question

freesurfer avatar image
freesurfer asked

What exactly does the "Weak AC" option do, from a technical viewpoint?

I understand that the option can be used if there is any trouble with using a generator or the multiplus often disconnects from its AC input.

But from a more technical standpoint, what exactly does it do?

The manual just states that it uses an "alternative charging algorithm" (or some wording along those lines) and that the efficiency and power factor will drop. But with no explanation of how the 2 different implementations work

Multiplus-II
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3 Answers
Guy Stewart (Victron Community Manager) avatar image
Guy Stewart (Victron Community Manager) answered ·

Hi @Freesurfer,

I've asked a Victron engineer for a bit more information and have done my best to understand.

In weak AC mode, we use a different way of driving the bridge (the circuit that converts AC <-> DC).

Normally the PWM has to be nicely in sync with the grid otherwise you get very weird currents flowing.


In Weak AC mode the phase of the mains voltage with respect to the generated PWM is no longer important. The resulting current will always be automatically in sync with the voltage.


However there will be more losses and the maximum power which can be reached this way is less so the charge current will be less.

The problem is that generators that require this setting sometimes produce really deformed voltages and as a result the current is also deformed.

The mention of the Power Factor not being 1 indicates that there is quite some variation in the current and it may be far from sinusoidal. It might be relatively ok when using a sinusoidal voltage.

It does not mean that there is a phase shift.

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Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

As far as I know with "weak AC" active the charger don't start with the full charging current immediately, it slowly increases the current from 0 to the maximum.

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JohnC avatar image
JohnC answered ·

Hi @Freesurfer

There's some mention of it in this article: https://www.victronenergy.com/live/multiplus_faq

"3.1 Derating of the power factor

Some generators have a waveform which makes it difficult to charge with the normal charge mechanism. Through activating the “Weak AC input” function, another mechanism is used which will enable the MultiPlus to charge under practically any condition.

Disadvantage of using this option is that the input power factor, normally equal or close to 1, will be degraded. This function should be used only when the preceding ones are ineffective.

This leading or lagging power factor will have the effect of demanding more kVa from the generator for the same or less DC Watts delivered to the batteries. You may need to further reduce the DC Current Limit in the Charger tab of VEConfigure if the generator continues to overload after changing this setting.

Effectively, when Weak AC is enabled, the total charge current possible will be less that the rated maximum charge current output of the charger (under normal conditions)."

There's certainly VAR's involved, as you can see a significant difference between a Multi's ac input Watts and dc charge Watts. And whatever my Multi is doing to the ac on it's input, it's enough 'backfeed' to override the 'economy' setting of my (avr) generator, and it goes from idle to full revs as soon as it's plugged in, some ~30 sec before switching through. And it switches through fast, no 'soft start' happening there.

Interestingly, that functionality is available with the Dynamic Current Limiter, which won't allow genny acceptance for me, so I have to leave it off. Presumably Power Assist can't deal with it??

I reckon too I can actually hear that power factor when the ac is passed through to a heater fan - it gets noisier..

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