Rahi generator mai i tētahi Rārangi Uta

How this is calculated#

A generator has to do two things. It has to carry the running load, and it has to hold its voltage when a large load starts. A small set can run a pump but still dip so far as the pump starts that lights flicker and contactors drop out. This tool works both and gives the larger as the size.

You list the loads with the power each draws, its power factor and the step it starts in. Loads in the same step start together. For the running load the tool adds the real power and the reactive power of every load, then takes the kVA from both. Loads at different power factors do not add as kVA. The set must also give the real power at its own rated power factor, which is often 0.8.

A motor draws more than its running current as it starts. The tool takes the motor's running kVA from the motor tool's full load current, then multiplies it by the start: six for direct on line, a third of that for star-delta, the multiple you enter for a soft starter, and one for a variable speed drive. For each step the tool finds the smallest set whose reactance keeps the dip within the limit you set. The largest need, running or starting, divided by what is left after any reduction for altitude and heat, is the size. The generator tool then gives the full load current and fault current of a set at that rating.

AmpSize holds no generator standard. The dip limit, the reactance and the soft starter multiple are inputs, and the result says so.

Worked examples#

• Farm lights, then a 7.5 kW water pump direct on line needs 50 kVA. The pump start sets the size, as Starting step 2, against a running load of 19.05 kVA.
• The same pump on a star-delta starter needs 22.92 kVA. The running load sets the size now, at a running load of 19.05 kVA.
• A dairy shed: milking plant, vacuum pump and hot water in turn needs 50 kVA. The vacuum pump's direct-on-line start, Starting step 2, sets it, against a running load of 34.96 kVA.
• A building's lighting, chiller on a drive and lifts, derated 10 % for heat needs 97.22 kVA. The running load sets the size at a running load of 79.44 kVA.

Limits#

The dip is worked from the set's subtransient reactance alone. It leaves out the voltage regulator's recovery, the engine's speed dip, and loads already running, which a maker's sizing program models. Use the size as a first pick, then check it on the maker's data.

The tool does not cover a set in parallel with the network, a non‑linear load's effect on the alternator, or the installation rules for a generating set.

See also: Generator, Motor and Maximum demand.