How it works
DOL starting applies full voltage directly; starting current = 5–7 × rated current, starting torque = 1.5–2 × rated torque. Star-delta starting connects stator in star during start (phase voltage reduced to 1/√3 of line voltage), switching to delta for run; starting current and torque both reduce by a factor of 3 compared to DOL delta start — the most important ratio to remember. Autotransformer starting uses taps of 65% or 80% of supply voltage; if tap ratio is x, both starting current and torque reduce by x². Soft starters use back-to-back SCRs to ramp up voltage gradually; electronic motor starters achieve stepless control. Resistance and reactance starters insert series impedance during start for wound rotor or slip ring motors, with external resistance giving the advantage of improved starting torque.
Key points to remember
The critical fact examiners probe: star-delta reduces starting torque to 1/3 of DOL value, not 1/√3 — this factor-of-3 comes from the combination of reduced voltage (1/√3) and reduced current (1/√3), giving torque proportional to V² which drops by 1/3. Autotransformer with 65% tap reduces starting current by (0.65)² = 0.4225 times the DOL inrush. DOL is only permitted below 5 kW in many Indian utilities to limit voltage disturbance. Soft starters provide the smoothest acceleration profile and allow adjustable current limiting, typically set to 2–3× full-load current. Slip ring motors uniquely allow external rotor resistance to simultaneously reduce starting current and increase starting torque — the star-delta method cannot do this.
Exam tip
Every Anna University electrical machines paper asks you to compare DOL, star-delta, and autotransformer starting methods in a table covering starting current, starting torque, cost, and applications — draw the table first, then fill it to ensure you don't miss any column.