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Contactor vs. Relay: What's the Difference and When to Use Each
A relay and a contactor do the same basic job: a small control current through a coil pulls in an armature that opens or closes contacts. The difference is scale. Relays switch control circuits: signals, small loads, other coils. Contactors switch power circuits: motors, heaters, lighting and other high-current loads.
Comparison
| Relay | Contactor | |
|---|---|---|
| Typical current | Up to roughly 10–15 A | From about 9 A to hundreds of amps |
| Poles | 1–4, often changeover (NO and NC) | Usually 3 main poles for three-phase, plus auxiliaries |
| Main contacts | NO, NC or changeover | Almost always NO |
| Arc handling | Minimal | Arc chutes and contact design to break high current |
| Rating for motors | Not intended | Rated by utilization category, e.g. AC-3 for starting and stopping motors |
| Mounting | Plug-in socket or PCB | DIN rail or panel, with accessories |
| Accessories | Few | Auxiliary contacts, overload relays, surge suppressors, interlocks |
What makes a contactor different
- Breaking high current: opening a loaded motor circuit draws an arc. Contactors use double-break contacts and arc chutes to extinguish it quickly, which protects the contacts.
- Utilization categories (IEC 60947-4-1): AC-1 for resistive loads, AC-3 for squirrel-cage motors starting and stopping normally, AC-4 for plugging and inching. A contactor's motor rating is its AC-3 rating.
- Auxiliary contacts provide status to a PLC or create interlocks. The Allen-Bradley 100-C09D01, for example, is a 9 A (AC-3) contactor with a 110/120 V coil and one NC auxiliary contact.
Motor starters
A contactor alone does not protect a motor from overload. A motor starter combines a contactor with an overload relay that trips on sustained overcurrent. Short-circuit protection comes from fuses or a circuit breaker upstream. A motor protection circuit breaker can provide both overload and short-circuit protection in one device ahead of the contactor.
Typical control arrangement
- A PLC output (transistor or small relay) switches a 24 V DC or 120 V AC control circuit.
- That circuit energizes the contactor coil.
- The contactor's main contacts switch the three-phase motor power.
- An auxiliary contact feeds back "running" status to a PLC input.
When a PLC transistor output cannot drive the coil directly (too much current, or AC), an interposing relay sits between them. See PLC outputs.
Choosing a contactor
- Size by the motor's full-load current at its voltage, using the AC-3 rating.
- Choose the coil voltage to match your control circuit (24 V DC, 120 V AC, and so on).
- Add auxiliary contacts for status and interlocks, and surge suppressors on the coil.
- For variable speed, use a VFD instead of switching the motor with a contactor. See how a VFD works.
Frequently asked questions
Can I use a relay instead of a contactor?
Only for loads within the relay's rating. Motors and other high-current or inductive loads need a properly rated contactor.
Why does my contactor hum?
AC coils can buzz from a dirty pole face, low coil voltage or a worn shading ring. Check the coil voltage first.
Find parts
We stock new surplus Allen-Bradley contactors, breakers and motor control. Call 1-800-724-4209 or request a quote.