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I'm doing a self build camper at the moment and have been setting up the electrics. Everything seems to be working well so far however I've been having some issues with my inverter.

I have two lithium batteries that are 220ah each and have a max continuous current rating of 200a which are connected in parallel. I then have this 3000w (6000w peak) pure sine wave inverter hooked up to the batteries: https://www.photonicuniverse.com/en/catalog/full/380-3000W-12V-pure-sine-wave-power-inverter-230V-AC-output-UK-sockets-with-remote-onoff-switch.html?gclid=Cj0KCQjwnqH7BRDdARIsACTSAdvLhoC0mDJEIrGYIunEEhhWvXBdQ8PC2mB1OBdWCFg_ZRHR-BsYVgYaAjyGEALw_wcB

Lower powered devices are running fine off the inverter however I wanted to run a microwave oven from it, specifically this one: https://ao.com/product/kmqfx33910-kitchenaid-microwave-stainless-steel-63833-50.aspx

It's rated at 900w microwave and 1200w grill so well below the 3000w of my inverter.

I originally had a 250a circuit breaker between the inverter and the battery but when I plugged the oven in and turned it on after about 30 seconds it tripped out. I thought that was a bit odd since 250a x 12v is 3000w so that would indicate the inverter was running at maximum capacity. I changed it for a 300a breaker instead but now when I plug the oven in and turn it on after 30 seconds the inverter trips instead and shuts itself off, presumably the over load protection is kicking in.

So it seems a bit odd to me that a 1200w appliance is maxing out a 3000w inverter. The inverter even says it can handle up to 6000w spikes for a few seconds so I'd have thought it should be more than capable of powering this oven.

I also have a Simarine Pico monitoring system and a shunt on the circuit so I can monitor the power consumption and see exactly what is going on. When the oven is on the draw on the batteries is around 130a so 130a x 12v = 1560w, so the inverter should only be running at about half of its maximum output. And the voltage when the oven is running drops slightly from 13.5v to 12.8v so decent enough voltage.

So I'm just a bit confused about the whole thing really, is my maths or understanding of any of these things wrong or could the inverter be faulty?

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  • \$\begingroup\$ I guess the main thing that crosses my mind, right now, is this: Is your microwave oven an older, heavier unit? More modern ones are made lots more cheaply now and don't have the monsterously big transformers that used to be used. These older ones may have a power-factor issue that accounts for the current limiting you are seeing. But I think modern ones should not have this problem (well, far less so, anyway.) \$\endgroup\$
    – jonk
    Sep 7, 2021 at 22:38
  • \$\begingroup\$ It's brand new I posted the link to it in my question \$\endgroup\$
    – geoffs3310
    Sep 7, 2021 at 22:43
  • \$\begingroup\$ @jonk that was my initial thought. But since he was able to measure the input current to the inverter, I don't think that's the main cause. \$\endgroup\$ Sep 7, 2021 at 22:43
  • \$\begingroup\$ The other thing that came to mind is how cheaper brands tend to advertise the "Max" wattage instead of the continuous. For example, some will say 3000W in big lettering on the inverter, but they can really only do 1500W continuous. Based on the datasheet for this inverter, that doesn't seem to be the case though. \$\endgroup\$ Sep 7, 2021 at 22:46
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    \$\begingroup\$ I'm measuring the voltage using the Simarine Pico monitor which has a cable connected to the positive and negative of the battery bank. I'll use a multimeter and measure it at the inverter instead and see that happens \$\endgroup\$
    – geoffs3310
    Sep 7, 2021 at 23:43

2 Answers 2

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I got to the bottom of this in the end. One of the connections between the inverter and the bus bar was bad and not making full contact which was reducing the voltage and therefore increasing the amperage. It had an m10 lug on an m8 terminal so it was a loose fit. I changed the lug to an m8 one and voila problem solved!

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Typical SMPS,Motor’s, uW ovens and incandescent bulbs have one thing in common. The peak start power may range from 5x to 12x max rated power. This exceeds you peak capacity and this is your problem.

A ZCS Triac suitably rated will help minimize I-peak .

  • microwave ovens tend to have very poor power factor with arc driven Thyratrons and welded MOT transformers with Eddy Currents and draw excessive reactive power with the very low ESR resonant 6u to 10uF capacitor.

Recommendation

Test the MF Cap and replace it or get a slightly bigger one. Your breaker is possibly tripping from saturated cores from excessive peak reactive currents

  • Heat is probably your biggest problem and the inverter possibly needs higher velocity turbulent airflow over the hotspots inside with exhaust.
    • you could boost the air flow with a bigger DC fan in the inverter and even check if it blows hot air at high velocity.
  • If the breaker trips in 30s it possibly is drawing much more (2x to 5x)than the rating unless previously hot from repeated retries.

When current exceeds the peak power limit , the inverter cores lose their high inductance and then become very inefficient low resistance thus overloading the breakers. This is accelerated with core temp rise, and the inverter simply too weak to handle the peak currents, unless the Cap is worn out in the oven.

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  • \$\begingroup\$ The link he gave is for a pure sine wave inverter so that's less likely to be that issue. With that being said, we can't really say for sure because we don't know what the waveform looks like under load. With cheaper inverters, who knows "pure" it actually is. \$\endgroup\$ Sep 8, 2021 at 4:29
  • \$\begingroup\$ MOT's are notoriously bad transformers with welded steel laminates increasing the eddy currents to stay quiet. Even an 8uF cap can't control the harmonic content but improves the power factor somewhat. When the caps fail as this one might have, they will easily trip 30A breakers . The THD is 3% intoa resistive load but unlikely into an arc resonant MOT driven Thryratron resonator. Perhaps I should drop the square wave comment and suggest a bigger MF cap. Thanks for your negative feedback. @bunker89320 The real problem is the breakers can trip off VAR's and his power meter might be true RMS \$\endgroup\$ Dec 31, 2021 at 4:32
  • \$\begingroup\$ @bunker89320 do you have any constructive feedback? \$\endgroup\$ Dec 31, 2021 at 4:40
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    \$\begingroup\$ I think your revised answer/ explanation makes sense. I don't really have anything to add to that \$\endgroup\$ Jan 6 at 4:19
  • \$\begingroup\$ @bunker89320 TY for your validation, but upvotes are worthy of your opinion \$\endgroup\$ Jan 6 at 22:52

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