Motor starters: DOL, star-delta, soft starter, VFD
Ways to start an induction motor, their starting current and torque, motor protection, and when a soft starter or VFD is the right choice.
Starting methods compared
| Method | Starting current (typ.) | Starting torque | Speed control | Notes |
|---|---|---|---|---|
| Direct on line (DOL) | 6–8 × rated | Full | No | Simple; mechanical shock and voltage dip |
| Star-delta | ≈ ⅓ of DOL | ≈ ⅓ of DOL | No | Motor must be rated for delta at line voltage; current peak at changeover |
| Soft starter | 2–4 × rated (adjustable) | Reduced, adjustable ramp | No | Thyristors ramp the voltage; soft stop for pumps (water hammer) |
| VFD | ≈ rated | Full torque at low speed | Yes | Highest cost; energy saving for pumps and fans |
Motor starter components
A classic DOL starter = short-circuit protection (fuse or motor protection circuit breaker) + contactor + overload protection (thermal or electronic relay, set to the motor rated current). A motor protection circuit breaker often combines short-circuit and overload protection. The overload relay's auxiliary contact goes to the PLC as "motor fault".
Soft starters in practice
- Set initial voltage/current limit and ramp time to the load: a loaded conveyor needs more starting torque than a centrifugal pump.
- Many soft starters close an internal or external bypass contactor after the ramp to avoid thyristor losses.
- They cannot reduce speed in operation — choose a VFD if speed must vary.
Choosing
- Small motors, few starts, strong supply → DOL.
- Large fans/pumps with weak supply, no speed control → soft starter.
- Process needs variable flow/speed, frequent starts, precise acceleration → VFD — see VFD basics.
Connections
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Sources
Updated 2026-10-02