Generator Load Budget for a Field Detachment
A detached flight is powered by one diesel generator rated 3.0 kW continuous and 4.5 kW for a maximum of 10 seconds on surge. The connected equipment is: a radio set drawing 400 W continuously; three laptop chargers at 50 W each; interior lighting totalling 300 W; a water pump whose motor is rated 1.1 kW but draws three times rated power for about 2 seconds at every start; and a refrigerator drawing 250 W while its compressor runs, with a 40% duty cycle and a start surge of 2.5 times running power. The detachment must run 12 hours overnight. The generator burns about 0.9 litres per hour at 80% load, and 20 litres of diesel are held.
1. Calculate the steady-state load with every item running, and express it as a percentage of the continuous rating.
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Add the continuous figures:
- Radio: 400 W
- Chargers: 3 × 50 W = 150 W
- Lighting: 300 W
- Pump running: 1100 W
- Refrigerator running: 250 W
Total = 2200 W = 2.2 kW.
Utilisation. 2.2 ÷ 3.0 = 73% of continuous rating.
Reading the number. 73% is comfortable for a steady load but leaves only 800 W of headroom. The fridge duty cycle does not help here: at 40% duty it averages 100 W, but the figure that matters for generator sizing is the load while the compressor is running, because the surge arrives at that moment. Judging the set by the average load would understate the risk in the next task.
2. The pump and the refrigerator compressor can start at any moment. Analyse the two possible start cases and say whether the generator will hold.
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Case 1 - pump starts while the fridge compressor is running. Replace the pump's 1100 W with its surge of 3 × 1100 = 3300 W: 400 + 150 + 300 + 3300 + 250 = 4400 W. Against a 4500 W surge rating: holds, with 100 W (2%) to spare.
Case 2 - pump and fridge compressor start within the same 10 seconds. Replace both: 400 + 150 + 300 + 3300 + (2.5 × 250 = 625) = 4775 W. Against 4500 W: exceeds by 275 W. The breaker trips and the whole detachment goes dark.
Judgement. The system is safe only if the two surges never overlap, and nothing in the installation enforces that. A 2% margin on Case 1 is not a margin at all - a cold morning or a slightly low supply voltage pushes the pump's actual starting current higher and Case 1 becomes a trip as well. The honest finding is: nominal load fine, surge case not controlled. This is a design fault, not an operator error, and it will present as a random total power failure that the operator cannot reproduce on demand.
3. Is the 20 litres of fuel adequate for the 12-hour run, and if a trip is to be avoided by load shedding, what should be shed and why?
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Fuel. At 0.9 L/h for 12 h: 12 × 0.9 = 10.8 L. At 73% load consumption is a little below the 80%-load figure, so actual use is roughly 10 L. Held: 20 L. Adequate, with about 9 L spare - fuel is not the constraint; the electrical margin is.
Load shedding, worked properly. Case 2 exceeds the surge rating by 275 W, so any shedding plan must remove at least 275 W:
- Chargers only (150 W): 4775 − 150 = 4625 W - still trips. Insufficient.
- Lighting only (300 W): 4775 − 300 = 4475 W - holds, but blinds the detachment at night for 2 seconds and, worse, removes the load that tells you the trip has occurred.
- Chargers + lighting (450 W): 4775 − 450 = 4325 W - holds with 175 W margin.
Better answer than shedding. Shedding is a workaround. The clean fix is an interlock or a standing order: start the pump only when the fridge compressor is off (it is off 60% of the time). That removes the simultaneous case entirely and keeps all loads alive. Recommend the interlock, use chargers-plus-lighting shedding only as the fallback until it is fitted.