Isolation Transformator loud humming with 230V powerstation input

Had something similar crop up at my cabin setup last year — worth sharing in case others have run into this.

Was running a small isolation transformer off a Bluetti AC200MAX and got a horrible low drone from it, almost like a 100Hz buzz rather than the usual 50Hz hum you'd expect. Thought the transformer was knackered at first.

Turns out the modified sine wave output from a lot of these portable powerstations is the culprit. Isolation transformers are pretty sensitive to waveform quality — a pure sine inverter barely causes a peep, but chuck a modified or "quasi-sine" signal at one and the core can vibrate like mad.

A few things I've found that help:

• Check the THD rating of your powerstation's output — higher distortion = louder hum
• Some units (EcoFlow, Bluetti etc.) have an AC output mode setting buried in the app — switching to "generator mode" or adjusting frequency tolerance sometimes helps
• The VA rating mismatch matters too — if the transformer is lightly loaded it can resonate more

Anyone else noticed this changes depending on load? Mine was noticeably worse when the transformer was running at under 20% capacity. Fully loaded it settled right down.

Curious whether anyone's tried running an isolation transformer off a Victron MultiPlus compared to one of these all-in-one units — I'd imagine the Victron's cleaner sine makes a difference but haven't done a proper back-to-back test myself.

Would be good to hear from anyone using a transformer in a static caravan or van setup where a powerstation is the only source. Is it just cosmetically annoying or are people seeing actual damage downstream?

@SaltyMaker did you ever figure out what was causing it? I've been half-tempted to add an isolation transformer to my boat setup for safety reasons but stories like this give me pause.

Was the humming constant or did it vary with load? I wonder if modified sine wave output from the Bluetti was the culprit — a lot of those powerstations produce something that's technically 230V but isn't quite the clean sine that transformers prefer.

Did you try running it off a proper pure sine inverter instead, like a Victron Phoenix, to see if that made a difference? That'd be the obvious comparison test wouldn't it?

Also curious whether the transformer itself was rated for the wattage you were pulling — undersized iron cores can resonate quite badly even with clean power.

@SaltyMaker classic "angry beehive in a metal box" noise — I've been there.

The culprit is almost certainly THD (total harmonic distortion) from the Bluetti's modified or quasi-sine output. Isolation transformers absolutely hate anything that isn't a clean sine wave — they'll drone like a pub regular who's

@SaltyMaker modified sine wave inverters and isolation transformers go together like cats and bath time — the transformer core magnetises unevenly on that stepped waveform and basically turns into a 50Hz tuneable buzzer.

Pure sine output sorted it for me on my shepherd's hut build; the Bluetti AC200MAX does output pure sine, so I'd actually be checking whether your transformer is undersized for the VA draw rather than blaming waveform quality straight off.

Also worth checking — is the transformer sitting on a hard surface? Mine was resonating against a wooden shelf and sounded twice as bad as it actually was. A bit of anti-vibration mat (the cheap foam stuff from Screwfix does the job) knocked it down to a barely noticeable hum.

@CamperCarl pure sine or go home — my Victron Multiplus and a decent Fogstar battery sorted this exact nightmare in the van, transformer's been whisper-quiet ever since.