Start-Stop Circuit Diagram

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A start-stop circuit diagram shows the classic 3-wire motor control scheme: a normally-closed Stop button wired in series with a normally-open Start button that is held in by the contactor's own auxiliary contact once the motor is running.

The 3-wire start-stop control circuit is the foundational motor control circuit in industrial electrical engineering. It appears in virtually every manufacturing facility, pumping station, conveyor system, and HVAC plant. Understanding it precisely — not approximately — is essential for any person working on industrial controls.

The circuit has two distinct sections: the power circuit and the control circuit.

Power Circuit: Three-phase supply (L1, L2, L3) feeds through a main circuit breaker or motor protection circuit breaker (MPCB), then through the three main poles of the main contactor (KM1), then through the overload relay heater elements, and finally to the motor terminals. This is a straightforward series connection. When the main contactor is energised, the motor is connected to the supply and runs. When the contactor de-energises, the motor is disconnected.

Control Circuit (3-Wire Scheme): The key to the 3-wire scheme is this specific arrangement:

1. The Stop pushbutton (SB1) is normally-closed (NC). It is wired in series in the control circuit — in line with the contactor coil. Pressing Stop breaks the circuit, drops the contactor, and stops the motor.

2. The Start pushbutton (SB2) is normally-open (NO). It is wired in parallel with the hold-in auxiliary contact. Pressing Start momentarily connects the contactor coil to the control supply, energising the coil and closing the contactor.

3. The Hold-In Auxiliary Contact (KM1-A): As soon as the main contactor KM1 closes, one of its auxiliary NO contacts also closes. This auxiliary contact is wired in parallel with the Start button. It therefore holds (seals, latches) the contactor coil in the energised state even after the Start button is released. The contactor stays energised through its own auxiliary contact — this is the hold-in (or seal-in) circuit.

The critical safety feature of the 3-wire scheme: if power is lost (voltage dip, power failure, or emergency stop activation), the contactor drops out. When power is restored, the motor does NOT restart automatically — the operator must press Start again. This is called undervoltage protection and it prevents unexpected automatic restarts of machines after a power interruption. This is the defining advantage of the 3-wire scheme over the simpler 2-wire (maintained contact) control scheme where a motor would restart automatically on power restoration.

The overload relay (F1) normally-closed contact is wired in series in the control circuit. If the motor draws excessive current for a sustained period, the overload relay trips, opening its NC contact, dropping the contactor, and stopping the motor.

The ladder diagram is the standard representation for start-stop motor control circuits in industrial settings, making it easy to read logic left-to-right across rungs — just like reading steps on a ladder. A typical start-stop ladder diagram shows the stop pushbutton (normally closed) in series with the start pushbutton (normally open), a contactor coil, and a sealing (hold-in) contact wired in parallel with the start button so the circuit latches once energised. You can build and annotate your own start-stop ladder diagrams free in the browser at Circuit Diagram Maker without any download.

How to wire start stop diagram

Understand the circuit topology before wiring Draw or print the 3-wire control circuit before touching any wire. The control circuit path must be: L1 (or control transformer secondary) → Stop button NC contact → (Start button NO contact in parallel with KM1 hold-in auxiliary NO contact) → overload relay NC contact → KM1 coil → N (neutral or control transformer return). Every component in this series path must be in the correct order and orientation. A misplaced NC or NO contact will cause the circuit to fail in a non-obvious way.
Select and size the main contactor and overload relay Choose a main contactor (KM1) with an AC-3 utilisation category rating at or above the motor's full-load current. Select a thermal or electronic overload relay adjustable to cover the motor's nameplate full-load current. Confirm the contactor coil voltage matches the control circuit voltage — this is typically 230 V AC (single-phase from the main supply) or 110 V AC (from a control transformer), or 24 V AC/DC in modern installations.
Wire the power circuit Connect three-phase supply through the MPCB or main circuit breaker to the main contactor (KM1) three input terminals (T1, T2, T3 on the supply side). Connect the three output terminals (T4, T5, T6 or 2, 4, 6 depending on manufacturer) through the overload relay heater assembly to the motor terminals (U, V, W). Verify phase rotation with a phase sequence meter if required by the driven machine (wrong rotation of a pump or compressor can cause damage).
Wire the Stop button in series in the control circuit Connect one wire from the control supply (L1 side or control transformer secondary) to one terminal of the Stop button (NC contact). Connect the other terminal of the Stop button to the junction point that feeds both the Start button and the hold-in auxiliary contact. The Stop button terminals are the NC terminals — verify with a continuity test (there should be continuity between them when the button is not pressed, open when pressed).
Wire the Start button and hold-in auxiliary in parallel Connect the Start button (NO terminals) between the Stop button output junction and the overload relay NC contact input. Connect the KM1 hold-in auxiliary contact (NO) in parallel with the Start button — one terminal to the Stop button output junction, the other terminal to the overload relay NC contact input. The auxiliary contact wires bridge across the Start button, not across the Stop button — a common wiring error to avoid.
Wire the overload relay NC contact and contactor coil Connect the overload relay NC contact in series: from the Start button / auxiliary contact parallel combination output, through the overload relay NC contact, to one terminal of the KM1 contactor coil. Connect the other coil terminal to the neutral or return side of the control supply. Confirm the coil terminal voltage rating on the contactor nameplate matches the supply being connected.
Test the circuit under power before connecting the motor With the motor disconnected but power applied to the control circuit, press the Start button. The contactor should click closed and remain closed when Start is released. Press the Stop button — the contactor must drop out immediately. Manually trip the overload relay test button — the contactor should drop. Reset the overload and verify the motor will not restart without pressing Start. Only after confirming correct control circuit operation should the motor be connected and a full-load trial run performed.

Specifications

Control scheme type	3-wire (momentary contact) — provides undervoltage protection (no automatic restart)
Stop button contact type	Normally-closed (NC) — fail-safe: open circuit stops the motor
Start button contact type	Normally-open (NO) — wired in parallel with hold-in auxiliary NO contact
Hold-in auxiliary contact type	Normally-open (NO) — wired in parallel with Start button; closes when contactor energises
Overload relay contact in control circuit	Normally-closed (NC) — opens on thermal trip, drops contactor
Contactor utilisation category for squirrel-cage motors	AC-3 (IEC 60947-4-1)
Applicable standards	IEC 60947-4-1 (motor starters), NFPA 79 (USA industrial machinery), IEC 60364, BS 7671, AS/NZS 3000
Stop button colour convention	Red (IEC 60947 / NFPA 79 / AS 61058); Start button: green

Safety warnings

All motor control panel wiring involving three-phase supply must be designed, installed, and commissioned by a qualified and licensed electrician in accordance with the applicable standard: IEC 60364 and IEC 60947-4 (industrial motor starters), NEC Article 430 (USA), BS 7671 (UK), or AS/NZS 3000 (Australia/NZ). The power circuit carries potentially lethal three-phase voltages (typically 400 V in Europe, 480 V in North America). Isolation, verification-dead using a proven instrument, and lockout/tagout is mandatory before any work on the power circuit.
The overload relay must be set to the motor's actual nameplate full-load current before the motor is run. An overload relay set too high will not protect the motor from thermal damage during an overload condition. An overload relay set too low will cause nuisance tripping during normal starts. Check the overload relay setting after installation and before the first run.
The Stop button must be a normally-closed (NC) contact device. Under no circumstances should a normally-open button be substituted for the Stop button. If the control cable to a NC Stop button is broken or disconnected, the circuit opens and the motor stops — this is the safe fail condition. A NO Stop button with a broken cable would leave the motor permanently running with no means of stopping it from that station.
The 3-wire start-stop circuit provides undervoltage protection (the motor does not restart automatically after a power failure) but does NOT provide protection against intentional or accidental bypassing of the Stop button or auxiliary circuit. For machinery where an uncontrolled start could injure a person, additional safety measures are required: safety-rated emergency stop devices, safety relays, and risk assessment per ISO 13849 or IEC 62061.

Tools needed

Insulated screwdrivers for contactor terminal connections
Digital multimeter for verifying control circuit voltages and contact continuity before power-on
Phase sequence meter for verifying correct motor rotation before full mechanical load
Clamp meter for measuring motor current during commissioning
Wire ferrule crimping tool for control circuit terminations
Torque screwdriver for tightening terminals to manufacturer-specified torque values
Lockout/tagout (LOTO) padlock set for safe isolation during wiring work

Common mistakes

Wiring the hold-in auxiliary contact in parallel with the Stop button instead of in parallel with the Start button — this would bypass the Stop button and make the motor impossible to stop by pressing the Stop button. The hold-in must be across the Start button only.
Using a normally-open contact for the Stop button — the motor will only run while the Stop button is held down in this configuration, since the circuit is only complete through the NO stop contact when it is pressed. The correct Stop button is normally-closed.
Not connecting the overload relay NC contact in series in the control circuit — if the overload relay's trip contact is omitted from the control circuit, the overload relay can trip mechanically but the contactor will not drop out, leaving the motor running in a potentially dangerous overloaded state.
Failing to set the overload relay to the correct current before energising — the factory default setting of the overload relay may be at the top of its adjustment range, providing no real protection. Always adjust to the motor nameplate FLC before the first run.
Forgetting to verify the auxiliary contact block is actually mechanically coupled to the main contactor before relying on the hold-in function — auxiliary contact blocks clip onto the contactor body and can occasionally be incorrectly seated. Verify the auxiliary contact opens and closes with the main contactor during the no-load control circuit test.

Troubleshooting

Motor starts when Start is pressed but immediately stops when Start is released: Cause: Hold-in auxiliary contact (KM1-A) is not closing — either the auxiliary contact block is not coupled to the contactor, a NO auxiliary contact has been wired instead of connected in parallel with Start, or the auxiliary contact itself is faulty Fix: With power removed, verify the auxiliary contact block is correctly seated on the contactor body. Using a multimeter in continuity mode, manually push the contactor core in (simulating energisation) and verify that the auxiliary contact intended for hold-in changes from open to closed (NO contact should now show continuity). Verify the wiring connects this contact in parallel with the Start button terminals — one wire to each side of the Start button.
Motor will not start — pressing Start has no effect: Cause: Open circuit in the control circuit — could be a blown control circuit fuse, a Stop button not returning to the closed position, a faulty Start button NO contact, a tripped and not reset overload relay, or an open contactor coil Fix: With power applied, use a multimeter in DC/AC voltage mode and probe across each component in the control circuit from the supply end to the coil. When probing across a component shows supply voltage, that component has an open circuit — there is voltage on one side but not the other. Check the overload relay reset button first — this is the most common cause after a trip event. Then check the Stop button continuity, then the Start button operation, then the control circuit fuse, then coil resistance.
Overload relay trips shortly after motor starts, even with no mechanical load: Cause: Overload relay set too low for the motor FLC, a single-phase supply condition causing the other two phases to carry excess current, or the motor itself has a developing winding fault causing excessive current draw Fix: Measure the motor's actual running current on all three phases with a clamp meter under the load condition that causes the trip. Compare to the motor nameplate FLC and the overload relay setting. If the measured current is within the motor's nameplate FLC, adjust the overload relay setting upward. If the measured current is above FLC, check the mechanical load and verify the supply voltage is balanced across all three phases — a phase voltage imbalance of more than 2% can significantly increase motor current.

Frequently asked questions

Why is the Stop button normally-closed (NC) and the Start button normally-open (NO)?

The normally-closed Stop button means the control circuit is complete (conducting) when the Stop button is not pressed. The motor only stops when the button is actively pressed, opening the circuit. This fail-safe design means that if the Stop button cable is broken, cut, or disconnected, the circuit opens and the motor stops — the safe condition. A normally-open Stop button would mean a broken cable leaves the motor running, which is dangerous.

What is the hold-in (seal-in) auxiliary contact and why is it essential?

The hold-in auxiliary contact is a normally-open (NO) auxiliary contact on the main contactor, wired in parallel with the Start button. Once the contactor closes (triggered by pressing Start), this auxiliary contact also closes, providing an alternative current path to the contactor coil that bypasses the Start button. The Start button can then be released without the contactor dropping out. Without this hold-in contact, the motor would only run while the Start button is physically held down.

What is undervoltage protection in a 3-wire start-stop circuit?

Undervoltage protection is the inherent behaviour of the 3-wire start-stop scheme whereby the motor does not restart automatically after a power interruption. When supply voltage drops or fails, the contactor coil de-energises and the contactor opens. When power returns, the contactor coil circuit is open at the Start button (which has been released) and cannot re-close without an operator pressing Start again. This prevents unexpected machine restarts after power failures, which is a significant industrial safety requirement.

What does the overload relay do in a start-stop circuit?

The overload relay protects the motor against sustained overcurrent caused by mechanical overload, a single-phase condition (one supply phase lost), or a developing winding fault. Its heater elements carry the motor current and trip a bimetallic contact when heat accumulated over time indicates overcurrent. The trip opens the NC contact in the control circuit, dropping the main contactor and stopping the motor. The overload relay must be set to the motor's full-load current rating.

Can multiple Start and Stop buttons be added to a single start-stop circuit?

Yes. Additional Stop buttons (NC) are wired in series with the existing Stop button — any one of them can stop the motor by opening the series circuit. Additional Start buttons (NO) are wired in parallel with the existing Start button and the hold-in auxiliary contact — any one of them can start the motor. This allows motor control from multiple locations on a production line without any additional relays or complex wiring.

How do I draw a ladder diagram for a start-stop motor control circuit?

In a start-stop ladder diagram, each rung is drawn as a horizontal line between two vertical power rails (L1 and L2 or L and N). The first rung contains the normally-closed stop button in series with the normally-open start button and the motor contactor coil (labelled M); a normally-open auxiliary contact of contactor M is wired in parallel with the start button on the same rung to provide latching (sealing). When Start is pressed, M energises, the sealing contact closes, and the motor runs even after Start is released; pressing Stop opens the circuit and drops out the contactor. Additional rungs below can control pilot lights or overload alarm contacts in the same format.

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