Contactor
Switchgear
An electromechanical switch rated for high current and frequent operation, used to switch motors, heaters, and lighting banks. Like a relay but built for industrial duty cycles and three-phase loads.
Schematic symbol
Relay
How contactor appears in a circuit diagram.
Types & variants
Key specs
Rated current (AC-3)
ACurrent for motor switching. Common: 9A, 12A, 18A, 25A, 32A, 40A, 50A, 65A, 80A, 100A, 150A.
Coil voltage
VControl voltage. Common: 24V DC, 24V AC, 230V AC, 400V AC.
Poles
—3-pole for three-phase, 4-pole for switched neutral, 1- or 2-pole for DC.
Utilization category
—AC-1 (resistive load), AC-3 (squirrel-cage motor, full voltage start), AC-4 (jogging/plugging).
Mechanical life
ops10 million operations typical. Electrical life 1–3 million at full load.
Markings
Rated current (e.g. 32A), coil voltage (e.g. 230V 50Hz), AC-3 category, manufacturer (Schneider LC1, ABB AF, Siemens 3RT), auxiliary contact codes (NO/NC).
Standard values
Common frame sizes: 9A (small motors), 18A (1.5kW), 32A (5.5kW), 50A (11kW), 65A (15kW), 100A (22kW), 150A (45kW). Coil power: 2–10VA hold.
How to choose
1) Pick AC-3 current ≥ motor full-load current × 1.25. 2) Match coil voltage to your control circuit. 3) Select 3-pole for 3-phase motors. 4) Add auxiliary contacts if your control logic needs status feedback. 5) Add thermal overload relay for motor protection. 6) For frequent reversing, use a mechanically-interlocked reversing pair.
Pinout & package
DIN-rail or panel mount. Top: L1/L2/L3 in, T1/T2/T3 out (3-pole main contacts). Side/bottom: A1/A2 coil terminals. Snap-on front: auxiliary NO/NC contacts (e.g. 13-14 NO, 21-22 NC).
Example circuits
- DOL (direct-on-line) motor starter with contactor + overload relay
- Star-delta motor starter (3 contactors + timer)
- Reversing starter for crane / hoist motor
- Banks of high-bay lights switched by a PLC output via contactor
- Heater bank control in industrial furnace
Common failures
Contact pitting / welding from arcing on switch-off (especially without arc suppressors). Coil burnout from overvoltage or holding at undervoltage. Mechanical jamming from dust or worn linkages. Buzzing/chattering from low coil voltage or dirty magnetic faces.
How to test
POWER ISOLATED. Check coil resistance (matches datasheet, usually 100–1000Ω). Manually press the armature — contacts should close with a solid click. Measure contact resistance closed (should be <0.1Ω). Inspect contacts for pitting. Apply rated coil voltage — should pull in cleanly.
Substitutes
Solid-state relay for DC or single-phase resistive loads (no moving parts but generates heat). Motor drive (VFD) if you need speed control as well as switching. Cannot substitute for high-current three-phase motor switching in most cases.