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Discuss Need some advice. - 12V to 240V in the Electrical Forum area at ElectriciansForums.net

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Hello! I've just joined the forum and am looking for a little advice.

- As what I am attempting to do will never be attached to the electrical grid (and why this is relevant will be obvious in a second) it should bypass any laws around such. (and if it doesn't, I have a licensed mate who will do any installs for me!)

Ok. Onto the problem!

I have a fridge in a caravan. It's a 240V "beer fridge" It's not 240/12V.

I have a 12V > 240V 400-1200W inverter.

It *is* enough to sustain the fridge (it's only a little fridge) but it doesn't have enough Peak power to charge up the compressors capacitor before it trips the inverters breaker.

It is *not* tripping any fuses.

and after resetting the inverters breaker it runs perfectly.

Obviously the time between the resets is giving that compressors capacitor enough juice to start the motor turning, and allowing the fridge to run.

Once the fridge's compressor is working, it works perfectly! Inverter isn't overheating/tripping/etc.

(I just spent an hour testing this.)

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Okay!

Here's the question.

Can I build a device that sits between the inverter and the fridge, that will build up a charge slower, as to not trip the inverters breaker, and still allow the fridge to start up?

and not ... do anything untoward. Switch off when the fridges demand drops to "running"

I'm thinking; something like a bank of capacitors? They charge up gradually, and when they hit a certain amount, they dump into the fridge, allowing to get going.

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Has anyone ever done anything like this before?

Yes I realise, it would be *far* simpler to just go and BUY an upright 12v-240V But I don't have a spare grand burning a hole in my pocket. I have an inverter I got for practically nothing, and a fridge that was; likewise; nothing.

Ignore the whole "12V" side of things please.
--

Basically - Can this be done?
Can it be done safely?
Can it be done cheaply (call this budget $100)

--

Thanks!
 
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Strima

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It could be done but cheaply and safely tend not to be in the same sentence. Do you know the peak rating of the inverter and the initial demand of the fridge?
 
It is nothing to do with the compressors capacitor. It is the starting current of the motor in the compressor that is causing the trip. A stationary motor has a big inrush current at start up.

So a bank of capacitors won't help.

Possibly a series resistor to limit the start current may help, this will need to be bypassed once started. Think a centrifugal switch.
 

telectrix

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maybe a thermistor type of device. high resistance cold, reducing as temp. increases.
 

telectrix

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think it's a NTC type. (negative temp. coeff. )
 

Lucien Nunes

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As above, not due to charging a capacitor slowly. On AC, you can't charge a capacitor slowly, it charges, discharges, charges up the other way and discharges again 50 times per second.

Split-phase induction motors as used in fridge compressors are notoriously hard for inverters to start. The starting current can be between 5 and 10 times the running current; the absolute minimum rating of inverter for reliable operation needs to be equal in VA to the locked-rotor current x voltage. This will still stress the inverter and a more generous rating is recommended. Some makers simply state that their inverters are not intended for powering fridges regardless of rating.

A soft-start arrangement of resistor that gets bypassed, or thermistor, will reduce the inrush current but also drastically reduce the starting torque, to the point where the motor might not start, or more likely will start under favourable conditions but remain stalled when the head pressure is high.

If the inverter has a true sine output, parallel capacitors across the motor could actually help, not by providing a boost of power (see above) but by correcting the power factor of the motor, which is usually low and lagging. Inverters have limits on both current (due to the H-bridge) and real power (due to the step-up converter). If it is tripping on current rather than power, then correcting the motor pf could help. But the 'quasi' or 'modified' sine that many inverters produce isn't actually sinusoidal, it's a stepped waveform with the same peak/mean ratio which screws up anything with parallel capacitors. Some inverter instructions even tell you to remove any capacitors from things like fluorescent battens as the peak currents that they pass are excessive and cause problems.

If it were mine, I'd modify the inverter load sensing circuitry to give it a response tailored to the start current profile of the motor; but doing that is beyond the scope of a forum thread and could easily just kill the inverter anyway. You could try a step-down autotransformer - knock 30% off the motor volts and see if that is enough to start it reliably. It has the advantage over resistance or a thermistor of not reducing the torque as much as it reduces the current. If it's a sine output inverter, try correcting the pf. You can look up the correct capacitance according to line voltage and motor VA; if that sounds like hard work for possibly no reward, get a bigger inverter.

E2A: You said not to mention the 12V side but be very sure your battery and cables are of low enough resistance. The peak current when starting the fridge could be 150A. Even if the battery, isolator switch, cables and connectors total just 0.01 ohms, the inverter will probably trip on low voltage.
 
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DPG

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People always underestimate the current required on the 12V side of inverters.
 

Lucien Nunes

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They also forget that allowable resistance follows the square of the voltage ratio. So with 240/12=20, the cable on the 12V side has to be 400 times the size for the same load power and percentage drop per metre. And hence why 24V is four times as user-friendly as 12V for heavy loads.
 

telectrix

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don'tknow aboutdownn in Oz, but in the UK there are several caravan brakers/scrappers scattered around the country. you can get a fridge that works on 240V, 12V or LPG for anywhere between £20 and £50.
 

Lucien Nunes

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I think the OP's point is that it's a specific drinks fridge that's not available as a 12V version. The problem with the 230/12/LPG fridges is that they are absorption coolers. To get the advantages of LPG operation and being silent, you have to put up with horrendous inefficiency when running on leccy. They need something like 10A from the 12V side, it's not even thermostatic it's a constant 10A load, only meant for when they are on the move and the LPG has to be turned off. Had one on the boat for years, only wired the 12V as an emergency backup in case the gas valve or burner failed. Changed to a 12V compressor fridge, motor has integral VFD, uses about 2A for 30-60% duty cycle. So about 10% of the consumption for a fridge three times the capacity. A 230V compressor fridge on an ordinary inverter will be somewhere between the two, if you have the oomph to start it.
 
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