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Kenneth

Axpert switching back to grid when starting electric motors.

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Good day

I have an issue that I hope someone can help me with. My system (Axpert) works fine until I switch on an electric motor. Anything above 1200w will make my system switch back to the grid which is very annoying. This happens all the time doesn't matter if the batteries are full or not. I know an electric motor has a surge when starting up but these inverters are supposed to handle a 10kw surge. I can switch on my geyser as well as my dishwasher which uses 4500w together without any issues. I can have no load on the inverter but if I start a motor bigger that 1200w it switches back to grid power and then later back to solar/battery power. Under normal conditions it is not a problem but when using my milling machine I have broken a couple of milling bits because of this issue. The interruption the switch back (from grid back to solar/battery) causes the drive motor on the mill to stop but the auto feeder keeps on feeding the work piece into the cutter causing broken cutters. 

I also have a second question. During the winter months my system is struggling to supply my load as well as charging the batteries fully come sundown. According to the specs of the MK2 it can handle 4500w input from the panels. I am already using 4900w of total panels without any issues. I want to add another 2X350w panels. Will this inverter handle that or not. If not is there a way of using more panels during low/short sun periods without damaging the inverter. 

Any comments will be appreciated.

Kenneth

My system:

Inverter         Axpert 5kva MK2 running with ICC to protech the pylontech's

Batteries       : 4 x Pylontect 3.5 KW

Solar panels : 14 x 350W (Total = 4900W)

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Posted (edited)

Hi Kenneth

My guess on your first question (let me venture far out of my comfort zone): The motor is an inductive load, so let's assume it has a power factor of 0.8. That means, to get "real power" of 1200W, it is actually pulling 1500kVA from the inverter. The surge of starting such a motor could be 6-7X its normal power, which puts you on an instantaneous load of 9000kVA to 10500kVA. Now on 9000kVA you should be fine (if absolutely no load is on the inverter), but the power factor could be lower and you could be sitting closer to 7X.

A Victron hybrid inverter would be an option, because it can blend grid, solar and battery (as I understand, I don't have solar on my hybrid yet), and can transfer 32-50Amps (depending on the Multiplus model, not considering the Quattros) from the grid, so you should not have such interruptions. However, don't take my word for it, I haven't tested such a system myself. It is a nice "trick" to start heavy loads from the grid (essentially) for that initial surge and the happily churn away on solar after that... 

Another possibility to pursue on your second question: If it turns out your inverter can't handle the extra panels, but you really just want to charge your batteries, you can look into a separate MPPT (like those Victron ones) which will come in on your DC side to charge the batteries. I am pretty sure it will work. Haven't tried it myself, but have seen others discussing it.

Edited by jykenmynie
Added a sentence.

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9 hours ago, Kenneth said:

doesn't matter if the batteries are full or not. I know an electric motor has a surge when starting up but these inverters are supposed to handle a 10kw surge. I can switch on my geyser as well as my dishwasher which uses 4500w together without any issues. I can have no load on the inverter but if I start a motor bigger that 1200w it switches back to grid power and then later back to solar/battery power. 

I suspect the battery is potentially where the trip up happens. The pylontechs have a 100A (100Ax48V = 4800W) peak limit and the surge of the motor starting (especially if there are already other loads connected or very little PV to help out) probably has the BMS disconnect the battery for protection. The geyser and dishwasher at 4500W are fairly resistive loads and do not have the same inrush current as a motor starting.

Do not know if there is an option to have the switch over to grid happen quicker. Hopefully members better versed in axpert will chime in....

I shouldn't play in the PV pool but will point out you need to look at more than the total watts of the panels. You need to check the MPPT max PV volts etc. and then also factor in temperature coefficient for cold mornings. I suggest you post the MPPT spec for the inverter (think there is a sticker/lable on the side of the inverter that gives the info) and include the specs for the solar panels so if better qualified posters come along they can advise you quicker.

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41 minutes ago, introverter said:

I suspect the battery is potentially where the trip up happens. The pylontechs have a 100A (100Ax48V = 4800W) peak limit and the surge of the motor starting (especially if there are already other loads connected or very little PV to help out) probably has the BMS disconnect the battery for protection.

The Pylontechs can handle 100A for 10 seconds, but he has four of them, so has more than enough peak current available to start the motor.

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Just now, jykenmynie said:

The Pylontechs can handle 100A for 10 seconds, but he has four of them, so has more than enough peak current available to start the motor.

yup...realised that...and was on my way back here to point out that kenneth should happily ignore my post 😳

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Hi Introvert

Thanks for the reply. What you are saying makes sense because when I installed the system I ran it without batteries for a while and with good sun I could start my compressor (2200w) and my beltsander (2200W) one after the other without switching back to grid. The only thing that bothers me is I also had a look at the Pylontec U3000 Plus specs and like you said it states that the batteries are supposed to take a discharge current of 100A for 15 seconds. The batteries are connected in parallel so doesn't that mean a total discharge current of 400A giving you 19200w ?

On my second question I just want to give more info. My inverter can handle a open circuit voltage from the the PV panels of 450 volts. I could not connect all of the panels in series because that would have given me 644 V. I have 14 x 350W panels connected to it. I have two banks of 7 panels each in series (326 V and 9A open circuit) and then the two banks gets connected in parallel to the inverter. That means 326 V at around 18 A. That gives me a max of 4900 W. That is already 400 W over the spec of the inverter.  On my ICC software I have seen 4948 W already on a good day but that rarely happens. In winter I am running max 3000 to 4000W from the panels and that is why I am looking at something extra during these low sun periods to supplement the wattage. I can add 1 or 2 extra panels to each bank but you are not always there to monitor the system so I am afraid that I might damage my inverter. I am looking at adding another bank of 7 panels that I can connect via a relay controlled by some software looking at the total ampage or wattage. If the amps/wattage are below a certain point it must connect the extra bank and if it goes above a certain point it must disconnect the extra bank. With this setup you will have ample power early and late into the day and during low sun periods like winter and cloudy days. Another thing I was wondering about is if there is not some product on the market that can limit the current or voltage from the panels to keep it in spec for the inverter at all times.Then you can add a couple more panels and you don't have to worry about damaging your inverter.

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1 hour ago, Kenneth said:

Thanks for the reply. What you are saying makes sense because when I installed the system I ran it without batteries for a while and with good sun I could start my compressor (2200w) and my beltsander (2200W) one after the other without switching back to grid. The only thing that bothers me is I also had a look at the Pylontec U3000 Plus specs and like you said it states that the batteries are supposed to take a discharge current of 100A for 15 seconds. The batteries are connected in parallel so doesn't that mean a total discharge current of 400A giving you 19200w ?

I highly doubt the batteries are the issue here. The inverter wouldn't care about the batteries if it is being run in SBU mode, and there is enough solar available.

The compressor and beltsander are high wattage induction loads, but don't they have speed control? In which case, I would assume that they wouldn't hit the inverter with such a high demand on start up as something like a cheaper pool pump? Though, I would assume that the milling machine also has the ability to start slowly?

Maybe someone line @Coulombwho understand these inverters better might have an idea?

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Posted (edited)
14 hours ago, Kenneth said:

I can have no load on the inverter but if I start a motor bigger than 1200w it switches back to grid power and then later back to solar/battery power.

Yes, starting large motors is a pain. It's possibly not helped by the fact that the inverter's logic that switches to grid mode doesn't seem to take into account whether the surge is withing the capability of the inverter or not; it seems to just work on instantaneous battery voltage. However, as I review the firmware, I see that it should not declare the battery weak (and thus trigger a switch to line mode) unless it sees three seconds worth of battery voltage measurements below a threshold. But my experience, and apparently yours too, is that it switches in a small fraction of a second. If I get time, I may study this more.

One possible solution is fully patched firmware with KettleKomp™; this uses battery voltage measurements (for the switch to/from line mode) that are compensated (to a degree) by the present battery load (or lift from charging). But there is no patched firmware for your model.

As already discussed, starting a 1200 W motor will be close to the surge rating of the inverter. I believe that the startup current is of the order of 7 times the running current, but it will be easier to work in apparent power. As @jykenmynie mentioned, a 1200 W motor will typically draw 1500 VA; 1500 x 7 = 10 500 VA, which is 5% above the surge rating. 

A workaround may be when using the milling machine, turn off the AC-in breaker to the Axpert. Then it can't switch to line mode. But being so close to the surge rating, this would seem to be tempting fate a little too much. [ Edit: plus, it's a pain having to do this, and to remember to switch it back on afterwards. ]

A second inverter in parallel might be a solution for both your problems. It will halve the surge current to each inverter, solving that issue IF it's just the peak load, not the battery sag. You would have a second solar charge controller, so you could add a third string of panels, or just add one or two panels to each string, and wire each string to its own inverter-charger.

Edited by Coulomb

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Posted (edited)
2 hours ago, Kenneth said:

when I installed the system I ran it without batteries for a while and with good sun I could start my compressor (2200w) and my beltsander (2200W) one after the other without switching back to grid. The only thing that bothers me is I also had a look at the Pylontec U3000 Plus specs and like you said it states that the batteries are supposed to take a discharge current of 100A for 15 seconds. The batteries are connected in parallel so doesn't that mean a total discharge current of 400A giving you 19200w ?

Correct, with the batteries in parallel the 100A should not be a problem (as pointed out by @jykenmynie ) so the battery peak should actually not be an issue (but this also assumes the installation is 100% with all the various bits knowing who else is present and who is the boss). If the battery voltage drops under load (which with only the 1 motor starting is maybe also clutching at straws) I suspect the inverter might still go back to grid.. (?)

@Kenneth go with what @Coulomb said..

Edited by introverter
back to school

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Thanks guys for all your input. I am still convinced that it has something to do with the batteries. I played around with it this morning by disconnecting the batteries and running my compressor (2200w) and then starting my belt sander (2200w). I mean that is a huge load and it does not switch the inverter back to grid power. As soon as I reconnect my batteries I have the same issue. It is a big hassle (disconnecting the batteries) but maybe that is what I should do to resolve the problem without dishing out more money. Maybe I can connect a big capacitor on these machines that can help with the surge. I don't know if that would be possible. Any ideas ?

Thanks again for all your input.

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to check if the battery voltage is dropping you could stick a multi meter on the batteries and see what happens to the voltage when you start one of those big/problematic loads or..

I have no idea between ICC and the inverter who makes the actual decisions so something that could be checked is what happens if you temporarily change the "back to utility" voltage (program 12?) and low battery cut-off voltage (program 29?) to something lower and then see what happens when trying to start the problematic load from battery BUT first check with the more axpert knowledgeable crowd and don't leave that setting low or you will end up with a low battery somewhere when you don't want it.

I have seen mention of motor soft starters but have no idea how effective they are and rather spending money towards more inverter/PV is maybe a more worthwhile spend?

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Thanks Introverter

I checked with a Fluke but the changeover occurs in like 10-20 milliseconds so the Fluke does not even pick up the voltage drop before it changes over to grid power. You did give me an idea with the ICC and inverter software and which one is making the decision to change back to grid. I am going to switch off my ICC software and check again. What I have also seen is that only motors with a fairly heavy starting load like the compressor is doing it. My pool pump (1500w) and even my milling machine (1500w) doesn't switch the inverter over to grid power. 

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55 minutes ago, Kenneth said:

I checked with a Fluke but the changeover occurs in like 10-20 milliseconds so the Fluke does not even pick up the voltage drop before it changes over to grid power. 

Don't know about ICC and the likes (watchpower?) but with the victron VRM a lot of the info that you don't see is logged (things like overload alarms, battery voltage etc.) and much of that info will be too quick - even for a fluke - but the info might be there somewhere.. 

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1 hour ago, Kenneth said:

What I have also seen is that only motors with a fairly heavy starting load like the compressor is doing it. My pool pump (1500w) and even my milling machine (1500w) doesn't switch the inverter over to grid power. 

With those loads already probably pushing the apparent surge capacity, other than wanting to know why it happens on battery vs PV, it might not be worth it if it risks releasing the magic smoke from either the inverter or a workshop machine..... current state of the world makes sourcing parts bit of a gamble?

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Well the other option is to totally switch back to grid power when I am working in my workshop. I wired my system in such a way that I can switch back to grid power without going through the inverter. I did that in case I have a system failure. Thanks for all your advice.

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10 hours ago, introverter said:

a lot of the info that you don't see is logged (things like overload alarms, battery voltage etc.) and much of that info will be too quick - even for a fluke - but the info might be there somewhere..

Indeed. If fault recording is turned on (default is off), you can send a command like QFAULT (and from poor memory QF, one of them gives more info than the other), and it will tell you information like the load measured at the time of the fault. But switching to grid isn't classed as a fault.

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