new TECO CU-18 Magnetic Contactor 35 Amp, 3 Phase, 110V Coil 3A1a1b (TAIAN CN-18)

TECO · Magnetic Contactor · CU-18 · 35A / 3-Phase · 110V Coil · In stock

What Is the TECO CU-18 Magnetic Contactor?

The TECO CU-18 (equivalent to TAIAN CN-18) is a 3-pole magnetic contactor rated for 35 amps at 3-phase AC voltage, with a 110V AC coil and a set of auxiliary contacts (3 normally-open + 1 normally-closed, designated 3A1A1B). TECO (Tongbao Electric Co.) and TAIAN are well-established Taiwanese manufacturers of industrial motor control components. A contactor is essentially a heavy-duty relay purpose-built for switching motors, heaters, lighting banks, and other high-power loads — it is the component you find inside motor starter assemblies, control panels, and power distribution cabinets.

In short: A 35-amp, 3-phase industrial contactor — the electromechanical switch that a PLC output or pushbutton energizes to start a motor, with built-in auxiliary contacts for control logic (seal-in circuits, run lights, PLC feedback).

Understanding Contactor Ratings and Auxiliary Contacts

1. AC-3 vs. AC-1 Ratings: 35A Means Different Things for Different Loads

The 35A rating on a contactor is typically the AC-3 rating (IEC 60947-4-1) — the current it can switch for a squirrel-cage motor (making 6x rated current, breaking at rated current). For resistive heating loads (AC-1), the same contactor might be rated for a higher continuous current (e.g., 50A or more). Always check the specific utilization category on the contactor nameplate. If you are switching a 3-phase motor, size the contactor by the motor's full-load current (FLC), not its horsepower. A 35A AC-3 contactor typically handles a 15-18.5 kW (20-25 HP) motor at 380-415V, or a smaller motor at 220V.

2. Auxiliary Contacts: 3A1A1B Explained

The 3A1A1B designation means this contactor includes six auxiliary contacts in addition to the three main power poles: three normally-open (3NO, or 3A in TECO notation) and one normally-closed (1NC, or 1B). Auxiliary contacts are low-current (typically 5-10A at 250VAC) and are used for control logic: a NO contact wired in parallel with the Start pushbutton creates a seal-in (holding) circuit that keeps the contactor energized after you release the Start button; a NC contact wired in series with the coil of another contactor provides electrical interlocking to prevent both contactors from closing simultaneously. Verify the exact auxiliary contact arrangement from the contactor nameplate — the 3A1A1B configuration may be fixed or may use add-on contact blocks.

3. 110V Coil: Control Voltage Considerations

The 110V AC coil is the control voltage that energizes the contactor. This is the voltage your control circuit must supply — from a control transformer, a PLC output module rated for 110VAC, or directly from one phase of your supply if it is 110V. Do not connect a 110V coil to a 220V or 24V supply. The coil has a significant inrush current (VA burden when closing) that drops to a lower holding VA once closed. If a PLC transistor output is driving this coil, use an interposing relay — the coil inrush can exceed a typical PLC output's current rating.

Key Specifications

Frequently Asked Questions

Is the TECO CU-18 a direct drop-in replacement for a TAIAN CN-18?

Yes — TECO and TAIAN contactors of the same frame size (CU-18 / CN-18) are mechanically and electrically interchangeable. They share the same DIN rail / screw mounting footprint, the same coil voltage options, and the same auxiliary contact arrangements. The CU-18 and CN-18 are essentially the same product under two brand names from related Taiwanese manufacturers. The main power terminal positions and the coil terminal positions will match. If your old contactor was a different brand (Schneider LC1-D, Siemens 3RT, ABB A-series), the physical footprint and coil voltage may differ — do not assume cross-brand interchangeability without measuring.

How do I wire a start-stop circuit using the auxiliary contacts?

The classic 3-wire control circuit: Wire a normally-open Start pushbutton from one side of the control voltage to the contactor coil terminal A1. Wire one of the contactor's NO auxiliary contacts in parallel with the Start button (this is the seal-in — once the contactor closes, the auxiliary contact holds the circuit energized even after you release Start). Wire a normally-closed Stop pushbutton in series with the coil circuit — pressing Stop breaks the holding current and the contactor drops out. Connect the other side of the coil (A2) to the control voltage return. Add a properly-sized fuse or circuit breaker in the control circuit. The remaining auxiliary contacts can be used for run indicators, PLC feedback inputs, or sequencing other equipment.

Can this contactor be used with a VFD output or soft starter?

This contactor is designed for across-the-line (Direct-On-Line / DOL) motor starting at full voltage and frequency. Do not install it between a VFD output and the motor — VFD outputs have high-frequency PWM waveforms with steep voltage rise times (dV/dt) that stress contactor insulation beyond its design rating. If you need an isolation contactor in a VFD system, place it on the input (line) side of the VFD, not between the VFD and the motor. For a soft starter, the contactor can serve as an isolation/bypass contactor on the line side, but sizing should account for the soft starter's bypass mode if applicable.

The contactor chatters or buzzes loudly — what causes this?

Contactor chatter (rapid opening and closing, or a loud 50/60 Hz hum) is typically caused by: (1) Low control voltage — if the 110V coil is receiving significantly less than 110V (due to undersized control transformer, long wire runs, or poor connections), the magnetic force is insufficient to hold the armature firmly closed. Measure voltage at the coil terminals while the contactor is energized. (2) Dirt or corrosion on the magnet faces — the armature and core pole faces must make clean, flat contact. If they are dirty, rusty, or have debris between them, the magnetic circuit has an air gap that causes hum. Clean the pole faces with a dry cloth. (3) A shading coil failure — AC contactors have a copper shading ring embedded in the magnet core that prevents the magnetic field from crossing zero at every half-cycle. If the shading ring is cracked or missing, the contactor will buzz loudly at 100/120 Hz.

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