TL;DR: A contactor is a high-current electromagnetic switch that turns a motor on and off. A motor starter is a contactor combined with a thermal or electronic overload relay — the overload protects the motor against sustained overcurrent (mechanical jam, single-phasing, prolonged overload) by tripping the control circuit before the motor burns out. Every motor circuit above the code threshold needs a motor starter. The contactor alone switches; the starter switches and protects.
The confusion is understandable because most physical motor starters look like a contactor with something bolted to the bottom of it — because that is exactly what they are. But the two components serve fundamentally different functions, and knowing which one you have (and which one you need) is essential for panel design and fault-finding.
For the full explanation of the difference between relays and contactors at the signal level, see the relay vs contactor post. This post focuses specifically on the motor starter combination and what the overload relay adds.
What a Contactor Does
A contactor is a heavy-duty electromagnetic switch designed for switching three-phase motor loads. The PLC output (or a control relay) energises the contactor coil — usually 24V DC. The coil creates an electromagnet that pulls in the armature, closing the main contacts. The main contacts switch the motor's 415V AC three-phase power supply.
A contactor does exactly one thing: switch the motor on or switch it off. It does not protect the motor against any fault condition. If the motor jams and draws 800% of its rated current, the contactor will keep it connected until the motor windings overheat and fail — or until a fuse in the power circuit operates.
A contactor is appropriate when:
- The protection is handled by another device upstream (motor circuit breaker, fuse + overload elsewhere in the circuit).
- The motor is below the code threshold requiring overload protection.
- The starter is built into a soft starter or VFD that provides overload protection internally.
For most direct-on-line (DOL) motor circuits, a bare contactor without overload protection is not code-compliant. You need a motor starter.
What a Motor Starter Adds
A motor starter is a contactor with an overload relay mounted to it — mechanically and electrically integrated. The overload relay measures the motor current through bimetal strips (thermal overload) or current sensors (electronic overload). If the motor draws more than its set full-load current (FLC) for longer than the thermal protection setting allows, the overload trips and opens a contact in the control circuit. The control circuit drops out; the contactor de-energises; the motor stops.
The overload relay provides:
- Motor protection against sustained overcurrent: jammed conveyor, closed discharge valve on a pump, motor with a failing bearing.
- Single-phase protection (electronic overload): if one phase of the three-phase supply fails, the motor draws excess current on the remaining phases — the overload trips it before it burns out.
- Trip class selection: Class 10 (trips within 10 seconds at 7.2× FLC) is standard for most motors. Class 20 is available for high-inertia loads.
- Manual reset requirement: after a thermal trip, the overload must be manually reset (button on the face of the relay) after the motor cools. This prevents automatic restart — important for fault investigation.
The Control Circuit
The overload relay has a normally-closed (NC) auxiliary contact that sits in series in the control circuit. This contact is wired in series with the contactor coil supply and in series with the Stop button:
+24V DC
→ Overload NC contact
→ Stop (NC pushbutton)
→ Start (NO pushbutton) [parallel with seal-in]
→ Contactor coil
0V DC
When the overload trips, the NC contact opens, the control circuit breaks, and the contactor drops out — even if the PLC or operator is holding the Start command active. The overload has authority over the entire control circuit.
The PLC reads the state of the contactor auxiliary contact (a small NO contact that closes when the main contactor closes) as motor-running feedback. The PLC also reads the overload trip state — typically via a separate auxiliary contact on the overload relay wired to a PLC input. This allows the PLC to:
- Detect that the motor stopped without a Stop command.
- Raise an "overload trip" alarm.
- Prevent a restart attempt until the overload is reset and the alarm is acknowledged.
This interlock sequence is the standard motor protection logic. Practice it in the motor start-stop scenario, which includes the seal-in rung and the auxiliary feedback input.
Side-by-Side Comparison
| Contactor Only | Motor Starter (Contactor + Overload) | |
|---|---|---|
| Switches the motor? | Yes | Yes |
| Overcurrent protection? | No | Yes — thermal or electronic overload |
| Single-phase protection? | No | Yes (electronic overload models) |
| Control circuit trip? | No — needs external protection | Yes — overload opens NC contact in series |
| Manual reset after fault? | No | Yes — required after thermal trip |
| Electrical code requirement? | Only if protection is elsewhere | Required for most direct motor circuits |
| PLC feedback | Auxiliary contact (motor running) | Auxiliary contact + overload trip contact |
| VFD application | Bypass contactor (VFD has own overload) | Often bare contactor — VFD provides protection |
When You Use a Bare Contactor (Without a Separate Overload)
There are legitimate cases for a contactor without an overload relay:
VFD bypass circuit: When a motor is normally driven by a VFD but has a bypass contactor for direct-on-line operation during VFD maintenance, the VFD itself provides overload protection in VFD mode. The bypass contactor may have a separately mounted overload relay, or the bypass circuit may be rated for occasional use only with upstream fuse protection.
Soft starter output: Many soft starters include internal overload protection. The run contactor on the output side of the soft starter is a bare contactor — the soft starter handles the protection.
Capacitor switching: Power factor correction capacitors are switched by contactors (with AC3 or AC6 duty ratings). There is no motor to protect from overload; the contactor is the right device.
Very small motors: Some national electrical codes allow motors below a threshold (typically 1 kW or 1.5 HP) to use fuse protection only. Check the applicable code — this is jurisdiction-specific.
Selecting a Motor Starter
Motor starters are selected on three parameters:
- Motor full-load current (FLC): from the motor nameplate. This sets the overload relay trip class range.
- Contactor duty current (AC3 rating): the contactor must be rated for the motor's FLC at the line voltage. AC3 rating covers squirrel-cage motors started DOL. AC1 rating (resistive load) is higher — do not substitute.
- Coil voltage: typically 24V DC for PLC-controlled circuits; sometimes 230V AC. The coil voltage does not affect the motor voltage.
Common motor starter series: Siemens 3RA (integrated), ABB AF/A (modular), Schneider TeSys D/F (modular), Eaton Moeller PKZM/DIL (modular), Rockwell 100-C/193 (modular with E300 electronic overload).
PLC Wiring Sequence
For a standard DOL motor starter controlled by a PLC:
- PLC digital output → contactor coil positive terminal.
- 0V → contactor coil negative terminal.
- Contactor main contacts → motor power circuit (L1/L2/L3 in; T1/T2/T3 out via overload relay current sensors).
- Overload relay NC contact (95/96) → in series in the control circuit.
- Overload relay trip contact → PLC digital input (overload fault feedback).
- Contactor auxiliary NO contact (13/14) → PLC digital input (motor running feedback).
The wiring lab Wiring 12 — Three-Wire Motor Starter walks through this exact circuit, including the seal-in rung in the PLC ladder and the overload interlock logic.
Frequently Asked Questions
Q: Can I use a contactor instead of a motor starter to save money?
A: Only if overload protection is provided elsewhere — upstream motor circuit breaker with an appropriate trip curve, or internal overload in a VFD or soft starter. A bare contactor switching a DOL motor with only fuse protection upstream is almost always a code violation and will eventually destroy the motor windings in an overload condition.
Q: What is the difference between a thermal and electronic overload relay?
A: A thermal overload relay uses bimetal strips that bend with heat to trip the relay. It is simple, reliable, and cheap. An electronic overload relay uses current transformers and a microprocessor to model motor temperature — it provides more accurate protection, single-phase fault detection, ground fault detection, and phase imbalance detection. Electronic overloads are preferred for larger motors and critical applications.
Q: Do I need a motor starter with a VFD?
A: When the motor is running via the VFD, the VFD provides overload protection. A line-side contactor (between supply and VFD input) is sometimes used for isolation and remote switching, but it does not need an overload relay. If there is also a bypass contactor for DOL operation, that bypass circuit needs its own overload protection.
Q: What does the overload relay trip class mean?
A: Trip class describes how quickly the overload trips at a given multiple of FLC. Class 10 trips within 10 seconds at 7.2× FLC (suitable for most standard squirrel-cage motors). Class 20 trips within 20 seconds at 7.2× FLC (for high-inertia loads with long acceleration times). Class 30 is for very high inertia applications. Using the wrong class risks either nuisance tripping on normal starts or inadequate protection during actual overloads.
Practice the PLC side of motor starter control — including the seal-in rung, auxiliary feedback input, and overload interlock — in the free browser simulator.
