I have a Multiplus powered by AC, acting as a UPS for a refrigerator and a freezer.
(Power Assist is off.)
This Multiplus also charges a battery bank, in Float mode. (The batteries are for when the grid goes down.)
If I understand it correctly, when AC is present at the input to the Mulitplus, that AC passes through the Multiplus (by way of a relay) directly to AC Out.
And when the Multiplus is in Float, it does not send any current to the batteries. That is, there shouldn't be any hum from the charger side of the Multiplus.
So then why do I hear the Multiplus hum noticeably, even when the refrigerator and freezer are idling? (25 Watts. (I do not have AES enabled.))
And more to the point, why do I hear the hum change (it becomes louder for a few seconds) when the 'fridge or freezer start up? It's as if the Multiplus is Inverting, even though AC In has power.
I expect this behavior when a Multiplus is acting as an Inverter. For example, I have another Multiplus, powering a small refrigerator/freezer, which is always inverting because it is not connected to AC. (It is powered by batteries + solar.) Naturally, when the refrigerator turns on, I hear the hum in that Multiplus grow louder for a few seconds as the in-rush current to the refrigerator increases during start. I expect this behavior for a Multiplus that is Inverting. But I do not expect it for a Multiplus that is simply passing-through AC from Input to Output.
I hypothesized that perhaps the first Multiplus hums because I had "UPS Function" enabled, and so the Multiplus would have to sync with 60 Hz AC.
But disabling UPS Function makes no difference in the quiescent hum, nor the "under load" hum.
This isn't a Safety Issue, nor a malfunction report. I'm simply curious to know why it sounds like it's Inverting when it's not.
P.S. I found someone asking the same question about a Quattro (from 2019). But no answer.