I. Preparations and Scheme Design
Clarify Motor Parameters and Requirements
Determine core motor parameters: voltage (e.g., three-phase 380V/single-phase 220V), power, rated current, starting method (direct-on-line starting/star-delta starting/variable frequency starting), and control functions (forward/reverse rotation, speed regulation, emergency stop).
Define application scenarios: e.g., workshop assembly line motors, conveyor equipment motors. Consider overload protection, short-circuit protection, and IP rating (waterproof and dustproof level).
Select Circuit Topology and Control Mode
Simple start-stop circuit: Suitable for low-power motors (≤7.5kW). Composed of circuit breakers, contactors, thermal relays, and buttons, it realizes motor start, stop, and overload protection.
Forward/reverse rotation control circuit: Uses two sets of contactors with interlock function to prevent phase-to-phase short circuit, enabling motor forward and reverse switching.
Star-delta reduced-voltage starting circuit: Suitable for high-power motors (≥15kW). It reduces starting current to protect the power grid and motor.
Variable frequency control circuit: Achieves motor speed regulation through frequency converters, featuring energy saving and high control precision. Ideal for speed-adjustable scenarios (e.g., fans, water pumps).
Electrical Component Selection
| Component Type | Selection Basis | Example |
|---|---|---|
| Circuit Breaker (QF) | Rated current is 1.2–1.5 times the motor rated current; breaking capacity matches the power grid | Molded Case Circuit Breaker (MCCB)/Miniature Circuit Breaker (MCB) |
| Contactor (KM) | Rated voltage matches the power supply; rated current ≥ motor rated current | AC contactor (e.g., CJX2 series) |
| Thermal Relay (FR) | Setting current = motor rated current, realizing overload protection | JR20 series thermal relay |
| Buttons/Switches | Control circuit voltage (usually AC220V/DC24V); select normally open (start)/normally closed (stop) types | LA38 series buttons |
| Wires | Current-carrying capacity ≥ motor rated current; consider laying methods (surface mounting/conduit installation) | Copper core cables (BV/BVR). Larger power requires thicker wire diameter |
| Protection Components | Fuses (short-circuit protection), surge protectors (lightning protection) | RT18 series fuses |
II. Draw Electrical Schematic Diagram and Wiring Diagram
Draw Electrical Schematic Diagram
Distinguish between the main circuit (power supply → circuit breaker → contactor → thermal relay → motor) and the control circuit (buttons → contactor coil → thermal relay normally closed contact).
Adopt national standard electrical symbols (e.g., GB/T 4728), mark component models, parameters, and terminal numbers to ensure clear logic (e.g., interlock and self-lock circuits).
Example (logic of simple start-stop circuit for three-phase motors):
Press the start button → contactor coil energizes → main contacts close → motor runs.
Contactor auxiliary normally open contact self-locks → motor continues running after releasing the button.
Press the stop button / thermal relay overload action → coil de-energizes → motor stops.
Draw Wiring Diagram
Draw according to the actual component layout, mark wire routes, wire diameters, and terminal block numbers for easy on-site wiring.
Wires for the main circuit should be thicker, while thinner wire diameters (e.g., 1.5mm²) can be used for the control circuit.
III. On-site Wiring and Assembly
Wiring Specifications
Lay the main circuit and control circuit separately to avoid interference; keep high-voltage circuits (380V/220V) away from low-voltage circuits (e.g., frequency converter communication lines).
Label wires clearly, ensure firm terminal block connections without looseness; maintain a moderate wire bending radius to prevent insulation damage.
Wire the motor junction box according to the nameplate marking (e.g., star Y/delta △ connection method). Incorrect wiring will burn the motor.
Wiring Steps
Fix electrical components: Install circuit breakers, contactors, thermal relays, etc., on the guide rails inside the distribution box.
Main circuit wiring: Power input → circuit breaker → contactor main contacts → thermal relay heating elements → motor terminals.
Control circuit wiring: Control power supply → stop button (normally closed) → start button (normally open) → contactor coil → thermal relay normally closed contact → control power supply negative pole.
Ground protection: Reliably ground the motor housing and distribution box housing (PE wire), with a grounding resistance ≤ 4Ω to prevent electric shock due to leakage.
IV. Commissioning and Acceptance
Pre-power Inspection
Use a multimeter to measure the insulation resistance of the main circuit (≥1MΩ) to prevent short circuits; check whether the wiring is consistent with the drawing, and whether there is wrong or missing wiring.
Manually test whether the contactor operates flexibly, and whether the thermal relay setting current is correct.
No-load Commissioning
Disconnect the motor from the load, press the start button after power on, observe whether the motor rotation direction and speed are normal, and check for abnormal noise or overheating.
Test whether the stop and emergency stop functions are reliable, and whether the thermal relay overload protection takes effect (short-circuit the thermal relay contacts to simulate overload).
Load Commissioning
Connect the load and gradually increase the load; observe whether the motor current is within the rated range and whether the component temperature rise is normal.
Record operating parameters (voltage, current, temperature) to ensure the circuit meets production requirements.
V. Safety and Maintenance Notes
All wiring and commissioning operations must be performed with power off. Hang a "No Switching On" sign to prevent electric shock.
The circuit must comply with national electrical installation specifications (e.g., GB 50254 Code for Construction and Acceptance of Electrical Installations of Low-voltage Electrical Apparatus).
Regularly check whether wires and components are aging or loose, and replace damaged parts in a timely manner to avoid faults.
