A 1 hp Solar Pump Trips Its 3 kW Inverter Thirty Seconds After Starting
A community borewell in a district of Uttar Pradesh is powered by an off-grid system: a 24 V, 200 Ah tubular bank (two 12 V monoblocs in series), a 3 kW single-phase inverter with a 6 kW surge rating for 3 seconds and a low-voltage disconnect at 21 V, and a 1.6 kWp array of four 400 Wp modules. The pump is a 1 hp (0.75 kW) capacitor-start AC submersible drawing 1.1 kW at 0.78 power factor. The battery-to-inverter DC cable is 2.5 mm2, three metres each way, and it is not fused.
The pump starts, runs for about thirty seconds, then the inverter trips on low battery. Dawn rest voltage is 24.6 V. A clamp meter records a 210 A peak for about 1.2 seconds at start and a steady 62 A while running at the inverter input. A min/max voltmeter on the inverter DC terminals logs a minimum of 19.6 V; the battery posts sag to 23.9 V under the running load.
1. Use the measured currents to separate the contributions of the cable, the battery and the inverter, and state which item is the primary cause of the trip.
Read the worked answer
Convert everything to volts at the inverter terminals, because that is what the trip logic uses.
Cable resistance. 2.5 mm2, three metres each way, is a six-metre loop: R = 0.0175 x 6 / 2.5 = 0.042 ohm.
Running condition. At 62 A the cable drops 62 x 0.042 = 2.6 V. With the battery at 23.9 V, the inverter sees about 21.3 V, which is only 0.3 V above its 21 V disconnect. Any further sag from pump load rise, a warm cable or a low state of charge pushes it over the edge after a few seconds. That matches the 30-second trip exactly.
Start condition. At the 210 A peak the cable would drop 8.8 V, so the inverter input momentarily collapses far below 21 V. The inverter's surge rating (6 kW, roughly 250 A at 24 V) lets it ride through the 1.2 second start by design, but the cable does not. The min/max record of 19.6 V is a sampling artefact of a fast event, not the true minimum.
Battery. Sag from 24.6 V to 23.9 V at 62 A is about 0.011 ohm for the bank, which is normal. The bank is not the primary culprit, but 24.6 V rest means it is already around 50-60% depth of discharge, so the system starts each pumping session with little margin.
Verdict: the cable is the primary cause; the partly discharged, undersized bank is a secondary contributor; the inverter is behaving correctly.
2. The owner wants to buy a 5 kW inverter instead. Evaluate that proposal against soft-start, a DC pump, or conversion to 48 V.
Read the worked answer
A bigger inverter will not fix this, because the fault is upstream of the inverter.
Why it fails. The trip happens because the DC input falls to 21 V, and that is set by cable resistance and battery state, not by inverter rating. A 5 kW inverter has a higher current draw for the same load and typically the same or a higher low-voltage disconnect, so the problem gets worse, not better. The money would be spent with no change in behaviour.
Ranked alternatives.
- Fix the DC path first. 35 mm2 cable (R = 0.003 ohm, about 1.8 V drop even at the 210 A surge), a 150 A DC fuse or breaker at the battery positive within 200 mm, and proper crimped lugs. Cheapest, addresses the actual cause.
- Restore battery condition. Equalise and capacity-test the bank; if it cannot hold two days of load, add capacity before anything else.
- Soft starter or VFD. Cuts the 210 A start to near running current, protects the pump and reduces surge. Good value, but does nothing for the running-voltage drop, so it must follow item 1.
- DC pump with MPPT controller. Removes the inverter and the AC motor's poor part-load efficiency, and is the most efficient long-term option on a solar borewell, but it is a full replacement of the pumping head and controller.
- 48 V conversion. Halves all currents for the same power, which is the correct architecture at the 3 kW level (24 V at 3 kW is about 125 A nominal, already marginal). Sensible at the next battery replacement, but it means a new inverter, new bank configuration and new cabling.
Recommendation: items 1 and 2 now, item 3 if the pump is old, and plan the 48 V move at end of battery life.