A motor starter is the standard way to control a 3-phase motor in industrial settings. It combines a contactor (a heavy-duty relay that switches the motor's power) with an overload relay (a thermal device that protects the motor from sustained overcurrent). Together with a start/stop control station, these three components form the most common motor control circuit you'll encounter in the field — on exhaust fans, conveyor belts, pumps, compressors, and packaging lines.

This guide walks through every connection in the circuit, explains why each wire is there, and links to an interactive simulator where you can practice the wiring yourself.

What's Inside a Motor Starter

A NEMA-rated combination motor starter has three main parts:

Contactor — three sets of main contacts (one per phase) that switch the motor's power on and off, plus one or more auxiliary contacts used in the control circuit. When the contactor coil is energized, an electromagnetic plunger pulls all contacts closed simultaneously.
Overload relay — a thermal protection device wired in series with the motor. Each phase passes through a heater element. If current stays above the relay's trip setting for too long, a bimetallic strip bends and mechanically opens the OL contacts (terminals 95-96), breaking the control circuit and shutting the motor down.
Control station — a separate enclosure mounted where the operator can reach it, containing a momentary START pushbutton (normally open) and a momentary STOP pushbutton (normally closed).

The Two Circuits

Every motor starter has two separate circuits that work together:

Power Circuit (High Current)

The power circuit carries the full motor current — often 10 to 50 amps or more. It runs from the three-phase supply (L1, L2, L3) through the contactor's main contacts, through the overload relay's heater elements, and out to the motor terminals (T1, T2, T3).

The wiring is straightforward:

L1 → contactor main contact → OL heater → T1
L2 → contactor main contact → OL heater → T2
L3 → contactor main contact → OL heater → T3

When the contactor is energized, all three main contacts close and the motor runs. When it de-energizes, the contacts open and the motor stops.

Control Circuit (Low Current)

The control circuit uses the same voltage as the power circuit (208V phase-to-phase in most training environments) but carries only a fraction of an amp — just enough to energize the contactor coil. This is called "line-voltage control" because the control circuit operates at the same voltage as the power circuit. In higher-voltage installations (480V), a control transformer typically steps the voltage down to 24V or 120V for safety — but the circuit logic is identical either way.

Here's the control circuit path, step by step:

L1 (hot) — power enters the control circuit from one phase
→ STOP button (NC) — normally closed; current flows through until pressed
→ START button (NO) — normally open; current flows only while pressed
→ Contactor coil (A1) — the electromagnetic coil that pulls the contacts closed
→ Coil terminal (A2) — the other end of the coil
→ OL contact (95-96) — normally closed; opens only when the overload relay trips
→ L2 (return) — completes the circuit back to the supply. In a 208V line-to-line circuit, L2 is another phase, not a neutral — both conductors are ungrounded.

When you press START, current flows through the closed STOP button, through the momentary START button, through the coil, through the OL contacts, and back to L2. The coil energizes and the contactor pulls in.

The Seal-In Contact

There's a problem with the circuit as described: the START button is momentary. Release it and the coil de-energizes, dropping the motor.

The solution is the seal-in contact — a normally-open auxiliary contact on the contactor, wired in parallel with the START button. When the contactor pulls in, this auxiliary contact closes. Now current flows through the seal-in contact even after you release START. The motor stays running.

To stop the motor, you press STOP. This opens the normally-closed STOP button, breaking the entire control circuit. The coil de-energizes, the main contacts open (motor stops), and the seal-in contact opens too. The circuit is now back to its resting state, waiting for the next START press.

Try it yourself — press START on the control station below, then release it. Watch the seal-in contact close and the current path shift. Then press STOP to break the circuit.

Interactive: 3-Wire Control Circuit

Click START

At Rest — press START to energize

This design gives you three built-in protections:

Low-voltage protection — if power is lost, the coil de-energizes and the seal-in opens. When power returns, the motor does not restart on its own. Someone must press START again.
Fail-safe stop — the STOP button is normally closed, wired in series. If the wire breaks or the button fails, the circuit opens and the motor stops. Fail-safe.
Overload protection — if the motor draws too much current for too long, the OL relay trips, opening the 95-96 contacts and shutting everything down.

Overload Relay Placement

You'll see two conventions for where the OL contacts (95-96) go in the control circuit:

Return-side (NEMA/JIC standard): OL contacts wired between the coil and the neutral return (L2). This is the most common placement in North American textbook diagrams and what most apprenticeship programs teach.
Hot-side (IEC 60204-1): OL contacts wired between the hot supply (L1) and the STOP button. This placement is standard in IEC installations and is becoming more common in North America.

Both placements work identically — the OL contacts are in series with the coil either way. The circuit doesn't care which side they're on. What matters is that they're there.

Wiring It Yourself

In the field, you would lock out and tag out (LOTO) the power supply and verify zero voltage with a multimeter before touching any terminals. In the simulator, that step is handled for you — but building the habit of thinking about it matters.

Reading about a circuit and wiring one are different skills. The best way to internalize this is to build it from scratch — connecting each wire, understanding why it goes where it does, then operating the circuit and verifying it with a multimeter.

Hands-On Practice

Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.

Wire this motor starter circuit yourself — free, no account needed →

The interactive trainer walks you through the circuit wire by wire before handing you a blank workbench. You'll wire the power circuit, build the control circuit with seal-in logic, commission the motor through a hands-on checklist, and then troubleshoot five common faults (stuck contacts, broken seal-in paths, tripped overloads) using a built-in digital multimeter.

Common Wiring Mistakes

If you're wiring a motor starter for the first time, watch out for these:

Forgetting the seal-in contact — the motor runs only while you hold START. Add the auxiliary NO contact in parallel with the START button.
Wiring STOP as NO instead of NC — the motor won't start at all. The STOP contact is open at rest, so the series path is permanently broken and pressing START has no effect. STOP must be normally closed, wired in series — that's the fail-safe design.
Swapping the OL heaters and OL contacts — the heater elements (wired in the power circuit) are not the same as the OL contacts (wired in the control circuit). Heaters carry motor current; the contacts carry only coil current.
Missing the neutral return — the control circuit needs a complete path back to L2. If the coil's A2 terminal has no path to the supply, nothing happens when you press START.

What's Next

Once you understand the basic motor starter, the natural next steps are:

Control transformers — stepping line voltage down to 24V or 120V for the control circuit, which is standard practice at 480V and common at 208V for added safety and longer control wire runs
Jogging circuits — adding a JOG button that runs the motor only while pressed, bypassing the seal-in contact without disabling it for normal RUN operation
Reversing starters — using two contactors with electrical interlocking to reverse the motor's direction by swapping two phases

Each of these builds directly on the 3-wire start/stop circuit covered here.