Complete Wiring Guide for Motor Self-locking (Self-holding) Circuito di controllo

The core of a motor self-locking circuit is to connect the auxiliary normally open contact of the contattore in parallel with the start button. This configuration enables continuous power supply after the start button is pressed, ensuring the motor remains in operation even when the button is released. It is the most basic one-way control circuit for motors. The following is a complete wiring scheme, selezione dei componenti, operation steps, and safety precautions.

1. List of Core Components (Universal Framework for Single-phase/Three-phase Systems, with Three-phase as the Main Focus)

1. Circuit Principle and Wiring Diagram (Diviso in circuito principale + Circuito di controllo)

(IO) Cablaggio del circuito principale (Three-phase Power Supply → Motor)

1. Three-phase power supply L1, L2, L3 → Incoming terminals of Circuit Breaker QF
2. Outgoing terminals of QF → Main contacts of Contactor KM (L1→KM1, L2→KM2, L3→KM3)
3. Outgoing terminals of KM main contacts → Main circuit input terminals of Thermal Overload Relay FR
4. Main circuit output terminals of FR → U, V, W wiring terminals of Three-phase Asynchronous Motor M
5. The motor housing must be connected to the PE protective grounding wire (yellow-green dual-color wire, not to be omitted)

(II) Cablaggio del circuito di controllo (Core of Self-locking, Optional Voltage: 380V/220V/24V)

Logica fondamentale: The normally closed stop button is connected in series in the control circuit, and the normally open start button is connected in parallel with the normally open auxiliary contact of KM.

1. Control power supply live wire L1 (or live wire output from the control transformer) → Stop Button SB2 (normalmente chiuso) → Start Button SB1 (normalmente aperto)
2. Connect the normally open auxiliary contact of Contactor KM in parallel here (ramo autobloccante, critical!)
3. After parallel connection → Normally closed auxiliary contact of Thermal Overload Relay FR (cuts off the control circuit in case of overload)
4. Then connect to coils A1 and A2 of Contactor KM (coil voltage must match the control power supply, per esempio., 220V/380V)
5. Connect coil A2 to neutral wire N (220V) or another phase power supply L2 (380V) to complete the control circuit

Wiring Key Points:

– The auxiliary contact must be a normally open contact (closes when KM is energized, forming a self-locking loop)

– The normally closed contact of the Thermal Overload Relay FR must be connected in series in the control circuit; do not reverse the connection

– The Stop Button SB2 must use a normally closed contact, and the Start Button SB1 must use a normally open contact

– Do not connect the coil voltage incorrectly (per esempio., a 220V coil connected to 380V will burn out)

(III) Simplified Wiring Diagram (Text Version)

Circuito principale: Three-phase Power Supply → QF → KM Main Contacts → FR Main Circuit → Motor M

Circuito di controllo: Control Power Supply → SB2 (Normalmente chiuso) → [SB1 (Normalmente aperto) ∥ KM Auxiliary Normally Open] → FR Normally Closed → KM Coil → Control Power Supply Neutral/Another Phase

III. Step-by-Step Wiring Procedures (Standard On-site Installation Process)

1. Power Off and Voltage Testing: Disconnect the main power supply, use a multimeter to confirm no voltage is present, and hang a “No Switching On” sign
2. Component Fixation: Fix the contactor, relè termico, buttons, interruttore automatico, ecc., inside the electrical cabinet, following the layout of the electrical schematic diagram
3. Cablaggio del circuito principale: Wire according to the L1-L2-L3 phase sequence, use wires of matching diameter, tighten the terminals securely to avoid heating caused by loose connections
4. Cablaggio del circuito di controllo

First connect the control power supply end, then wire in the sequence of “Stop → Start + Self-locking → Thermal Relay Normally Closed → Coil”

Use a multimeter to measure the on-off status of the auxiliary contacts and confirm the correct wiring of the KM auxiliary normally open contact

Check the wiring of coil A1 and A2 to ensure the voltage matches

5. Protective Grounding: Connect the housing of all metal components and the motor housing to the PE grounding wire; do not misuse the yellow-green dual-color wire
6. Insulation Testing: Use a megohmmeter to measure the insulation resistance between the main circuit, control circuit, and ground; a value ≥1MΩ is considered qualified
7. No-load Test Run

Close QF, press SB1: Contactor KM energizes, and the motor starts; release SB1: KM remains energized (autobloccante riuscito)

Premi SB2: KM de-energizes, and the motor stops

Test the thermal overload relay’s overload protection function (simulate overload conditions and observe if tripping occurs)

During operation, check for abnormal noise, overheating, or unusual odors

1. Common Fault Troubleshooting and Solutions

1. Safety and Optimization Suggestions
2. Safety First: All wiring must be performed with the power off, after voltage testing; it is recommended to use 24V safe voltage for the control circuit to avoid electric shock
3. Component Matching: The rated current of the contactor, relè termico, and circuit breaker must match the motor power; do not use undersized components
4. Brand Replacement: Schneider and Chint components can be directly replaced (per esempio., LC1D09 ↔ NC1-0910, XB2 buttons ↔ NP2 buttons); ensure the number of auxiliary contacts and coil voltage are consistent
5. Wiring Technology: Color-code wires according to the phase sequence (L1 yellow, L2 green, L3 red, N blue, PE yellow-green dual-color), clearly label the terminals, and arrange wires neatly in trunking
6. Redundant Protection: Additional protection devices such as fuses, emergency stop buttons, and residual current circuit breakers can be added to improve circuit safety