A relay and a contactor do the same fundamental thing: a coil generates a magnetic field, that field moves a mechanical armature, and the armature opens or closes a set of contacts. Both are electromagnetic switching devices. The question "what is the difference between a relay and a contactor?" is really a question about scale.
Relays are small, rated for signal-level currents in control circuits. Contactors are large, rated for motor currents in power circuits. Knowing which one to reach for — and why — is a basic competency for any PLC technician or controls electrician.
How a Relay Works
A relay has a coil and a set of contacts. Apply the rated voltage to the coil — typically 24V DC in modern panels — and it energises the electromagnet, which pulls the armature down and changes the contact state. Remove the voltage and a return spring pushes the armature back.
The contacts are rated for the maximum current and voltage they can safely switch. A typical DIN-rail relay used in control panels has contacts rated at 5A at 250V AC or 5A at 30V DC. That is plenty for:
- Switching the coil of another relay or contactor (coils draw 0.05A to 0.2A).
- Driving a 24V indicator lamp or buzzer.
- Connecting a PLC output card to a higher-current device (PLC outputs typically source or sink a maximum of 0.5A; the relay allows switching larger loads from that low-current output).
Most DIN-rail relays in industrial panels are plug-in designs: the base mounts permanently on the DIN rail with the wiring connected to it, and the relay module clicks into the base. When a coil burns out after years of cycling, you press the tab, pull the old relay, snap in the new one. No rewiring required. This is deliberate design — relay bases get wired once, relays get replaced as consumables.
Modern plug-in relays often have a LED indicator built in. When the coil is energised, the LED lights. This is your first fault-finding tool: glance at the relay in the cabinet and you can immediately see which coils are energised without touching anything.
How a Contactor Works
A contactor uses exactly the same coil-and-armature mechanism as a relay. The difference is everything else.
Contactor main contacts are designed to carry motor-level current — 9A, 22A, 40A, 63A, 115A and beyond — repeatedly over hundreds of thousands of operating cycles. The contacts are large, with silver-alloy tips designed to handle the arcing that occurs when you interrupt current under load. The main contacts in a contactor are three-phase: they switch all three phases of the motor supply simultaneously.
Contactors also have auxiliary contacts — smaller contacts in the control circuit. These are typically one or two normally-open and one normally-closed contact pair. The normally-open auxiliary contact is used for the seal-in function: wired in parallel with the Start pushbutton so the contactor holds itself energised after the button is released. The normally-closed auxiliary is often used in interlock circuits — for example, ensuring the forward and reverse contactors cannot both energise at the same time.
The coil voltage on a contactor is a separate specification from the main contact rating. A common contactor might have 40A main contacts but a 24V DC coil. The PLC output or relay drives the 24V coil; the coil pulls in the contactor; the main contacts switch 415V AC at 40A to the motor. This is exactly the separation between the control circuit and the power circuit in physical hardware form.
The Ratings That Tell You Which to Use
The simplest decision rule: look at the current the load draws.
| Current | Device | Typical application |
|---|---|---|
| 0–10A | Relay | Coils, lamps, PLC I/O interfacing, small solenoids |
| 9–800A | Contactor | Motors, large heaters, capacitor banks |
If you are switching a signal, a coil, or a light load — reach for a relay. If you are switching a motor or any three-phase load — reach for a contactor.
There is an overlap zone: some large relays (solid-state relays, for instance) can switch motors up to 25A. But in a traditional panel, 9A is roughly where you cross from relay territory into contactor territory. The Siemens 3RT and Rockwell 100-C contactor series start at 9A; most relay datasheet maximums are 10A. This is not a coincidence.
Motor Starter = Contactor + Overload Relay
A motor starter is a contactor combined with a thermal overload relay. The overload relay protects the motor against sustained overcurrent — a jammed conveyor, a pump running on a closed discharge valve, a motor with a failing bearing. For the full breakdown of what the overload relay adds and when a bare contactor is sufficient, see the motor starter vs contactor comparison. It does this by measuring the heat accumulated in a set of bimetal strips carrying the motor current. When the strips overheat (because the motor is drawing too much current for too long), they deflect, trip the overload, and open a contact in the control circuit. The control circuit opens, the contactor de-energises, the motor stops.
The overload relay has two key settings:
- Full load current (FLC) setting: dial it to the motor's nameplate FLC. This calibrates the thermal model to the specific motor.
- Trip class: typically Class 10 (trips within 10 seconds at 7.2× FLC), used for most standard motors.
Reset: after a thermal trip, the overload relay must be reset manually (a button on the face of the relay) after the motor has cooled. This prevents automatic restart of a faulting motor, which could be dangerous if someone is investigating the fault. Some overload relays can be configured for auto-reset in specific applications where that is safe.
The overload relay also has a normally-closed contact wired into the control circuit. When it trips, this contact opens, dropping out the contactor. The PLC sees the motor de-energise (the auxiliary contact feedback goes low), triggers a fault alarm, and prevents the operator from restarting until they reset the overload and acknowledge the alarm. This is the standard motor protection sequence.
When Each Belongs in a Panel
Use a relay when:
- A PLC output card needs to switch a load above its current rating (most PLC transistor outputs source 0.5A max — insufficient to drive a 40A contactor coil directly; an intermediate relay is used).
- You need to galvanically isolate two circuits at different voltages (24V DC control circuit switching a 240V AC solenoid valve coil, with the relay as the isolation barrier).
- You need to multiply one signal into several switched outputs (one PLC output drives one relay coil; that relay's multiple contacts switch several downstream devices simultaneously).
- You are hardwiring a safety interlock that must be proven and certified (safety relays with force-guided contacts are a specific category of this).
Use a contactor when:
- You are switching a three-phase motor.
- You are switching any single-phase load above roughly 10A.
- You need the sealed auxiliary contact for the motor seal-in and status feedback to the PLC.
- The application requires the load to be switched many thousands of times per year (motors in cycling applications). Contactors are built for duty; relays are not.
Use a motor starter (contactor + overload) when:
- You are controlling a motor directly-on-line (DOL). In most countries, thermal or electronic overload protection is a code requirement for motor circuits above a certain size.
Ladder Logic and the Contactor Seal-In
The relationship between the contactor's auxiliary contact seal-in and the PLC seal-in rung is worth understanding explicitly, because beginners often confuse the two.
In a hardwired control circuit (no PLC), the motor seal-in is done entirely in hardware. The contactor auxiliary contact is wired in parallel with the Start button. When the operator presses Start, the coil energises and the auxiliary contact closes, holding the coil energised after the button is released. Stop button or overload trip opens the circuit and drops the coil.
In a PLC-controlled circuit, the seal-in is implemented in the ladder program:
|--[Start_PB]--+--[/Stop_PB]--[/Overload]--( K1_Coil )--|
| |
|--[K1_Aux]---+
The K1_Aux contact in the PLC's input card gives feedback from the contactor's physical auxiliary contact. When the contactor pulls in, this input goes HIGH, and the PLC seal-in rung holds the K1_Coil output energised. The physical hardwired seal-in is no longer needed — the PLC logic replaces it.
Understanding both versions is important because you will encounter both in the field. Older machines are hardwired; newer ones are PLC-controlled. Some machines use both (a hardwired safety interlock plus a PLC control circuit).
Practice Relay and Contactor Logic
The relay and contactor concepts above — seal-in rungs, interlock logic, overload trips — are exactly what the Electrical Foundations lessons on this platform cover. The first two lessons, including the relay vs contactor lesson, are free.
Start the Electrical Foundations lessons free →
If you want to build on the PLC control side, practice motor start/stop seal-in rungs and overload interlock logic in the free simulator — the same logic your PLC runs when it controls a physical motor starter.
Practice motor control ladder logic free →
Frequently Asked Questions
What is the main difference between a relay and a contactor?
Both use a coil and electromagnetic armature to switch contacts. The difference is scale and rating. Relays are small, rated for 5–10A in control circuits — used for signal switching, PLC output isolation, and coil driving. Contactors are large, rated for motor currents (9A to 800A), designed to switch three-phase motors and large loads repeatedly over many thousands of cycles.
Can a relay be used instead of a contactor?
For small loads under 10A, a relay can switch directly. For motors and three-phase loads, no — a relay is not built for the current, arcing, or cycling duty. Using a relay to switch a 415V AC motor circuit will destroy the relay contacts rapidly and is unsafe. Use a contactor rated for the motor's full load current.
What is a motor starter?
A motor starter is a contactor combined with a thermal or electronic overload relay. The contactor switches the motor's three-phase power supply. The overload relay protects the motor against sustained overcurrent by tripping the control circuit if the motor draws more than its rated full load current for longer than the protection setting allows.
What does the auxiliary contact on a contactor do?
Auxiliary contacts are smaller contacts that change state when the main contactor pulls in. The normally-open auxiliary contact is typically wired in parallel with the Start button to create the motor seal-in — it holds the contactor coil energised after the button is released. It is also wired to a PLC input to give the program feedback that the contactor has physically closed.
What is an overload relay and why is it needed?
An overload relay measures motor current using bimetal or electronic sensing. If the motor draws above its full load current for longer than the thermal protection allows — due to mechanical overload, low supply voltage, or a developing fault — the relay trips, opens the control circuit, and stops the motor before it burns out. It is required by electrical codes for motor circuits above a certain size in most countries.
Get hands-on with relay and contactor logic in the Electrical Foundations track. The relay vs contactor lesson and the control circuit lesson are free. No credit card. Start building the electrical fundamentals your PLC programming already assumes you know.
