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hummingbear asked

Multiplus makes loud noise when switching from generator input back to inverter

I have a 24V Multiplus 5000VA connected to my battery system. I also have a 10KW Generator connected to AC Input.


If I'm using the generator to charge my system and afterwards turn it off, the Multiplus makes a very loud hum/vibration that slowly diffuses after 5 or so seconds. This happens almost every time I turn off the generator. I even programmed the multiplus to stop taking AC Input from the generator before powering it down.


Is this sound part of normal operation when switching off AC Input to the Multiplus?


Edit: Video of what I'm describing

MultiPlus Quattro Inverter ChargerGenerator
pxl-20210130-0143117852.mp4 (77.8 MiB)
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klim8skeptic avatar image klim8skeptic ♦ commented ·

I would hope that you have flipped the generator isolator, before shutting down the generator.

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hummingbear avatar image hummingbear klim8skeptic ♦ commented ·

@klim8skeptic "flipped the generator isolator"

Where would I find this and how would I flip it? Is this something that would be on the generator or in my multiplus? My generator only has on/off controlled by a relay or switch.

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alan avatar image alan hummingbear commented ·

Turn of the breaker on the genset or on the input cable before you turn off the genset.


What is probably happening is when you turn the genset off the voltage tapers down to 180v before the multi disconnects, this is the stock AC input voltage settings, then the multi has to work hard to get back to 230/40v.

You could also raise the disconnect voltage in the multi.

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hummingbear avatar image hummingbear alan commented ·

Thanks for the response @Alan - I have the Multiplus programmed to stop taking AC Input, wait 1 minute and then turn off the generator. The Multiplus has already switched over to inverting for 1min, it's only when the generator shuts off that this noise happens

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2 Answers
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hummingbear answered ·

@Paul B @Alan

Monitoring the Voltage and Frequency of both AC In and AC Out when the generator shuts down (not using a breaker) it brings the frequency of AC In to 45 and AC Out matches that. That's essentially the cause of the issue/noise I'm hearing, and explains why some of the LED lights in my house would flicker as it brings the frequency back up to 60Hz (North America).

Unchecking the setting Accept a wide input frequency range on the Multiplus fixed the issue.

It's interesting that the Multi is still synchronizing the frequency even though it's not accepting AC Input. Does anyone know if that's expected behavior?

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alan avatar image alan commented ·

It makes sense that it would, as it has to sync to the incoming AC before it connects the AC input relay.

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hummingbear avatar image hummingbear alan commented ·

@Alan I agree that it does have to sync before accepting power from AC Input, but wouldn't it make more sense for the multiplus to start syncing when the Multi is put into charger mode rather than when it's in Inverter Only mode?


Even when it's in Charger mode and you give it AC Input it takes about 30s before it actually switches over to AC Input. It seems like a flaw that the Multi is still syncing to AC Input when in Inverter Only mode

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klim8skeptic avatar image klim8skeptic ♦ commented ·

Google generator motoring.

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Paul B avatar image Paul B commented ·

the multi will always try to sync and thus its first course is to get the frequency's to match, so yes it will always try to follow the ac in power.

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Paul B avatar image
Paul B answered ·

As far as I know there is no feature that turns the multi to invert 1 min before you turn the generator off. so please explain how you are doing that

Alans comment is more than likely still correct.


also if you have any motors running then as the gen goes off these will likley restart so there will be a high ac current as the start current for these kicks in.

try increasing the AC low disconnect voltage to 205 volts this may help


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hummingbear avatar image hummingbear commented ·

@Paul B Correct, there is no built in feature that supports this out of the box. I'm using NodeRed on my VenusOS device and configured it through there. With MQTT you can tell the inverter to go into inverter only mode which disconnects AC Input. After 1min then I turn off the relay for the generator which shuts it off.

I'm not sure Alan's comment would be correct if I'm not accepting input from AC1 at the time the generator shuts off?

I've uploaded a video to my original Question so you can hear exactly what happens.

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hummingbear avatar image hummingbear commented ·

@Paul B so I tested turning the breaker off from Generator to Multiplus before shutting down the generator. You are both correct that it did not make the noise I described.


Though this leaves me with even more questions:
1) If the Multiplus is no-longer accepting AC Input from the generator and there are no other loads on the generator, why would it cause issues (this noise) when shutting the generator down?

2) What actually is this noise, what's going on inside the inverter here?

3) If I have to manually flip the generator breaker on/off, this defeats the purpose of having auto start/stop functionality? What am I missing here, there are for sure others who have a similar setup. My previous inverter charger (and old Trace) did not have any issues like this.

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alan avatar image alan hummingbear commented ·

Maybe, while AC1 input is being ignored the multi is still dropping/syncing the output voltage to match? Even though it not connected it would still see the input.

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hummingbear avatar image hummingbear alan commented ·

In that case, what is the difference between cutting the breaker from Gen to Multi vs. shutting down the generator?

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alan avatar image alan hummingbear commented ·

Turning off the breaker, drops the input while the genset is at its rated output. (230/50?)

Switching off the genset kills the engine which continues to turn the alternator at a lower rev producing a lower than rated output for a very short time before it drops to below the multi disconnect setting/shuts off.

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