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I have got a 3 kVA (2400 watt) solar hybrid inverter (pure sine wave) which is connected to a 1280 watt PV array and 2 x 12 volt 150 Ah batteries. It is supposed to switch to utility mode if the combined voltage of PV and batteries goes below 23 volts.

This afternoon (during plenty of sunlight), I tried to power my refrigerator using the system but as soon as the compressor started, I could hear some knocking sound from the fridge, and at the same time, the LED lights in the house (also powered by the inverter) started to flicker.

Next, I tried using a voltage stabilizer on the fridge to see if that made any difference but the result was the same (knocking sound from compressor, and flickering lights).

The display on the inverter showed that it was producing enough output at that time for the required load (sometime like 550 watts). To validate this, I connected a 1200 watt halogen heater and ran a 100 watt ceiling fan, and it was able to power this load without any issues. The display on the inverter showed a load of 1.1 kW.

My batteries are about 2 years old and normally don't give a long backup. Even on a 150 watt load I might get about 10 to 15 minutes out of them. So, could the problem be that the refrigerator's compressor produces a sudden spike in the required load and the batteries are not able to handle this?

Or, do I need more solar panels to be able to handle this spike? I'm planning to run my DC inverter air conditioner on this during summer and want to know before I replace the batteries or buy more solar panels.

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    \$\begingroup\$ This may be an issue with the motor starting behavior more than the steady state run condition. Motors draw a lot more current than usual on startup, and things like phase shift capacitors used at start could make for a problematic load for the inverter. Unfortunately there isn't really enough engineering detail about the components themselves for this to be answered in the context of this site - you are probably more in the domain of seeking the knowledge of those with the experience of such usage. \$\endgroup\$
    – Chris Stratton
    Mar 10, 2019 at 17:11
  • \$\begingroup\$ Also consider refrigerator motors are always connected to the compressor so that is part of the load... \$\endgroup\$
    – Solar Mike
    Mar 10, 2019 at 18:46
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    \$\begingroup\$ UPS and sine-wave converters hate asymmetric loads such as capacitive start/run type motors. The issue is not the wattage consumed, it is the voltage/current lag the motor (large inductor) and capacitor creates. UPS and sine-wave converters attempt to correct for sine wave distortion, which never goes away, so the inverter is always running extra hard to supply power, and to correct the phase lag. A large resistor can 'hide' some of this phase shift, but it cuts down on motor start torque, which can be another issue. \$\endgroup\$
    – user105652
    Mar 10, 2019 at 19:28

2 Answers 2

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As mentioned in the comments, fridge motors take a huge current surge when they first start up. This overloads the inverter, which momentarily shuts down.

There is no cheap/easy solution. Get a 12V fridge and run it straight off the batteries, or get a bigger inverter.

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To run your fridge off an inverter, the safe assumption is that the inverter is approximately able to provide seven times the running wattage of the fridge at least for a few seconds. For instance, if your fridge is rated 300W, use at least an inverter that can provide peak wattage of 2100W and above. That way you will be safe. Fridge draws a lot of current at start up

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