I’ve been putting this off because the water has “worked fine.” Fine until the kettle’s on, the fridge decides to cycle, the router brick warms up, and the borehole pump jumps in with all the grace of a mule. Today was the day I gave the pump a soft-start and stopped pretending those little chirps from the inverter were “normal”.
Starting point
- Single-phase borehole pump, nameplate 1.1 kW, 230 V.
- Pressure tank at 2.2 bar cut-in, 3.2 bar cut-out.
- Inverter 5 kVA with plenty of headroom for steady loads, but sensitive to stacked starts.
- Cables run about 24 m from plant room to well head, then drop. Copper, 2.5 mm² on the pump leg.
- Running current ~5.2–5.6 A. Clamp meter shows cold, dry start inrush at 28–32 A. That’s the problem.
If you don’t measure, you guess. I put the clamp on L, set the logger in the inverter app, and did three controlled starts with every non-essential load off. The trace looks like a quick mountain: 0.2 seconds to ~30 A then down to the 5–6 A plateau. It is short, but when it coincides with the hob or the AC compressor you get the wobble.
Which box and why
I went soft-starter, not VFD. Reasons: it is a simple single-phase motor with a start capacitor, I don’t need variable flow, and I want fewer things to fiddle with. A VFD would give pretty curves and pressure control, but also harmonics, shielding, and a whole afternoon of parameter menus. Soft-start is a ramp, not a lifestyle.
Box is rated 20 A continuous with a short-term start allowance well above my needs. DIN rail mount. I checked the datasheet for minimum supply voltage under ramp and made sure the inverter can hold that without dipping, then checked motor compatibility with soft-start on single-phase. All green.
Install notes that actually mattered
- Locked out the inverter output, bled the line, and dropped the pressure down to just above cut-in so the first start wouldn’t fight head height.
- Put the soft-start as close to the pump feed breaker as possible to keep lead lengths short.
- Verified the pressure tank pre-charge while the line was empty: 1.9 bar with my cut-in at 2.2 bar. Close enough, nudged it to 2.0 bar.
- Re-made the crimp lugs. I thought they were fine. They were not fine. A lazy crimp is an invisible heat source.
Wiring is line in, line out, neutral pass-through on this model, start ramp set to 1.5 s, stop ramp disabled. I left kick-start off for the first run.
Results
Three starts, same test conditions as the morning:
- Peak current now 13–15 A instead of 28–32 A.
- Ramp time 1.5 s to the running plateau at ~5.4 A.
- Inverter stayed silent. No little chirp, no brown note, no twitch on the lights.
- Line voltage sag reduced from a 6–7 V blink to 2–3 V. The fridge didn’t even flinch.
I ran a real-world stack next: pump cut-in with the kettle at 85 °C and the induction on low. No complaint. I would not do that every day, but it no longer punishes small mistakes.
Side quests while I had the tools out
- Moved the router and Starlink brick to a smart plug and set a sleep schedule that matches when we are actually asleep. That shaved an honest 12–15 W overnight.
- Cleaned the pump strainer and wrote the date on it with a paint pen. Future me forgets these things.
- Labeled the plant-room breakers in plain language. “Pump,” “kitchen sockets,” “workshop,” not “QF-3.”
What changed because of today
- Kitchen timing is calmer. I can run water while cooking without doing mental maths about peaks.
- The AC can start during a hand-wash without flipping my focus to the inverter display.
- Generator stays a cold backup, as it should.
Numbers I will watch for a week
- Daily starts. If I see more than 25, I will raise cut-out slightly or chase a small drip.
- Peak current at first start of the morning. If it creeps up, something is binding or the ramp is too gentle.
- SOC at sunset on “normal” days. If today’s work did anything useful, I should land a few points higher without changing habits.
What didn’t work
I tried a 2.5 s ramp. The motor sounded like it was thinking about starting rather than starting. Back to 1.5 s. Also tried “kick-start 10 % for 100 ms.” Unnecessary for this pump. Off again.
Costs, plain
Soft-starter box was cheaper than a panel plus rails and a morning on the roof, and today it saved me more grief than another panel would. I like panels. This was the better first move.
Next small things
- Swap the last two Schuko plugs on the plant-room for blue commando and stop pretending I enjoy wobbly earths.
- Bring the dishwasher onto the “daytime only” relay like the washing machine and hob. One more small peak pushed into sunshine.
- Put a paper tag on the pressure tank with today’s numbers. Digital notes are nice until your phone dies.
That is it. One quiet box, one ugly current spike tamed, and a house that feels less fragile when the tap runs.