Siemens 3TF6844-0CM7 AC Contactor – SIRIUS 3TF6

Siemens 3TF6844-0CM7 — 250 A Heavy-Duty AC Contactor for High-Current Motor Switching

The Siemens 3TF6844-0CM7 is a three-pole AC contactor rated at 250 A under AC-3 utilization category, belonging to the SIRIUS Innovation 3TF6 contactor family. It is designed for direct-on-line and star-delta motor starting in motor control centers, pump stations, compressor drives, and large HVAC systems where switching reliability under sustained thermal and mechanical stress is non-negotiable. The -0CM7 suffix encodes the coil voltage variant: 200–220 V AC at 50/60 Hz, placing this unit in the mid-voltage control circuit range commonly specified in Asian and European industrial installations.

Within the 3TF6 series, the 3TF6844 frame designates the 250 A current rating with four integrated auxiliary contacts (4 NO + 4 NC), eliminating the need for bolt-on auxiliary contact blocks in standard wiring configurations. The contactor operates on a laminated silicon-steel electromagnet core with short-circuit rings to suppress AC hum, a design detail that reduces audible noise in enclosed switchgear panels. Main contact material is silver-cadmium oxide (AgCdO) or silver-tin oxide (AgSnO₂) depending on production batch, both of which maintain stable contact resistance across the rated switching life of ≥1,000,000 mechanical operations and ≥500,000 electrical operations at AC-3 duty.

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Technical Parameters

Hardware Logical Analysis

The 3TF6844-0CM7 employs a double-break main contact geometry per pole. Each pole contains two contact bridges in series, which halves the arc voltage per break point and distributes thermal energy across a larger contact surface area. At 250 A AC-3, the inrush current during motor starting can reach 6–8× the rated current for 50–100 ms; the double-break design ensures the arc is extinguished within the arc chamber before the contact material reaches its melting threshold.

The electromagnetic actuator uses a C-frame laminated core with a copper short-circuit ring embedded in the pole face. This ring induces a phase-shifted flux component that prevents the magnetic force from dropping to zero at each AC half-cycle, eliminating the 100/120 Hz mechanical chatter that would otherwise accelerate contact wear and generate panel noise. The coil winding is encapsulated in thermosetting resin, providing Class F (155 °C) thermal insulation and resistance to condensation in humid industrial environments.

EMC performance is addressed through the arc chamber geometry: the arc runners and splitter plates are arranged to stretch and cool the arc rapidly, limiting the rate of rise of recovery voltage (RRRV) and reducing high-frequency conducted emissions back into the control bus. For installations requiring additional coil transient suppression, Siemens 3TX6 varistor modules clip the inductive kick from the coil de-energization to below 1,000 V peak, protecting adjacent PLC digital output cards from overvoltage stress.

The 4 NO + 4 NC auxiliary contact block is mechanically linked to the main armature via a rigid actuator bridge, ensuring zero differential travel between main and auxiliary contact state transitions. This eliminates race conditions in PLC ladder logic that depend on auxiliary contact feedback for interlock confirmation — a critical requirement in reversing starter and star-delta transition circuits where simultaneous main contact closure would cause a phase-to-phase short circuit.

System Integration Benefits

• Direct SIMATIC S7 Digital Output Compatibility: The 200–220 V AC coil voltage aligns with standard 220 V AC control circuits driven by Siemens S7-300/400/1500 relay output modules (e.g., SM 322 DO 16×AC 120/230 V), enabling direct coil drive without intermediate relays.
• Star-Delta Transition Precision: The rigid auxiliary contact actuator bridge provides deterministic contact state feedback within 10 ms of main contact actuation, supporting tight star-delta transition timing in S7 timed sequence programs without auxiliary relay buffering.
• Overload Relay Coordination: The 3TF6844-0CM7 is dimensionally and electrically coordinated with Siemens 3RU2 thermal overload relays and 3RB3 electronic overload relays for Type 2 coordination per IEC 60947-4-1, meaning the contactor and overload relay are tested together to withstand short-circuit currents without hazardous damage.
• Reversing Contactor Interlock: Siemens 3TK mechanical interlock kits are factory-matched to the 3TF6 frame, providing positive mechanical exclusion between two contactors in a reversing configuration — a hardware-level safety measure independent of PLC software interlocks.
• Diagnostic Transparency via Auxiliary Contacts: The four independent auxiliary contact pairs allow simultaneous wiring of: (1) PLC feedback input, (2) local panel indicator lamp, (3) SCADA hardwired status, and (4) spare for future expansion — all from a single contactor without add-on blocks.
• Panel Space Efficiency: The 3TF6 frame integrates auxiliary contacts within the contactor body footprint, reducing panel depth by approximately 40 mm compared to configurations requiring separate auxiliary contact blocks, a measurable advantage in compact MCC cubicles.
• Wide Voltage Tolerance: The coil operates reliably between 85% and 110% of rated voltage (170–242 V AC), accommodating the voltage fluctuations common in industrial distribution networks without nuisance dropout or coil overheating.
• Surge Suppressor Compatibility: The 3TX6 RC and varistor suppressor modules snap directly onto the coil terminals without tools, reducing installation time and ensuring consistent suppressor placement across multiple identical panels.
• IEC/UL Dual Certification: Simultaneous IEC 60947-4-1 and UL 508 listing allows a single part number to be specified for both European CE-marked and North American UL-listed panel builds, simplifying global project procurement.
• Thermal Derating Data Availability: Siemens publishes full thermal derating curves for the 3TF6 series, enabling panel designers to calculate accurate current derating at elevated ambient temperatures without conservative over-sizing.

Quality Assurance & Global Logistics

Every Siemens 3TF6844-0CM7 unit supplied by siemensplc.com is sourced through verified industrial distribution channels. Each shipment undergoes label authenticity verification — checking holographic seals, date codes, and factory markings against Siemens published authentication criteria — before dispatch. Coil continuity and contact resistance are spot-checked on incoming stock. Units are individually packed in anti-static foam inserts within corrugated export cartons rated for international air and sea freight handling.

Shipments originate from Xiamen, China, a major export hub with direct access to DHL Express, FedEx International Priority, and consolidated sea freight services to Southeast Asia, the Middle East, Europe, and the Americas. Standard DHL Express transit times from Xiamen are 2–4 business days to most destinations. Export documentation — commercial invoice, packing list, certificate of origin, and MSDS where required — is prepared in-house and provided with every shipment. For project orders requiring third-party inspection or specific country-of-origin certificates, arrangements can be made upon request. All units carry a 12-month warranty against manufacturing defects from the date of shipment.

Contact Information

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