question

pmikep avatar image

Why does my Multiplus hum (& "strain") with AC Power and Float?

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.

MultiPlus Quattro Inverter Charger
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2 Answers
JohnC avatar image
JohnC answered ·

There was a firmware issue detected and repaired more recently. Depending on your firmware it may/not be relevant to you.. https://community.victronenergy.com/questions/104858/excessive-noise-while-charging-via-new-multiplus-i.html

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I have the latest firmware, v 487. So that didn't make a difference for me.


But you are correct that it is the Charger part of the circuitry making the hum.

After reading your message last night, and while I was drifting off to sleep, it occurred to me that there was a simple test.

This moring I disabled the Charger function using Victron Connect and the MK-3. After I did that, the hum stopped.

So, Victron, if you're reading this, a Request for Enhancement: When the Charger is in Float, and when there is AC 1 IN, a user setting for the charger circuit to disable itself. (It would be nice for quiet sleeping in a camper van when on Shore Power.)

I have the latest firmware, v 487. So that didn't make a difference for me.

The latest (with the potential fix) is 489.

The latest version of VictronConnect doesn't offer v. 489 for my Multiplus. (Not a Multiplus II.) It might be that, because the original Multiplus uses two toroids, and the new Multiplus II uses one, the resonant frequency of the two is different than the one, and the fix for the M II doesn't apply to the original M.


There was something about downloading 489 from Dropbox. But I didn't see the link. And I'm not a member of Victron Professional.

For now I'll wait for the next version of Victron Connect to be released. The hum isn't very loud on my units. My question was out of curiosity - how things worked.

And I'm not a member of Victron Professional.

You can make an account. It's free (no charge).

nigelfxs avatar image
nigelfxs answered ·

Hi @pmikep

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.

The Multiplus continues to operate in "inverting mode", even though it is supplying almost no power in this situation. It does this so that it can instantly take over supplying power to your loads if the incoming AC supply fails.

The Multiplus therefore has to synchronise its own AC output to the incoming AC - so it can take over in an instant. The audible hum is caused by the Multiplus trying to match the the incoming AC waveform, which may be slightly distorted (i.e. not a perfect sine-wave). Connecting a hair-dryer on its "half power" setting will illustrate this rather well ;-)

The main source of audible hum is usually from the windings of the large toroidal transformer in the Multiplus.

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I agree with you that when either the Power Assist or UPS function is enabled, then the Mulitplus has to sync with the AC sine wave. (In fact it says this for the UPS function in the Manual.)


However, when neither Power Assist nor UPS function is enabled (and I disabled them for my testing), then I do not believe that the Multiplus is in sync. In this case, when the Mulitplus detects a loss of AC IN, a relay snaps and the Mulitplus begins inverting. (The Manual discusses a delay here, which is consistent with a mechanical relay switching.)

BTW, my testing is with our local power company, which is hooked up to the nation's grid. That is, I'm fairly sure that the sine wave is not distorted here.

Whatever the case, I found that switching the Charger off stopped the hum.