Turkish
A three-phase motor draws five to eight times its full-load current while it runs up to speed. That inrush is not a fault — but it is the reason starting method is a design decision rather than a default. Start a large motor directly on line and you get maximum torque, maximum current and maximum mechanical shock. Start it softly and you protect the supply, the coupling and the driven machine, at the cost of money and panel space.
This guide compares the three methods you will actually be choosing between — direct-on-line, star-delta, and electronic soft starters — on the criteria that decide it: torque, current, mechanical stress, cost, and what each one demands of the switchgear.
Three things happen when a motor starts across the line:
A small motor on a stiff supply makes all three invisible. A 90 kW pump on a weak supply or a long feeder makes all three the dominant design constraint.
DOL connects the motor straight to the supply at full voltage. It is the simplest, cheapest and most robust starting method there is: one contactor, one overload relay, one circuit breaker.
DOL is correct whenever the supply can absorb the current surge and the load can absorb the torque. In practice that means smaller motors — in many installations up to roughly 5–7.5 kW as a matter of course, and larger where the supply is stiff and the load tolerates a hard start. Pumps, fans and compressors started unloaded or against a closed valve are frequently DOL at ratings well above that.
The contactor must be sized for the starting duty, not just the running current — this is the AC-3 utilisation category question we cover in our AC kontaktör seçim kılavuzu. And the protective device must be selected so it does not trip on the inrush: a breaker with an instantaneous set too low will nuisance-trip every time the motor starts. Our LC1D 25–38 A ve LC1D 40–65 A contactors cover the bulk of DOL applications.
Star-delta starting reduces the voltage across each winding during run-up. The motor is initially connected in star, which puts each winding across the phase-to-neutral voltage rather than the phase-to-phase voltage — a factor of 1/√3, or about 58%.
Both starting current and starting torque scale roughly with the square of the applied voltage. At 58% voltage that means about one-third of both: starting current drops to roughly 2–3 × full-load current, and starting torque drops to roughly one-third of the DOL value.
Star-delta suits motors that start light — fans, centrifuges, unloaded compressors, pumps started against a closed valve. It is a poor choice for anything that needs meaningful torque to break away, because one-third torque may simply not move the load.
Three contactors instead of one, interlocked mechanically and electrically, plus a timer. The motor must have all six winding terminals brought out and must be designed for delta running on the supply voltage. There is also the transition spike: when the motor is momentarily disconnected and reconnected, it can draw a current transient as it resynchronises, which partly offsets the reduction you were trying to achieve.
A soft starter uses anti-parallel thyristors to ramp the voltage up from a set initial value to full voltage over a controlled time. Torque and current follow the voltage, so both can be shaped to suit the load.
| Criterion | Direct-on-line | Star-delta | Soft starter |
|---|---|---|---|
| Starting current | 6–8 × FLC | 2–3 × FLC | Adjustable, typically 2–5 × FLC |
| Starting torque | Full (1.5–2.5 × FLT) | About 1/3 of DOL | Adjustable, set above breakaway torque |
| Mechanical shock | Highest | Moderate, plus transition spike | Lowest of the three |
| Voltage dip on supply | Largest | Reduced | Controlled and limited |
| Main components | 1 contactor + overload | 3 contactors + timer | Electronic controller + bypass contactor |
| Motor cable requirements | 3 cores | 6 cores | 3 cores |
| Capital cost | Lowest | Low–moderate | Highest of the three |
| Panel space | Smallest | Moderate | Larger, with heat to manage |
| Speed control during running | No | No | No — that requires a VFD |
| Best suited to | Small motors, stiff supply, tolerant load | Larger motors starting light | Where current limit, smooth ramp or soft stop is needed |
Work through four questions in order:
Whichever method you choose, the protective device has to be set around the starting current, not the running current. An MCCB with an instantaneous pickup set too low will trip on every start; set too high and it stops protecting the cable. That is a trip unit setting question, and it is covered in detail in our guide to MCCB trip unit settings (Ir, Isd, Ii, Ig).
Contactors also need selecting for the duty. Frequent starting and stopping — and star-delta switching in particular — is a harder duty than simply holding a motor in circuit. If you are commissioning or troubleshooting a starter, our AC kontaktör arıza giderme kılavuzu covers the failure modes that show up most often. For larger motors, the LC1D 80–95 A ve LC1D 115–150 A ranges cover the upper end.
There is no universal kW threshold — the limit is set by the supply’s ability to absorb the inrush and the load’s tolerance of full torque. As a rough guide many installations use DOL up to 5–7.5 kW as standard practice, and larger where the supply is stiff and the load starts light.
Both torque and current scale approximately with the square of the applied voltage. In star each winding sees 1/√3 of the line voltage, so both fall to roughly one-third of the DOL value. That is exactly why star-delta only works on loads that start light.
Usually yes, and it removes the six-core cable requirement and the open transition. The trade-offs are higher capital cost, panel heat, and the need to confirm the load does not need speed control during running.
Most installations use one. Once the motor is up to speed the contactor bypasses the thyristors, eliminating their heat dissipation during normal running and extending their life.
Yes. The breaker’s instantaneous and short-time settings must sit above the starting current to avoid nuisance tripping, while long-time protection still has to protect the cable. Lower starting current makes that balance easier to strike.
The deciding question is almost never “which is cheapest” — it is “which one will actually start this load on this supply.” Answer the torque and inrush questions first, and the cost question usually resolves itself.
Building motor starters and need contactors and protection matched to the duty? Send us the motor ratings and starting method. Contact our engineering team and we will confirm the right contactor and MCCB combination.
