Contactor Wiring Diagram: Main Contacts, Coil and Auxiliary Contact Circuits
A contactor wiring diagram shows how three-phase line supply connects through main contacts to the motor load, and how the 24 V or 230 V control circuit energises the coil via push-button stations, auxiliary contacts and overload relay contacts.
A contactor is an electromechanical switching device designed for frequent operation — switching loads at duty cycles far exceeding those tolerated by standard circuit breakers. Unlike a circuit breaker, a contactor provides no overcurrent protection; it is a pure switching element that must be paired with a thermal overload relay or electronic motor protection relay for motor applications. The main contacts are the large contacts that interrupt the main load current. In a three-phase motor circuit, three main contacts — labelled 1/L1-2/T1, 3/L2-4/T2, 5/L3-6/T3 — connect line supply L1, L2, L3 to motor terminals T1, T2, T3 when the coil is energised. Main contacts are rated in AC-3 utilisation category (for squirrel-cage motor switching) or AC-1 (for resistive loads). The coil circuit, sometimes called the control circuit, is entirely separate from the main circuit and typically operates at a reduced voltage — 24 V AC or DC, or 230 V AC — to protect the control panel from the high voltage of the main circuit. The coil terminals are labelled A1 and A2. Energising A1–A2 at rated coil voltage causes the coil to generate a magnetic field that pulls the armature and closes the main contacts simultaneously. Auxiliary contacts are small-rated contacts — typically 10 A — that are mechanically linked to the main contact armature and change state simultaneously with main contacts. Normally-open auxiliary contact 13-14 is used as a hold-in or seal-in contact wired in parallel with the start push-button, maintaining the coil circuit energised after the start button is released. Normally-closed auxiliary contact 21-22 is used in interlocking circuits — for example, in reversing starters, the NC auxiliary of the forward contactor is wired in series with the coil of the reverse contactor, ensuring both cannot energise simultaneously. Overload relay contact 95-96 (NC) is wired in series with the coil supply to trip the control circuit and de-energise the contactor if the overload relay detects excess current.
How to wire contactor wiring diagram
- Mount contactor and overload relay DIN rail mount the contactor and clip the overload relay directly onto the contactor base. Confirm the overload relay rating matches the motor FLA range. Set the overload dial to motor nameplate FLA.
- Connect main circuit Connect L1, L2, L3 incoming supply to contactor terminals 1, 3, 5. Connect outgoing terminals 2, 4, 6 to overload relay input. Connect overload relay output to motor terminals T1, T2, T3 using appropriately rated cable.
- Wire control circuit supply Connect control supply (24 V or 230 V AC) through the stop push-button (NC contact) to one end of the start push-button (NO) and to terminal A2 of the contactor coil. Connect the other end of the start push-button to coil terminal A1.
- Wire hold-in auxiliary contact Connect auxiliary contact 13 to the junction of stop button and start button. Connect auxiliary contact 14 to coil A1. This parallel path maintains the coil after start button release.
- Wire overload relay trip contact Wire overload relay NC contact (terminals 95-96) in series with the coil supply, between the control supply and the stop push-button. Overload trip opens this contact, de-energising the coil and stopping the motor.
Specifications
| Main contact rating | AC-3 utilisation category, matched to motor kW rating |
|---|---|
| Coil voltage options | 24 V AC/DC, 110 V AC, 230 V AC (specify on order) |
| Auxiliary contacts (standard) | 1 NO (13-14) and 1 NC (21-22) standard; expandable |
| Overload relay setting | Set to motor nameplate full-load amps (FLA) |
Safety warnings
- Main contactor contacts operate at full line voltage — confirm main circuit is de-energised separately from the control circuit before touching contactor main terminals.
- Never manually push the contactor armature to close contacts with voltage on the main terminals — arc flash at rated current causes severe burns.
- Coil voltage must exactly match the supply voltage applied to A1-A2 — applying 230 V to a 24 V coil destroys the coil insulation immediately.
Tools needed
- Clamp-on ammeter for measuring main contact current against motor nameplate
- Digital multimeter for coil voltage and auxiliary contact resistance checks
- Insulated screwdriver for torquing main terminal and coil terminal screws
- Contact resistance tester for main contact drop measurement during maintenance
Common mistakes
- Wiring the overload relay contact in series with the main circuit instead of in the control coil circuit, leaving the motor unprotected from overload.
- Applying the wrong coil voltage — wiring a 24 V coil to 230 V control power causes immediate coil failure and potential fire.
- Failing to wire a seal-in auxiliary contact in parallel with the start button, causing the motor to stop as soon as the operator releases the start push-button.
Troubleshooting
- Contactor energises but motor does not start
- Cause: Open main contact on one or more phases, or loose main terminal connection Fix: De-energise main circuit. Inspect main contact faces for pitting or welding. Measure contact drop across each main contact with millivoltmeter — above 50 mV indicates worn contacts needing replacement.
- Motor starts but overload trips quickly
- Cause: Overload relay set too low, or one phase open causing unbalanced current Fix: Verify overload relay setting matches nameplate FLA. Measure current on all three phases — imbalance above 5% indicates phase loss. Correct open fuse or contact and re-verify current balance.
- Coil burns out repeatedly
- Cause: Contactor coil voltage incorrect or contact face not seating properly Fix: Measure actual control supply voltage at A1-A2. Verify it matches coil rating label. Clean armature contact faces with dry cloth. Check shading ring integrity. Inspect for mechanical obstruction preventing armature from fully closing.
Frequently asked questions
What is the difference between a contactor and a circuit breaker?
A contactor is designed for frequent switching operations — millions of cycles — but provides no overcurrent protection. A circuit breaker is designed primarily for overcurrent and short-circuit protection and is rated for infrequent operation (hundreds of operations before replacement). Motor circuits require both: a circuit breaker or fuses for short-circuit protection, and a contactor for normal start-stop switching, with an overload relay for thermal protection.
What does AC-3 utilisation category mean?
AC-3 is the IEC contact utilisation category for switching squirrel-cage induction motors under normal conditions — making at locked-rotor current (up to 6-8 times full-load amps) and breaking at running current. This is the most demanding standard contactor duty cycle. The contactor must be rated at least AC-3 for motor switching applications. AC-1 is for resistive and slightly inductive loads, which requires lower contact interrupt capability.
How does the hold-in (seal-in) auxiliary contact work?
The normally-open auxiliary contact 13-14 is wired in parallel with the start push-button (NO). When the start button is momentarily pressed, current flows through the button to energise the coil. As the contactor closes, auxiliary contact 13-14 also closes, providing an alternative current path that maintains coil energisation after the button is released. The stop push-button (NC) in series with this circuit breaks the coil current, releasing the armature and opening all contacts.
Why does my contactor chatter or hum loudly?
Humming or chattering indicates the coil is not holding the armature firmly closed. Causes include: coil voltage below minimum (check supply voltage under load), dirt or corrosion on the armature face preventing full contact, or a failed shading ring on the armature pole face. The shading ring is a copper ring embedded in the pole face that provides a phase-shifted magnetic flux component to eliminate the twice-per-cycle zero-force point that would otherwise cause 100/120 Hz armature vibration.
How do I test auxiliary contact function?
With the contactor de-energised, measure continuity between terminals 13 and 14 (NO contact — should be open) and between 21 and 22 (NC contact — should show continuity). Energise the coil with rated voltage and repeat: 13-14 should now show continuity and 21-22 should be open. Any deviation indicates a worn or failed auxiliary contact block requiring replacement.