question

justmurph avatar image
justmurph asked

Multiplus 5k ignores input current limit with power assist

I have a Multiplus 24/5000 that I am using to run the dive compressor on my boat (2.7kw) and a Honda 2kw generator connected to the AC1 input to provide the power when solar doesn’t keep up.

The Multiplus runs the dive compressor (3 phase motor driven by a single to three phase VFD) easily on its own.

When I’m not running the dive compressor, the multiplus honors the 8.5A input current limit I’ve set, however when I fire up the dive compressor it seems to ignore that limit and will try and pull all of the power from the generator, with little to no power assist.

Running the compressor at full speed the generator will overload and trip. If I lower the compressor speed with the VFD I can get it to a point where the genny can just keep up at around 10A, with the multiplus saying it’s assisting by maybe 200W on top of the 2000W from the gen. It's not ideal running the compressor this slow.

So it appears that power assist is doing a little bit, but no where near enough and the input current limit is being ignored. I can turn the input current limit right down to 4 and the behaviour doesn't change at all.


The minimum input current limit for my 5k is supposed to be 3.9A and I’m on firmware 430. Any ideas?

MultiPlus Quattro Inverter ChargerGeneratorpowerassist
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4 Answers
Matthias Lange - DE avatar image
Matthias Lange - DE answered ·

I don't think you can load you 2kW Honda with a 2kw inductive load.
Take a look in the manual/datasheet of the generator.

I guess you have to lower the input current more to 6-7A.

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I've updated the original question to be a bit more clear.

That is exactly the point. The multiplus ignores the input current limit. I can lower the input current limit to 4A and it doesn't make any difference to the behaviour, the Multiplus just ignores it and puts almost all of the load to the generator. It does say it is assisting, but not by much (maybe 200W).

When just charging, will limit the load on the generator to whatever I set. It's only when you have a larger AC load that it ignores the input current limit.

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First you should try to update the firmware to the latest version. 430 is about 3 years old. The latest version is 481.

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psp-maxi avatar image
psp-maxi answered ·

Hi justmurph,

may i ask you what type of dive compressor do you have? and what type of VFD?

I have 2x Multiplus 3000 in parallel and the dive compressor companie BAUER say, it is not possible to start the BAUER compressor with 2KW in this setup.

Thank you for your help

Thomas

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It's a Bauer Junior II, which I've swapped to a 3 phase motor and a single to three phase 3kW Siemens V20 VFD. Before this I tried to make it work with the single phase motor using an adaptive soft starter, but the startup current was still too high.

The setup with the VFD works brilliantly, but there is a bit of a loss of efficiency.

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justmurph avatar image
justmurph answered ·

I've updated to firmware 481, no change.

What I've worked out is that it seems to be an issue with how the multi measures the input current when the load is highly reactive, as it is here with the VFD. When I put a 2kW resistive load on, the multi will limit the input current to whatever I set.

Interestingly, with the input current set all the way down to 4A, and the compressor turned down the point where the generator juuuuust keeps up (about 11A input current), if I plug that 2k resistive load in as well as the compressor (now 4kW......but less reactive) the multi now pulls less current from the generator (8A).

So....where to from here?

- I'm guessing that there is no settings change that will make it work.

- Is this something that could be fixed in a future firmware, or is this a hard limitation of the hardware in the multi?

- Maybe with a front end harmonic filter on the VFD I can get the power factor up to point where the multi can control the input current. That's an expensive test.

Any help is appreciated. :)


A few figures:

AC IN

V = 232V

I = 9A

Real Power = 1550W

Apparent Power = 2088W

Power factor = 0.74


AC OUT (including ~200W of power assist)

V = 232V

I = 12A

Real Power = 1750W

Apparent Power = 2784W

Power factor = 0.62

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Hm... I think the bad power factor could cause that problem but I never done something like that.

I don't think this can be solved with a software update, I guess it is a hardware limitation.

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justmurph avatar image justmurph Matthias Lange - DE ♦ ·

In this case what I think the issue is the harmonics (because the VFD will only draw power from the inverter when the voltage of the AC waveform is above 200V), so I am going to try the line filter that Siemens recommends for this drive, as it isn't too expensive.

I suspect that this could be resolved in software though, as the multi does report the correct input current (via VE Config), it's just it seems that it doesn't control directly from that variable. It seem like maybe it's back calculating the input current from the real power (rather than directly controlling from the actual input current or apparent power).

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@JustMurph Reading true rms power is a complicated thing and requires special hardware. Just look up true rms meters and see the cost. Any one worth buying and with a good sample rate is expensive. Definitely the Victron does not have the built in hardware functions for that.

In any case power factor is a variable thing, so it is also not a great thing to base calculations on. Watts is volt amps but apparent power also has trigonometry involved. So much more complex to work into an inverter system.

With wave distortion from generator plus wave distortions from a VSD drive it is a tall order. The Victron would be working hard to synchronise and run the load.

Do you have the wide frequency accept also checked that makes power factor worse, also if battery charging is also on it adds an extra degree of difficulty and extra load.

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Thank you for the input Alexandra. Understood that measuring RMS power is difficult and what I'm asking of the inverter is also difficult.

Do we need to measure RMS power to make this work? The Multi does report what appears to be the actual input current, so shouldn't it be possible for it to control for that input current? Or is it hardware limitation that the multi simply can’t control the input current when the power factor is so poor?

I'll give it a go with wide frequency accept turned off later today and see how it goes.

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Unfortunately turning wide frequency accept off made no change.

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@JustMurph

So with bad power factor the amperage curve is not following the voltage curve, so it could be sampling at the peak of the curve of voltage but the amp draw is out of sync so maybe correct at that point, but not a true representation of the amp draw.

Just a theory at this point. Have you tried running it just off the multi then charging up after? Or is battery cycling an issue?

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You might be onto something there. The input current the multi reports is a bit high. The max for the generator should be 8.5A but the multi reading will bounce between 8 and 12.

We can run the compressor and generator at different times. The multi does run the compressor easily. It's just not idea from the point of view of additional battery cycles and also if we're doing a big day of diving, then you really need to be charging and filling cylinders at the same time.

I'll have the line filter some time this week and will see how that goes.

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I've now tested it with the newer 2.2kW Honda generator. With the VFD at 34Hz the multiplus can keep the generator at the input current limit I've set of 6.5A (~1500W). With the 2kW generator it was running flat out at 10A. However, if I run it faster than 34Hz the input current starts to come up. At 40Hz we're now at 1850W.

The line filter just arrived today, so I'm hoping that will make a further improvement.

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Righto, I have some more data.

So, did some testing this morning with the EU22i and the EPCOS line filter installed. The short of it is the new generator definitely runs it a bit better and the line filter makes no difference. With the new gen I can go from running the VFD at 34Hz to 38Hz for the same load on the generator.

That said, even at 34Hz we're over the continuous rated output current rating of the gen (7.5A or 1800W with a power factor of 1). The power factor gets better as I increase the VFD speed, but once we get to 45Hz the generator voltage starts to sag and the inverter isn't propping it up. I tried to ramp it to 50Hz but the VFD cut out on under voltage.

Next step I will get a scope on it to see what the wave form looks like. I don't think a line reactor will help as I don't think this is a lead/lag issue, but more likely harmonics from the VFD front end.

Is there any input from the Victron technical team?

One solution would be to use a DC generator rather than an AC one, however I can't find a small and quiet DC generator. The only ones available seem to be the very loud construction site types and that's not an acceptable solution for me.

screen-shot-2021-05-09-at-121410-pm.png

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@JustMurph

The voltage sagging from the generator really is an issue then. As the Victron will synchronize both its htz, wave form (to a certain degree) and voltage to the generator. It is in an AVR.

Do you have weak ac enabled? Possibly part of the issue.

3.1 Derating of the power factor

https://www.victronenergy.com/live/multiplus_faq#in_the_charger_tab_of_veconfigure

The VFD will definitely be unhappy under 220V incoming voltage.

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Weak AC is not enabled, but I have tried with it turned on.

The voltage from the generator is sagging because it's being overloaded, which is because the Multi is not assisting as much as it should. In the above tests the input current limit was set at 7A for all tests. The only time the multi actually limited the input current was when it was only charging.

I will try and get a scope on it soon to see what the wave form looks like and see how much lead/lag we have. My thinking at this stage is that the multi isn't able to effectively synchronise to the poor wave form, which is why it ends up overloading the generator.

Has this been escalated to the Victron engineers?

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cjhwhite7 avatar image
cjhwhite7 answered ·

I have had this same issue with Air Con, also a compressor basicly.

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