Motor starter selection guide: DOL, reversing and star-delta starters, with contactor and overload relay sizing tables and wiring rules.
A motor starter is not a single component — it is the assembly that lets an operator safely switch a three-phase induction motor on and off while protecting it from overload. Every starter in this guide is built from three functional blocks: a contactor that makes and breaks the power circuit, a thermal overload relay that trips on sustained over-current, and a control circuit (typically start/stop pushbuttons and a holding contact). Understanding these blocks is the key to choosing the right starter for your panel.
DOL is the simplest and most common starter: the contactor applies full line voltage to the motor the instant it is energised. It is the correct choice for the vast majority of small and medium motors, roughly up to 4–5 kW on a healthy supply, because it is cheap, compact, and gives maximum starting torque.
Sizing a DOL starter means matching the contactor's AC-3 rated current to the motor's full-load current (FLC) with a margin, and setting the thermal overload relay to the motor FLC. A contactor rated for 9 A AC-3 covers a ~4 kW motor at 400 V; a 12 A or 18 A frame covers 5.5–7.5 kW. Never set the overload relay above the motor's service-factor current.
To reverse a three-phase motor you swap two of the three supply phases, which is done with two contactors — a forward contactor and a reverse contactor — fed from the same bus. The danger is that both contactors must never close at once, or you create a phase-to-phase short. This is prevented by a mechanical and electrical interlock: a mechanical flag blocks one contactor while the other is engaged, and an auxiliary contact in the control circuit prevents simultaneous coil energisation.
Reversing starters are used on conveyors, hoists, and mixers. Select both contactors to the same AC-3 current as a DOL starter, add the interlock, and size the overload relay on the common output.
Above roughly 4 kW, the inrush of a DOL start (typically 6–8× FLC) can cause an objectionable voltage dip on a weak supply or trip upstream protection. Star-delta starting uses three contactors — a main, a star, and a delta contactor — plus a timer. During start the motor windings are connected in star (phase voltage drops to 58%, current to about one third), then after a timed transition the windings switch to delta for running.
Star-delta cuts starting current but also cuts starting torque to one third, so it suits low-inertia loads such as pumps, fans, and compressors — not lifts or heavily loaded conveyors. The two transition contactors are sized to the reduced star current, while the main contactor carries full running current.
| Motor power (400 V) | FLC (A) | Contactor AC-3 (A) | Overload relay range (A) |
|---|---|---|---|
| 1.5 kW | 3.5 | 9 | 2.5–4 |
| 3 kW | 6.5 | 9 | 4–6.3 |
| 5.5 kW | 11 | 12 | 9–13 |
| 7.5 kW | 15 | 18 | 13–18 |
| 11 kW | 22 | 25 | 16–25 |
| 15 kW (star-delta) | 30 | 32 | 22–32 |
Always verify the contactor's utilisation category. AC-3 is for normal squirrel-cage starting; AC-4 (frequent inching or reversing) requires a larger frame because the break arc is harsher. For reversing duty, move one category up.
The contactor coil is rated for a control voltage that is almost always different from the motor supply — commonly 230 V AC, 24 V DC, or 110 V AC. Choose the coil voltage to match your panel's control transformer or switching power supply, and keep it consistent across the starter and any pilot devices. A 24 V DC coil is safest for wet or hazardous environments because it limits shock risk, while 230 V AC simplifies wiring when no control transformer is fitted. Mismatched coil voltage is a leading cause of field calls where the contactor chatters and will not pull in.
The most frequent errors are: sizing the contactor to starting current instead of running FLC (over-spec, wasted cost), leaving the overload relay at default (nuisance trips or no protection), omitting the mechanical interlock on reversing starters (catastrophic short), and forgetting that star-delta reduces torque. Get these right and the starter will outlast the motor it protects.
DOL applies full line voltage immediately and is used for motors up to about 4–5 kW. Star-delta first connects the windings in star to limit starting current and torque, then switches to delta for running, suiting larger motors on weak supplies.
Set the thermal overload relay to the motor's full-load current from the nameplate, within the relay's adjustable range. The contactor is sized to the same running current at AC-3; never set the overload above the motor service factor.
Because it uses two contactors to swap two supply phases. If both closed together they create a phase-to-phase short. A mechanical and electrical interlock guarantees only one contactor can be energised at a time.
No. The thermal overload relay protects against sustained over-current and single-phasing; short-circuit protection must come from upstream fuses or an MCB/MCCB coordinated with the contactor.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.