I have a 600W modified sine wave inverter and am trying to run a 70w bar fridge.
It almost starts the fridge but overloads. I have been told that a 600w pure sinewave inverter would probably do the job but they are 2 to 3 times the price.
My 2000w inverter easily handles the load.
Pure sinewave inverters usually have a decent output transformer so my question is if I could get a couple of cheap 240v transformers connected back to back would this possibly help in starting the fridge motor.
Hi, Norm. Compressor motor startup current is almost certainly the culprit. That isn't going to be helped by supplying a more pure sine wave.
Look at the inverter specs on peak current. You'll have to choose one that has greater peak current than the startup current of your fridge compressor.
At 70W input consumption the frig is most likely a Peltier cooled device a-la Sicao BC46A
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Since Peltier devices run from DC the likely culprit is the input conversion circuitry to convert from mains power down to the required DC. Connecting back-to-back transformers at the input of your frig is not going to help the situation imo. A higher powered DC - AC converter (not necessarily sine-wave) is the only way you can overcome the switch-on surge requirement.
Look at the inverter specs on peak current. You'll have to choose one that has greater peak current than the startup current of your fridge compressor.
Cheers Chris
Hi The "salesman" told me that because the pure sine wave inverter had a larger tranformer it would be better able to handle the surge. It definitely was much heavier than my modified sinewave inverter.
The modified sine wave inverters have tiny ( and I mean tiny) transformers which means there is less energy stored in the iron.
Did he also tell you Saddam had WMDs? Same reasoning is being used by the salesman.
Let's first look at numbers. For example, this modified sine wave output is two 200 volt square waves with a spike of up to 270 volts between those square waves. This is perfectly fine for computers because computers are so robust. But this may also damage some small electric motors due to what is in those output waves - ie harmonics.
There is no pure sine wave inverter. There are just 'cleaner' outputs. Increased harmonics means hot spots inside a motor. That is completely irrelevant to your problem - starting that fridge. It may be 70 watts. But it also has a massive 'power on' surge. Electric motors (unlike electronics) don't like low voltage during power up - a slow response to a load change. Can your inverter supply new current demands quickly?
Meanwhile, other problems include what controls that motor. Are its electronics made upset by electricity provided with multiple frequencies (60 Hz, 180 Hz, 300 Hz - all provided simultaneously)?
Long before anyone can answer, first the problem must be identified. It does not start? If symptoms don't come with 'whys', then every action will only be speculation. Is this fridge nothing more than a motor and a mechanical thermostat? Or does it contain other components? What do specs say for that fridge? Unfortunately, most numeric specs for refriges are poor. Still, required are those numbers and facts.
There is no bigger transformer. If the unit is outputing 200 watts, then it has a 200 watt transformer in both modified or pure sine wave. Inverter must supply not just 70 watts but also the massively higher power required, fast on demand, during startup. The pure sine wave inverter is only three times more? Then what is it missing, or is that modified inverter better than most? We don't know since even numbers such as THD for both units were not provided. Start with manufacture specs for everything. What kind of load is this refrig?
It all comes down to the surge capacity of your inverter. They are rated for this i.e. how much peak wattage they can output for a short period of time. Generally most 600W inverters can handle a surge of about 1000W but you need to spec your inverter to check that. The modified vs pure sine is most likly not an issue in this case as a fridge should run without to much trouble on a modified wave. It is only generally important to have a "clean" wave when running more sensitive equipment such as computers, printers, and home entertainment systems. As for a pure sine wave inverter it being heavier doesn't mean it can handle a big surge. It's heavier because it takes a more complicated transformation process to produce the wave form, therefore more components.
Again this is a case of a salesman not really knowing the physics of what is going on in the product he sells. They read a product sheet and spit it out with out really knowing what your needs may be. Bottom line is to spec out the component and match it to the specs on your loads, generally a little online research can do this much more than some guy who just needs to make the sale.
Anyway just run the fridge off your larger inverter as it can handle the surge.
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