO contact closes, enforcing a break-before-make sequence that prevents cross-conduction in reversing starter configurations.

EMC performance is addressed through the coil circuit design. The coil winding is encapsulated to prevent moisture ingress and vibration-induced wire fatigue. For DC coil variants (where applicable), a freewheeling diode or varistor is integrated or recommended across the coil terminals to suppress the inductive kickback voltage spike (typically 10–50× supply voltage) that occurs at de-energization — protecting the PLC output transistor from overvoltage damage. For AC coil variants, the natural zero-crossing of the AC supply limits the peak transient energy.

System Integration Benefits

• Deterministic switching latency: Operate time (coil energized to main contacts closed) is typically 15–25 ms; release time is 10–20 ms. These fixed windows allow PLC programmers to set accurate interlock timers without empirical tuning.
• Direct PLC output compatibility: 24V AC/DC coil variants draw ≤ 7 VA sealed, within the current sourcing capacity of standard PLC transistor output modules without relay interposing stages.
• Modular auxiliary expansion: Add-on auxiliary contact blocks (1NO, 1NC, or 2NO+2NC) clip onto the contactor body without rewiring the main circuit, allowing field reconfiguration for star-delta, reversing, or multi-feedback schemes.
• Thermal overload relay integration: The S-K20 mechanically mates with the Mitsubishi TH-K20 thermal overload relay, forming a complete motor protection unit with a single DIN rail footprint and direct terminal connection — no external wiring between contactor and overload relay.
• Reversing starter compatibility: Two S-K20 units with cross-wired NC auxiliary contacts form a mechanically and electrically interlocked reversing starter. The NC contact of each contactor is wired in series with the coil circuit of the opposing contactor, enforcing hardware-level mutual exclusion independent of PLC logic.
• Star-delta transition support: In a three-contactor star-delta starter, the S-K20 handles the main, star, and delta switching positions. The 1NC auxiliary contact of the star contactor interlocks the delta contactor coil circuit, preventing simultaneous closure during the open-transition period.
• Diagnostic transparency: The 1NO auxiliary contact wired to a PLC input enables the control program to distinguish between: (a) coil energized but contacts not closed (mechanical fault), (b) coil de-energized but contacts still closed (welded contacts), and (c) normal run state — providing three distinct diagnostic states from a single feedback point.
• High-density panel layout: The S-K20’s compact body width allows side-by-side mounting of multiple units on a standard 35mm DIN rail without additional spacing, maximizing usable panel real estate in space-constrained enclosures.
• Voltage range flexibility: Rated for operation up to 690V AC, the S-K20 is suitable for both 400V TN-S distribution systems common in Europe and Asia, and 480V systems used in North American industrial installations, without derating.
• Long service interval: With ≥10 million mechanical operations, the S-K20 does not require scheduled contact replacement in typical motor control applications running 100 starts per day — equating to over 270 years of theoretical mechanical life at that duty cycle.

Quality Assurance & Global Logistics

Every S-K20 unit stocked at our Xiamen, China facility is sourced through verified Mitsubishi Electric authorized distribution channels. Prior to storage, each unit undergoes incoming inspection covering OEM labeling integrity, date code verification, hologram authentication, and packaging seal condition. Units showing any sign of repackaging, label inconsistency, or abnormal weight are quarantined and returned to the supply chain.

Storage conditions are maintained at controlled temperature (15–25°C) and humidity (<60% RH) to preserve contact surface integrity and coil insulation resistance over extended shelf periods. For project orders requiring documentation, we provide Certificate of Conformance (CoC), packing list, commercial invoice with HS code 8536.49 declaration, and — upon request — third-party inspection reports.

Shipping from Xiamen is executed via DHL Express, FedEx International Priority, and UPS Worldwide Expedited. Standard dispatch lead time is 1–3 business days from order confirmation. For urgent requirements, same-day dispatch is available for orders confirmed before 14:00 CST. Typical transit times: Southeast Asia 2–4 days; Europe 3–5 days; North America 4–6 days; Middle East 4–7 days. All shipments include tracking numbers and export documentation compliant with Chinese customs regulations.

The 12-month warranty covers manufacturing defects and premature failure under normal operating conditions as defined by IEC 60947-4-1. Warranty claims are processed with photographic evidence and, where required, return of the defective unit for failure analysis. Replacement units are dispatched within 5 business days of claim approval.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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