Why does my inverter keep tripping?

Question

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?

Answer 1

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!

Answer 2

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.