3-phase MultiPlus-II 48/6k5 – loud growling and overload when starting a DOL compressor motor (inte

Been wrestling with a similar headache on my cabin setup — though mine's single-phase, the underlying physics is the same and it might help narrow down your culprit.

DOL (direct-on-line) motor starts are brutal. A compressor motor can pull 6–10× its rated FLA in the first few cycles, and the MultiPlus-II's overload headroom, whilst genuinely impressive on paper, is time-limited. Victron's own documentation specifies the 6kVA unit can sustain roughly 2× rated current for a short window before the protection trips. If your starting transient pushes beyond that envelope — even briefly — you'll get exactly the growling and shutdown behaviour you're describing.

A few things worth checking:

• Battery internal resistance — a weak or undersized bank collapses under the inrush spike before the inverter even has a chance to respond. What's your DC bus voltage doing at the moment of failure? Log it via VRM or a Cerbo if you have one.
• AC coupling configuration — are all three units properly phase-synchronised? A misconfigured assistant can cause one phase to bear disproportionate load during motor magnetisation.
• Star/delta or soft-starter — honestly, DOL on a Victron stack of this size is always going to be marginal. A basic electronic soft-starter (even a cheap Invertek or similar) transforms the inrush profile dramatically.

On my own 48V Fogstar/Victron setup I added a soft-starter to a 2.2kW pump and the difference in battery sag was immediately visible on the Cerbo graphs — went from alarming dips to barely noticeable.

What's the compressor's rated kW and pole count? That'll help

@Paddy cheers for chiming in — yeah the physics absolutely translates across.

One thing worth flagging specifically for the 3-phase MultiPlus-II setup though: with DOL starts you're not just seeing that brutal inrush current, you're potentially seeing it hit all three phases simultaneously and unevenly depending on where in the AC cycle each phase sits when the contactor closes. That can really confuse the inverters' overload detection and trigger a fault even when the total load is theoretically within spec.

Have you looked at soft-starter options? Even a fairly modest electronic soft-starter dropped in ahead of the compressor motor makes an enormous difference — smooths that inrush right down. Alternatively, if the compressor has a star-delta starter option that's worth exploring too.

What's the kW rating on the motor you're running?

@Paddy @EdGrant91 — DOL motors and inverters are basically the electrical equivalent of asking someone to sprint before they've even stood up 😄

Jokes aside, the inrush on a DOL compressor can be 6-8x running current — your MultiPlus-II is essentially having a minor existential crisis every time that thing

@MoorLee that sprint analogy is spot on 😄

Worth mentioning — on my shepherd's hut setup I had similar grief with a small compressor. Switched to a soft starter (grabbed a cheap Shihlin one off eBay) and the Victron stopped complaining almost immediately. Inrush dropped massively.

Also check your DC side before assuming it's purely the inverter — weak battery connections or undersized cable will make the overload situation way worse when that inrush hits. Fogstar cells are solid but even they can't save you from a dodgy busbar connection.

If soft starting isn't an option, a VFD is the proper fix for compressors — controls ramp-up speed entirely. Bit more outlay but worth it.

@Paddy @EdGrant91 — one thing nobody's touched on yet: check your DC bus voltage sag during that startup spike. Even with a healthy battery, high internal resistance (especially if your cells are cold) will cause the 48V rail to dip sharply the moment the inverter tries to deliver that inrush current. The MultiPlus-II reads that as an overload condition and faults out before the motor even gets close to running speed.

Measured mine on a cold morning last winter with a Victron SmartShunt — rail dropped from 52V to 44V in under a second. New Fogstar Drift cells sorted it, but interim fix was pre-warming the battery bank and reducing the load before starting the compressor.

Also worth checking your DC cable sizing — undersized cables add resistance and make the sag worse.

@RetiredPlumber raises the point I was going to make about DC sag — but I'd go further and say log it properly rather than just checking. A Victron BMV-712 or Cerbo GX will show you exactly how far the bus is drooping and for how long.

On my tiny house setup I found the inverter was crying wolf on overload because the DC sag was triggering the protection before the AC side even got close to its limits. Turned out to be a marginal connection on one of the battery terminal lugs — resistance was only slightly elevated but under inrush current it was catastrophic.

Worth torquing every DC connection and checking terminal resistance with a decent clamp meter before assuming the MultiPlus itself is the problem.