Small motors, up to about 7.5 HP (5.5 kW), are usually started DOL. Larger three-phase motors are usually started star-delta, which cuts the starting current to about one third. Star-delta needs a motor with six terminals that is rated to run in delta at 415 V.
How they differ
| DOL starter | Star-delta starter | |
|---|---|---|
| How it starts | Full voltage on the motor at once | Starts in star, then changes over to delta |
| Starting current | About 6 to 8 × full-load current | About one third of DOL |
| Starting torque | Full | About one third, so best for pumps, fans and light starts |
| Motor terminals | 3 or 6 | 6 (both ends of each winding) |
| Typical use | Up to about 7.5 HP | About 7.5 HP and above |
Electricity supply companies and the site's own supply may set limits on starting current, so larger motors on weak supplies often need star-delta or a soft starter even below 10 HP. A semi-automatic star-delta (SASD) starter is changed over by hand; a fully automatic one (FASD) uses a timer.
Starter and overload relay by motor size (415 V, three-phase)
| Motor | Approx. full-load current | Usual starter | Overload relay |
|---|---|---|---|
| 1 HP (0.75 kW) | 1.8 A | DOL | 1.6–2.5 A range, set 1.8 A |
| 2 HP (1.5 kW) | 3.4 A | DOL | 2.5–4 A range, set 3.4 A |
| 3 HP (2.2 kW) | 4.8 A | DOL | 4–6.3 A range, set 4.8 A |
| 5 HP (3.7 kW) | 7.5 A | DOL | 6–10 A range, set 7.5 A |
| 7.5 HP (5.5 kW) | 11 A | DOL or star-delta | DOL: set 11 A · star-delta: set 6.4 A |
| 10 HP (7.5 kW) | 14.5 A | Star-delta | set 8.4 A (0.58 × FLC) |
| 15 HP (11 kW) | 21 A | Star-delta | set 12 A |
| 20 HP (15 kW) | 28 A | Star-delta | set 16 A |
| 25 HP (18.5 kW) | 34 A | Star-delta | set 20 A |
Full-load currents are typical for standard 4-pole motors; always use the current on your motor's nameplate. In a star-delta starter the overload relay sits in the winding circuit, so it is set to about 0.58 × the full-load current (full-load current ÷ 1.73).
Submersible and single-phase pumps
For three-phase submersible pumps use a pump controller with a single-phasing preventer, such as the L&T MU-G. Single-phase submersible pumps need a controller with the right start and run capacitors, such as the L&T MR-G2 or MR-Gi.
Starters we supply
- All motor starters & pump controllers
- L&T MK1 starter (DOL, SASD, FASD)
- L&T MU / MB starter
- L&T MN / ML starter in sheet-steel enclosure
- Siemens RAJA+ agriculture starter
- L&T MU-G submersible pump controller
Need help choosing?
Send us the rating, the load or a photo of the nameplate on WhatsApp, and we will confirm the right model, availability and rate.
Values are typical guidance for Indian 230 V single-phase and 415 V three-phase supplies. Always follow the equipment nameplate, the manufacturer's instructions and your licensed electrician, who is responsible for the final selection and installation.