1. Soft Starter Cabinet Main Circuit Wiring (incoming line → soft starter → bypass → motor)
The soft starter cabinet main circuit hastwo parallel paths:
| Path | Path | Component | Conduction timing |
|---|---|---|---|
| Soft start path | incoming line → soft starter → motor | thyristor valve group | Startup phase |
| Bypass path | incoming line → bypass contactor → motor | bypass contactor KM | Operating stage |
2. Soft Starter Control Terminal Wiring (ATS480 as the example)
Schneider ATS480 control terminals fall intoinput terminals, output terminals, and communication terminalsthree groups:
- Digital inputs: RUN (start), STOP/RESET (stop/reset), LI1 (local/remote switchover), LI2-LI4 (programmable inputs, such as preset start curve selection).
- Relay output: R1 (fault relay, actuates on fault), R2 (run relay, actuates while running), R3 (programmable output, such as rated speed reached signal).
- Analog input: AI1 (current feedback 0–20 mA), AI2 (programmable 0–10 V or 4–20 mA).
- Analog output: AO (motor current indication 0–20 mA).
- Communication terminals: MODBUS RTU (RJ45 or terminals D+/D-).
3. Bypass Contactor Logic (Start → Full Speed → Bypass)
The bypass contactorKMlogic is the core control circuit of a soft starter cabinet:
- Start command: Press the start button SB2 → the RUN terminal is energized → the soft starter begins phase-angle firing, and the output voltage rises gradually from 30% Ue.
- Full-speed detection: The soft starter detects that the motor has reached rated speed (output voltage = 100% Ue), and internal logic issues the bypass command.
- Bypass action: The R3 programmable relay output closes → the KM coil is energized → the bypass contactor main contacts close → current transfers to the bypass circuit.
- SCR dropout: After the bypass contactor confirms closure, the soft starter stops the firing pulses, and the SCRs drop out of operation.
- Stop: Press SB1 → soft starter stop command + bypass contactor opens → the motor coasts to a stop or performs a soft stop.
4. Start/Stop Control Circuit Wiring (Standard Interlock Logic)
The control circuit usesstandard start/stop + self-hold + interlocklogic:
Start circuit: Stop button SB1 (normally closed) → start button SB2 (normally open) → soft starter RUN terminal. SB2 is paralleled with the run relay R2 normally open contact for self-hold.
Stop circuit: SB1 opens the RUN terminal supply → the soft starter stops → relay R2 drops out → bypass contactor KM opens → the motor stops.
Fault circuit: R1 fault relay (operates on fault) → fault indicator light + buzzer + reset button SB3.
Thermal protection: The thermal relay FR normally closed contact is wired in series at the front end of the control circuit; on overload it opens the RUN terminal supply.
Interlock: While the soft starter is running, KM must not open (to prevent breaking the bypass under load); when KM is closed, the soft starter may stop its firing pulses.
5. Common Wiring Errors in Soft Starter Cabinets (4 red lines)
① The soft starter output terminals and the bypass contactor output terminals havemismatched phase sequence→ the motor sees a reverse-direction shock at the instant of bypass transfer; ② the bypass contactor coil is not wired to the R3 programmable relay → no bypass after startup completes, and the SCRs run continuously and overheat; ③ the RUN terminal and STOP terminal share the same button → start/stop logic becomes confused; ④ the bypass contactor main contacts are undersized → size them at 1.25 times the motor rated current.
FAQ FAQ
Can the soft starter output terminals and the bypass contactor output terminals be connected to different phases?
NoThe soft starter output terminals U/V/W and the bypass contactor output terminals must be connected to the motor'ssame set of terminals(U1/V1/W1), with phase sequence exactly matching. Otherwise, the phase sequence changes at the instant of bypass switching, the motor reverses, and severe mechanical shock occurs.
When does the bypass contactor in a soft starter cabinet close?
After the soft starter detects that the motor has reached rated speed (output voltage 100% Ue), the internal logic issues a bypass command → relay R3 closes → the bypass contactor closes. The transfer is typically<50ms, and the motor does not notice it.
Must the bypass be opened first when a soft starter cabinet stops?
Yes. When stopping, open the bypass contactor first, then let the soft starter perform a soft stop (optional) or simply stop the trigger pulses. If the bypass contactor opens under load, it will arc, so choose a contactor with AC-3 utilization category.
Can a soft starter be used without a bypass contactor?
Not recommended. Without bypass, the thyristors carry current continuously, power loss is high (~1.5V × 3 × I voltage drop), heating is severe and requires larger heat dissipation, and harmonics persist. Bypass can be omitted only for low power (≤7.5kW) or short-time duty.
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