A contactor is a magnetically operated switch. A low-current coil pulls an armature, the armature closes a set of heavy contacts, and those contacts connect line power to a load that would destroy an ordinary switch. Energize the coil and the load runs. De-energize it and the load stops. Bolt an overload relay onto it and the contactor becomes a motor starter.
Look at what operates the equipment in an industrial panel. A pushbutton station switches a control signal measured in milliamps; the motor it starts can be rated in hundreds of horsepower. A thermostat contact closes on a temperature change and a bank of electric heaters comes on. None of those control devices could survive switching their loads directly. The contactor is the interface: the small, safe control signal on one side, the heavy power circuit on the other, and an electromagnet in between doing the physical work of connecting them.
Close It by Hand First
Set the coil aside for a moment. A contactor is a mechanical device you can operate without any control circuit at all. Many NEMA-style contactors expose a test tab or armature bump: press it with the control circuit dead, and the armature moves, the contact bridge travels, and the main contacts close exactly as they would under coil power. Line connects to load because you pushed the mechanism closed.
Instrument: Bare Contactor — Manual Bump
Manually operated switches and manual contactors ran motors before magnetic coils took over, and the magnetic contactor is that same switch with an electromagnet standing in for your hand. The manual press also survives as a diagnostic move — it separates power-circuit problems from control-circuit problems in one step, which is the subject of its own article.
The Coil: Two Circuits in One Device
The electromagnet is what makes the contactor useful. Apply rated voltage across the coil terminals, A1 and A2, and the coil builds a magnetic field that pulls the armature in. The armature carries the contact bridge, so the main contacts (L1-T1, L2-T2, L3-T3 on a three-pole device) close together and connect the line to the load. Remove coil voltage and a spring returns the armature; the contacts open and the load drops out.
That means every contactor contains two electrically separate circuits. The control circuit powers the coil and carries only the coil's small current. The power circuit runs through the main contacts and carries whatever the load demands. They meet mechanically, at the armature, and nowhere else. A 24V coil can close contacts switching 480V three-phase, with the line voltage handled by the contactor and kept away from the operator's control devices. That separation is part of why the device exists at all.
The coil voltage is a nameplate rating, not a fixed convention. 24V and 120V control circuits are common; some installations drive the coil at line voltage. The coil rating and the main-contact rating are independent of each other.
Auxiliary Contacts: The Same Armature, a Second Job
Alongside the main poles, most contactors carry auxiliary contacts: small, pilot-duty contacts mounted on the same armature. They are numbered by IEC convention: 13-14 for the first normally open pair, 21-22 for a normally closed pair. They switch signal-level current only and are never used to carry the load.
What matters is that they ride the armature. When the coil pulls in, every contact on that armature changes position together: the main poles close, and the auxiliaries flip with them.
That is what makes the three-wire control circuit work. Press START and the coil energizes through the button. The armature pulls in, the three main contacts connect the motor, and at the same moment the 13-14 auxiliary closes and completes a second path to the coil, in parallel with START. Release the button and the coil stays energized through its own auxiliary contact. The contactor is holding itself in through the 13-14 auxiliary; the main power poles play no part in the holding path. Press STOP, the coil path breaks, the armature drops, and every contact on it opens together.
Instrument: Contactor Seal-In — 3-Wire Control
Auxiliary contacts do the platform's other signaling work too: pilot lights that report whether the contactor is pulled in, PLC inputs that track its state, and the normally closed pairs that interlock one contactor against another in reversing circuits.
Relay, Contactor, Starter
A relay and a contactor share the same mechanism: a coil, an armature, contacts that move together. What separates the three names is what each device is built to switch — and in the starter's case, what is bolted on.
| Device | What it switches | The physical difference |
|---|---|---|
| Control relay | Signal-level loads, generally under 15 A | Small contacts |
| Contactor | Power loads — motors, heating banks, lighting, transformers | Heavy main contacts plus arc suppression built to break load current |
| Motor starter | A motor, with running protection | A contactor with an overload relay added |
The relay-to-contactor line is current capacity. A relay's contacts switch pilot devices and coils; a contactor's main contacts are built to make and break load current above roughly 15 A, and the extra hardware (larger contact faces, arc chutes) exists to survive the arc that interrupting that current draws.
The contactor-to-starter line is the overload relay. A bare contactor switches the load but does not protect it. A motor starter is a contactor with an overload relay added: the OL's sensing elements sit in the power circuit, and its 95-96 contact sits in series with the coil, ready to drop the contactor out when motor current stays too high for too long. For loads that do not need running protection (lighting banks, resistance heating, transformers) a bare contactor is the correct device.
Where You'll Wire One
The distinction stops being abstract the first time you build with it. In the control relay jog trainer, the seal-in contact deliberately lives on a control relay instead of the contactor, which forces the question of whose armature a given contact rides. In the reversing hoist trainer, the forward and reverse contactors are bare contactors, and the overload relay is a separate device you wire yourself — assembling a starter from its two halves rather than reading that a starter contains them. And in every three-wire circuit on the platform, the 13-14 auxiliary is the difference between a motor that runs while you hold the button and one you can walk away from.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Wire the coil, press START, and watch the seal-in hold in the free trainer →Frequently asked questions
What is a contactor?
A contactor is a magnetically operated switch for power loads. A low-current coil pulls an armature that closes heavy main contacts, connecting line power to a motor, heating bank, or other high-current load. The control circuit that powers the coil is electrically separate from the power circuit the contacts switch, so a small, safe signal can control a load far beyond what the signaling device itself could carry.
What is the difference between a contactor and a relay?
The difference is current capacity, not mechanism. Both are coil-operated switches, but relay contacts are built for signal-level loads, generally under 15 A, while a contactor's main contacts are built to make and break load current above that range. The physical consequence of the higher rating is visible hardware: larger contact faces and arc suppression that relays generally lack.
What is the difference between a contactor and a motor starter?
The overload relay. A motor starter is a contactor with an overload relay added. The contactor switches the motor; the overload relay protects it by opening its 95-96 contact in the coil circuit when motor current stays above the set point too long, dropping the contactor out. A bare contactor with no overload section switches the load but provides no running protection.
What do the numbers 13 and 14 mean on a contactor?
They identify the first auxiliary contact and mark it normally open, following IEC numbering convention. The first digit is the contact's position in sequence; the second digit gives the function: 3 and 4 mark a normally open pair, 1 and 2 a normally closed pair. So 13-14 is the first auxiliary contact, normally open, and 21-22 is the second, normally closed. It is a labeling convention, not a clause from a standard.
Can a contactor switch loads other than motors?
Yes. Contactors switch lighting banks, resistance heating, transformers, and capacitor banks — loads that draw heavy current but do not need the motor-style running overload protection an overload relay provides. Their circuits still carry their own short-circuit protection upstream. Motor loads add an overload relay to the contactor, and the combined device is a motor starter.