Inverter problem

[brian_374334686]

Hi,

I have tried to use a 300 Watt 12 Volt -220 Volt inverter to run a small freezer ( 69 Watts) but the wires just get hot and the freezer does not start. What am I doing wrong?

Errr.. please keep it simple!

Regards Brian

 

[ProfJohnL]

Hello Brian,

I would be very surprised if your freezer is only rated at 69W.

That point aside, I assume the freezer uses a mechanical compressor, then the start up power to turn the compressor over is probably substantially more that the quoted running power. My suspicion is that the inverter is not big enough to cope with start up load.

Some domestic fridges may have a quoted running consumption of 150W, but may need 1500W or more to actually start the compressor. Once it is running the power consumption drops.

The other possible factor may the size of 12V power cables you are using. 300W at 12V needs cables capable of carrying 24A. From memory this would requite a cable of at least 4mmCSA, and for any length over about 1 M you would need something bigger probably 6mmCSA.

It is unusual to use a 12V source and an inverter to run a fridge yet alone a freezer, perhaps you are asking too much of the system.

 

[Rod_5041370]

I have a Waeco chest fridge and Waeco chest freezer in my car.

I use a 300watt inverter running from a seperate 120A/H battery.

The fridge which is an absorption type runs perfectly for up to 8 hours but the freezer which is a compressor type doesn't even start.

It is not to do with starting current as it also doesn't work on my 2000 watt inverter.

I believe that it is more to do with the output waveform.

 

[ProfJohnL]

Hi again Brain,

Rod has made an interesting point. Just in case you were not aware, our normal mains supply is pure sine wave (apart from interference) and is generally kinder to appliances with wound components such as transformers, and motor coils.

The vast majority of cheap inverters produce what is called a modified sine wave; in simple terms it is not a true sine wave but a highly distorted one. The distortions can interact with the inductance characteristics of transformers and motors and mess about with the voltage and current deliveries of the inverter.

Some inverters are more tolerant that others to these interactions. Generally the cheaper ones will be less tolerant, and the more expensive inverters may offer a true sine wave output, which is the best output form to have.

There is a trade off between purity of output and inverter efficiency. The modified sine wave devices can offer efficiencies in the high 90%'s. True sine wave unit are more likely to be about 70% efficient - swings and roundabouts!