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240v Fridge off seperate inverter


baldlimey

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Has anyone tried running a 240v fridge off a seperate inverter. I'm wonderin if this would use less/more/the same power as a 12v fridge?

 

We have the regular inverter charging but I believe the resting power consumption for these are quite high so I was wondering about putting a small (500w?) inverter with a very efficent 240v fridge or would I be eating through more power than a 12v fridge?

 

Any advice gratefully received.

 

Pete

 

 

 

 

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I did some experiments some years ago with a kind loan from Bimble and tested several types of pure sine wave invertes.

The results were that anything less than 1200w just wouldn't start the comprssor. Once running there was little significant increase in load between a small inverter and one rated at 2000W.

What folks forget / ignore is the standby current of any inverter which over a day is much more than the power consumed by the fridge when it is running.

Further none of the standby saving options would kick the inverter into starting.

 

I did publish the results on here but nobody was particularly interested...

 

Soo to ansqwer your question a smaller inverter won't start a mains fridge

There will be no power saving by having two separate inverters.

 

It's better to have a vert good qiality mains fridge (mine is a Liebherr Fridge Freezer) than a poorly cased 12/24v one. But then folks get panicky......

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This is actually a very complicated subject and I don't think anybody has done the definitive experiment to give the answer.

A 12 volt compressor should be more efficient than an AC one, it will be designed to be  efficient (rather than cheap) and (some) DC motors are more efficient than AC ones.

Most 12 volt fridges are converted 240v models and to keep the cost down (because its pretty high to start with) they tend to use cheaper models.

So first we need back to back measurements from the same fridge in its 12 and 240 volt versions.

Even if the 240 volt is better then there are the inverter losses to factor in. If you already have an inverter on 24/7 then you are getting the quiescent loss anyway so that goes to another "budget".

A dedicated inverter is a nice idea but I reckon you then need a dedicated thermostat to turn the inverter on and off, and maybe separate thermostats on the fridge and freezer parts. This is not trivial as switching on an inverter can be quite a demanding job that might be beyond what a simple thermostat can do so then its maybe thermistors/thermocouples driving a big relay with maybe a bit of electronics. Its on my ever growing todo list.

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14 minutes ago, Tracy D'arth said:

Wire the inverter on/off switch to the thermostat in the fridge instead of the compressor. Short the old stat wires together so that the fridge stat switches the inverter on when required.

No idling current on the inverter.

 

Thats probably the way to do it as long as you are happy messing with the inverter innards. I don't think the on-off switch is a proper on-off that handles the full current, it just sends the inverter into a deep sleep.

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4 minutes ago, dmr said:

 

Thats probably the way to do it as long as you are happy messing with the inverter innards. I don't think the on-off switch is a proper on-off that handles the full current, it just sends the inverter into a deep sleep.

Correct, there is very little current on that switch, certainly less than the fridge compressor and its a noninductive load too.

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1 minute ago, Tracy D'arth said:

Correct, there is very little current on that switch, certainly less than the fridge compressor and its a noninductive load too.

I reckon my inverter has a whole load of capacitors upstream of the on-off switch. If I disconnect it and reconnect it later, while turned off, I still get a whacking spark.

 

We have a fridge freezer with a single thermostat and I suspect this is a bit of a compromise, I would like to know what the gain would be of having two thermostats....it would also fix the issue of the freezer defrosting if we leave the boat unheated for a night in winter.

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I run mine via a Mastervolt 1500 true sine wave inverter which is on 24/7 The inverter is idling  for about 16 hrs a day, some of that time the engine is running of course. Without putting an multimeter in series with the inverter I don't know what the quiescent current as it doesn't register on my monitor.

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Beware that if you have two inverters on the boat the two AC supplies could be 180 degrees out of phase, meaning +240V on one live wire and -240V on the other. If you touch both at once you would be subject to an electric shock of twice mains voltage. And that would probably be fatal.

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1 hour ago, David Mack said:

Beware that if you have two inverters on the boat the two AC supplies could be 180 degrees out of phase, meaning +240V on one live wire and -240V on the other. If you touch both at once you would be subject to an electric shock of twice mains voltage. And that would probably be fatal.

 

Interesting thought. Very unlikely to touch both at once but I think you are correct, in fact they will run at slightly different clock speeds so certainly will be out of phase for some of the time. Where I used to work, due to constant space re-allocations and shifting of partitions, we had an office where the 13 amp sockets on a single work bench were run off two different phases (in a university Electronics dept too😀)

 

A very quick www search turns up nothing about multiple inverter shock risks.

 

.......and the TravelPower has 120v on the live and 120 on the neutral.

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13 minutes ago, ditchcrawler said:

Much safer, only 120 volts to earth maximum. We use to have safety circuits that ran on 50 volts, only 25 volts to earth 

 

I wonder why "out of phase 120volts" has not been adopted for domestic wiring?? As long as we avoid appliances with earth-neutral bonding the change-over could be gradual.

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5 minutes ago, dmr said:

 

I wonder why "out of phase 120volts" has not been adopted for domestic wiring?? As long as we avoid appliances with earth-neutral bonding the change-over could be gradual.

Ideally all switching would need to be changed to double pole

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20 minutes ago, dmr said:

Interesting thought. Very unlikely to touch both at once but I think you are correct, in fact they will run at slightly different clock speeds so certainly will be out of phase for some of the time. Where I used to work, due to constant space re-allocations and shifting of partitions, we had an office where the 13 amp sockets on a single work bench were run off two different phases (in a university Electronics dept too😀)

 

I believe that there is legislation that states that the minimum distance between two sockets on different phases / power sources must be a minimum of 2 metres apart to ensure that one person cannot touch both at the same time.

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13 minutes ago, Alan de Enfield said:

 

I believe that there is legislation that states that the minimum distance between two sockets on different phases / power sources must be a minimum of 2 metres apart to ensure that one person cannot touch both at the same time.

 

If so then that's not nearly enough, loads of people have those 4 way extension leads with a long lead on them.

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19 minutes ago, Alan de Enfield said:

 

I believe that there is legislation that states that the minimum distance between two sockets on different phases / power sources must be a minimum of 2 metres apart to ensure that one person cannot touch both at the same time.

 

That used to be the case, but I believe this requirement was deprecated in recent wiring regs.

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28 minutes ago, dmr said:

 

I wonder why "out of phase 120volts" has not been adopted for domestic wiring?? As long as we avoid appliances with earth-neutral bonding the change-over could be gradual.

 

I assume you mean 180 degrees or balanced supply?

 

In the US they have 120V/240V into most domestic properties. Here we normally have a single live wire of 230V. If you want more power then you have two or 3 phases or 230V that some would call 400V 3-phase.

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2 minutes ago, Mikexx said:

 

I assume you mean 180 degrees or balanced supply?

 

In the US they have 120V/240V into most domestic properties. Here we normally have a single live wire of 230V. If you want more power then you have two or 3 phases or 230V that some would call 400V 3-phase.

 

Yes, that's how the TravelPower works, 120 volts on live and neutral out of phase (or balanced). I supposed strictly 180 shifted is not quite the same thing but thats getting very technical. 

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6 hours ago, dmr said:

 

Yes, that's how the TravelPower works, 120 volts on live and neutral out of phase (or balanced). I supposed strictly 180 shifted is not quite the same thing but thats getting very technical. 

 

Does this highlight a terminology problem with the term "neutral"? Neutral in a three phase star configured circuit is at the centre of the star IIRC, and in your Travel Power it is in the middle of the single phase, but in household wiring one side of the phase is held down to the  "neutral" terminal while the other terminal cycles up and down the whole magnitude of the sine wave. 

 

So the two types of "neutral" appear to be different things to me, and should be given different names IMV. Or am I misunderstanding something basic?

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2 hours ago, MtB said:

 

Does this highlight a terminology problem with the term "neutral"? Neutral in a three phase star configured circuit is at the centre of the star IIRC, and in your Travel Power it is in the middle of the single phase, but in household wiring one side of the phase is held down to the  "neutral" terminal while the other terminal cycles up and down the whole magnitude of the sine wave. 

 

So the two types of "neutral" appear to be different things to me, and should be given different names IMV. Or am I misunderstanding something basic?


If you want to be pedantic, the travel power doesn’t have a neutral, it has two 120v lives. As do some (centre tapped) inverters.

 

This is why the ISO for small boat ac systems requires double pole breakers etc. if you have such a supply (called an “unpolarised supply”).

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4 minutes ago, nicknorman said:

This is why the ISO for small boat ac systems requires double pole breakers etc. if you have such a supply (called an “unpolarised supply”).

 

Since you raised the subject - it is also a requirement in ISO 13297 (AC Electrical Systems for Small Craft) to have double-pole breakers in the shore line supply (7:2:1)

How many do ?

 

 

Screenshot (687).png

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5 hours ago, MtB said:

 

Does this highlight a terminology problem with the term "neutral"? Neutral in a three phase star configured circuit is at the centre of the star IIRC, and in your Travel Power it is in the middle of the single phase, but in household wiring one side of the phase is held down to the  "neutral" terminal while the other terminal cycles up and down the whole magnitude of the sine wave. 

 

So the two types of "neutral" appear to be different things to me, and should be given different names IMV. Or am I misunderstanding something basic?

Your two household mains input wires are connected to one of the three phases and the neutral of the three phase supply cable in the street, so it is the same neutral. At your local substation the neutral conductor is connected to an earthing rod so is at earth potential there. But at an individual property there will be a volt drop in the neutral conductor (unless the loads on all three phases are exactly equal) which means the domestic neutral conductor may be a few volts different from earth potential.

Edited by David Mack
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6 minutes ago, David Mack said:

Your household mains input wires are connected to one of the three phases and the neutral of the three phase supply cable in the street,

 

It may not be common, but our household input IS 3-phase. When we had it built we wanted to future proof it for any electrical demands.

 

 

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20211107_124907[1].jpg

Edited by Alan de Enfield
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13 hours ago, Alan de Enfield said:

 

I believe that there is legislation that states that the minimum distance between two sockets on different phases / power sources must be a minimum of 2 metres apart to ensure that one person cannot touch both at the same time.

Good practise but not sure if the same regulations fall onto boaters.

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