Three-Phase 400 V Generators

A three-phase set has an alternator producing three voltages spaced 120 degrees apart, so its power socket supplies 400 V while phase-to-neutral gives 230 V at 50 Hz; a single-phase unit delivers 230 V only. The practical gap shows in deliverable power: a 16 A three-phase socket handles about 11 kVA against roughly 3.6 kVA from a 16 A single-phase outlet. Induction motors in construction machinery are built for 400 V and will not start without three phases, and spreading the load across three circuits keeps currents lower.

Three-phase sets have single-phase 230 V outlets beside the power socket, each fed from one phase. The limit is how output is divided: one phase normally gives no more than a third of rated power, so about 3 kVA on a 9 kVA unit. Loads must be spread evenly across the three phases — imbalance beyond roughly 20% pulls the phase voltages apart and overheats the windings. The phases also cannot be summed to feed one large 230 V load. If the work is mainly hand tools, 230 V generators come out cheaper.

Sizing starts by converting kVA into kW: active power equals apparent power times the power factor, typically 0.8 for generator sets, so a 10 kVA unit yields around 8 kW of usable output. On top of that comes the starting current of an induction motor, which for a fraction of a second reaches 2.5–3 times rated current. A workable rule: the largest single machine should stay within 40–50% of the generator rating and continuous load at 70–80% — that margin absorbs starting surges and keeps the set out of overload.

Fuel choice follows annual running hours. Petrol units weigh tens of kilograms less, cost less to buy and suit intermittent duty — a few hours a day, moved around the site by hand. Diesel sets burn roughly 20–30% less fuel per kilowatt-hour and are built for continuous operation. Past about 500 operating hours a year, a diesel's higher purchase price is recovered on fuel. For standby duty on a building, also compare emergency power generators.