Soft starter wiring diagram — basics
Wiring a soft starter is no harder than wiring a contactor. Three phases in, three phases out to the motor, plus control signals. But there are details that affect safety and equipment life.
What you need
- Soft starter rated for your motor current
- Circuit breaker (D curve, 1.5-2x motor I_rated)
- Power cable
- Contactor (if remote disconnect is needed)
- Start/stop buttons or PLC signals
Step 1: power wiring
Mains → breaker → (contactor) → soft starter → motor.
- Connect mains to input L1, L2, L3
- Connect motor to output T1, T2, T3
- Ground all enclosures
Contactor goes BEFORE the soft starter only, never after.
Step 2: control signals
Buttons
- Start — DI1, normally open
- Stop — DI2, normally closed (NC)
PLC
Same as buttons, via PLC digital outputs.
Modbus
RS-485: A+ and B-, 120 Ohm terminator at end of line.
Step 3: outputs
- RUN — relay closed when motor is running
- FAULT — relay closed on error
- BYPASS — bypass mode signal
Step 4: parameter setup
- Motor rated current
- Start ramp (pumps 10-20, conveyors 5-10, compressors 8-15 sec)
- Stop ramp (pumps 10-30 sec for anti-water-hammer)
- Starting voltage (40-70%)
- Current limit (300-400%)
Common wiring mistakes
- Contactor after soft starter — damages thyristors on opening under load
- No grounding — electric shock risk
- Power and signal cables together — interference
- Wrong phase sequence — swap two phases at input (not output)
- Stop button NO instead of NC — motor will not stop on wire break
Bypass contactor
For soft starters without built-in bypass. Wired in parallel, controlled by BYPASS output.
FAQ
Can I skip the contactor?
Yes, if there is a circuit breaker. Contactor is for remote disconnect only.
What cable?
Copper, sized for rated current. 22 kW (45A) — min 10 mm sq. 37 kW (75A) — 25 mm sq.
Is an EMC filter needed?
Usually not — a soft starter generates less noise than a VFD.
Can the motor be reversed?
Not with a standard soft starter. Needs a contactor scheme or a model with built-in reversing.
How long does wiring take?
1-2 hours for an experienced electrician.
Summary
Mains → breaker → soft starter → motor. Contactor before, never after. Stop = NC contact. See how soft starters work. Browse our soft starter catalogue.
