Start-Stop Motor Control Diagram
This is a free printable start stop motor control diagram: download the diagram as SVG or open it and print to paper or PDF.
A start-stop motor control diagram shows how a 3-wire control circuit uses a normally-closed stop button, a normally-open start button, and a contactor hold-in auxiliary contact to latch a motor on until deliberately stopped.
The 3-wire control circuit is the foundation of virtually every industrial motor starter installation. Understanding why it is called '3-wire' clarifies the entire topology: three conductors run from the control station to the magnetic contactor coil circuit — a common (neutral or earth reference), a stop wire, and a start wire. This is distinct from a 2-wire circuit, which uses a single maintained-contact device such as a float switch or thermostat and provides no low-voltage protection.
In a 3-wire circuit the normally-closed (NC) stop pushbutton is wired in series with the contactor coil. The normally-open (NO) start pushbutton is wired in parallel with the contactor's own auxiliary NO contact, which is mechanically coupled to the main power contacts. When the start button is momentarily pressed, current flows through the NC stop button, through the now-closed start button, and energises the coil. The coil pulls in the main contacts (connecting the motor to the three-phase supply) and simultaneously closes the auxiliary contact. That auxiliary contact, often called the hold-in, seal-in, or memory contact, bridges across the start button. When the operator releases the start button, current continues to flow via the auxiliary path — the motor keeps running.
Pressing the stop button opens the NC contact, breaking coil current. The coil drops out, the main contacts open, and the auxiliary contact opens simultaneously. The circuit has no memory — it cannot restart without a deliberate press of the start button.
This behaviour provides inherent low-voltage protection (also called no-volt protection). If the supply voltage drops or fails entirely, the coil de-energises, the contactor drops out, and the motor stops. When power is restored the motor does not restart automatically, protecting personnel and machinery. This is why 3-wire control is mandatory in most industrial and commercial motor applications under IEC 60947-4-1 and NFPA 79.
How to wire start stop motor control diagram
- Isolate and verify dead Switch off the main circuit breaker feeding the motor control centre or panel. Use a calibrated voltage tester to verify all phases at the contactor line terminals are at zero volts. Apply lockout-tagout before touching any wiring.
- Wire the main power circuit Connect the three-phase supply (L1, L2, L3) to the top (line) terminals of the contactor. Connect the bottom (load) terminals through the overload relay heater elements to the motor terminals (U, V, W). Size conductors to the motor full-load current per NEC Table 310.16 or IEC 60364-5-52.
- Wire the control transformer or use line voltage For 120 V or 24 V control, install a control transformer between one phase and neutral. Fuse the secondary with a control circuit fuse (typically 2–6 A) on the ungrounded leg. Route control voltage to the first terminal of the stop pushbutton.
- Wire the stop button in series Connect the control voltage wire to one terminal of the normally-closed stop pushbutton. Run a wire from the other stop terminal onward. This is the point where additional NC stop buttons or the overload relay contact join in series before reaching the coil.
- Wire the start button and auxiliary contact in parallel Connect the normally-open start pushbutton between the wire leaving the stop button and the wire entering the coil. Connect one terminal of the auxiliary NO contact to the same junction as the top of the start button, and the other terminal to the junction at the coil input — in parallel with the start button.
- Complete the coil circuit Run the remaining coil terminal to the neutral or common return of the control circuit. Double-check polarity if using DC coils. Verify coil voltage rating matches the control supply before energising.
- Test the circuit Restore power. Press start — confirm motor runs and continues after releasing the button. Press stop — confirm motor stops. Simulate overload trip if possible. Simulate a power interruption and verify the motor does not restart automatically on power restoration.
Specifications
| Control circuit type | 3-wire, momentary-contact with hold-in auxiliary |
|---|---|
| Stop button contact type | Normally-closed (NC), momentary action |
| Start button contact type | Normally-open (NO), momentary action |
| Auxiliary hold-in contact | Normally-open (NO), mechanically coupled to main contactor |
| Overload relay integration | NC trip contact wired in series with coil circuit |
| Low-voltage protection | Inherent — coil drops out on supply failure; motor cannot auto-restart |
| Typical control voltages | 24 V AC/DC, 120 V AC, 230 V AC (match to contactor coil rating) |
| Applicable standards | IEC 60947-4-1, NFPA 79, NEC Article 430, AS/NZS 3000 |
Safety warnings
- Always isolate and verify dead using a calibrated voltage tester before opening any panel or touching terminals. Apply lockout-tagout (LOTO) per OSHA 1910.147, EN ISO 50110, or local equivalent. Stored energy in capacitors or motor back-EMF can be lethal.
- A 3-wire circuit does NOT protect against deliberate bypassing of the stop button. Never jump out the stop button for testing without full awareness of what machinery will start. Always warn personnel near the driven equipment before a test run.
- Overload relay settings must be configured to the motor's nameplate full-load current. An improperly set overload protects neither the motor nor the wiring and may create a fire hazard.
- Control wiring must be rated for the control voltage and run in conduit or trunking separate from high-voltage power cables where practicable, to prevent inductive interference and insulation damage.
- Installation and modification of motor control circuits must be performed by or under the direct supervision of a licensed or registered electrician and must comply with local wiring codes (NEC/NFPA 70, BS 7671, IEC 60364, AS/NZS 3000, or applicable national standard).
Tools needed
- Calibrated voltage tester (CAT III rated)
- Lockout-tagout kit
- Insulated screwdrivers (flat and Phillips)
- Wire stripper
- Crimping tool with ferrule set
- Continuity and resistance multimeter
- Cable labels and permanent marker
- Torque screwdriver (for terminal torque specs)
Common mistakes
- Wiring the stop button as normally-open instead of normally-closed — the motor will not run at all, but more dangerously a failed-open stop button will not stop the motor in an emergency.
- Forgetting the hold-in auxiliary contact — without it the motor runs only while the start button is held, and low-voltage protection is lost.
- Connecting the overload relay NC contact in the wrong leg of the circuit, so a trip does not de-energise the coil.
- Using the incorrect auxiliary contact type — confusing NO for NC or using a contact from a different manufacturer block that does not mechanically interlock with the contactor correctly.
- Undersizing the control transformer — the VA rating must cover the coil inrush current as well as any indication lamps or other loads on the secondary.
- Failing to label start and stop wires at both ends, making future troubleshooting hazardous and time-consuming.
Troubleshooting
- Motor does not start when start button is pressed
- Cause: Open circuit in the control loop — stop button, overload relay contact, coil, or control fuse Fix: With power on and LOTO protocol respected, use a voltmeter to trace control voltage from the supply through each series element in turn. The point where voltage is present upstream but absent downstream is the open. Replace the faulty element.
- Motor starts but stops immediately when start button is released
- Cause: Auxiliary hold-in contact is not closing — faulty auxiliary contact block, contact not installed correctly, or wrong contact designation (NC instead of NO) wired Fix: Verify the auxiliary contact block is correctly fitted to the contactor. Measure continuity across the auxiliary contact terminals with the coil energised — it must be closed. Replace the auxiliary block if defective.
- Motor will not stop when stop button is pressed
- Cause: Stop button wired in parallel instead of series, welded main contacts, or coil circuit bypassed Fix: Immediately isolate supply at the upstream breaker. Inspect wiring against the diagram. Check main contacts for welding. If contacts are welded, replace the contactor. Rewire the stop button in series.
- Overload relay trips repeatedly under normal load
- Cause: Overload relay set too low, supply voltage imbalance, motor single-phasing, or high ambient temperature inside enclosure Fix: Verify relay setting equals motor nameplate FLA. Check supply voltage on all three phases under load using a true-RMS meter. Investigate motor current draw with a clamp meter. Ensure enclosure ventilation is adequate.
- Motor restarts automatically after a power interruption
- Cause: Circuit is wired as 2-wire (maintained contact) rather than 3-wire, or hold-in contact is connected in a way that bypasses the start button permanently Fix: Review the wiring against the 3-wire diagram. The auxiliary contact must bridge the start button only — not the stop button. A maintained-contact device wired where the start button should be must be replaced with a momentary pushbutton.
Frequently asked questions
Why is it called a 3-wire control circuit?
Three conductors connect the remote control station to the contactor coil circuit: a common return wire, a wire from the normally-closed stop button, and a wire from the normally-open start button. This distinguishes it from a 2-wire circuit, which uses only two conductors and a maintained-contact device.
What is the purpose of the auxiliary contact in a start-stop circuit?
The auxiliary contact (hold-in or seal-in contact) is a normally-open contact mechanically linked to the main contactor. It closes when the coil energises and bridges across the start pushbutton, keeping the coil energised after the operator releases the momentary start button.
What happens if the supply voltage drops suddenly?
The coil de-energises when voltage drops below its drop-out threshold. The main and auxiliary contacts both open. The motor stops and cannot restart automatically when power returns — this is called low-voltage or no-volt protection, and it is a primary safety feature of 3-wire control.
Can multiple stop or start stations be added to one circuit?
Yes. Additional stop buttons (NC contacts) are wired in series with the existing stop button; additional start buttons (NO contacts) are wired in parallel with the existing start button and the auxiliary contact. Each station can independently stop or start the motor.
What overload relay role does the diagram typically show?
The thermal or electronic overload relay NC contact is wired in series with the stop button and the coil. If the motor draws excess current long enough to trip the overload, its contact opens, de-energising the coil and stopping the motor — identical in effect to pressing stop.
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