Can I automatically switch a fridge between 2 sockets?

[matmaxgeds]

Hi,

I am trying to understand whether it is possible to put together a circuit to allow a fridge to switch between two sockets.

I live in Sierra Leone and the power company here rations power by switching of power to some houses. We also have two mains connections fed to two different circuits in the house. Luckily, they are rarely rationed at the same time hence I would like to find a way to plug the fridge into both circuits and have it automatically select whichever has power.

I have found expensive automatic transfer switches meant for generators advertised online but I wondered if anything cheaper was possible given that the load from one fridge will be much less.

Thanks in advance,

Matt

 

[rjm2k]

This guy made something like what you want.... You can probably buy something similar too, Note that I doubt it would be wise to join the Earths as he has if you are talking about 2 separate main supplies though so that would probably need switching like the Live and Neutral.

http://www.theaquariumwiki.com/DIY_Emergency_Power_Backup

No idea of the safety/legality of such a device, am sure others will be along to comment soon......

 

[dhutch]

I have no idea why the guy who wrote that artical didnt just an of the shelf ups system, as far as Im aware they do not all turn off after 20minutes.
- This may also be an option for you, if they are avilable with two supplies, or fo the powers never off that long.

Otherwise, theres no reason why it should be safe to use a relay like the one he has made that I can see although I havnt heard of it being done (also waits for others to comment) although I would be aiming to make a little less badly, in a dedicated box, cable restraints on the incoming/outgoing etc. Its also essential that the relay is of a 'break before make' type, ie, can never be momentrally connected to both supplies bridging them. Do we know if they are different phases, might well be?

See if anyone else can shed more.


Daniel

 

[bernardgreen]

A simply change over relay switching the fridge from one supply to the other will work.

BUT it may not be safe when both supplies are available. The two supplies may be on different phases which would have contacts in the same relay on different phases. That is NOT a good idea as the arc that would occur if the relay switched while the fridge was running could become an arc between two phases.

Much safer to have two contactors electrically ( and mechanically ) interlocked so that it is impossible for both to operate at the same time.

The earth is another matter and more information about how the "earths" are delivered to the property is needed. Joining two "earths" from two different supplies could ( often does ) result in high currents flowing in the cable connecting them. Where supplies are rationed by cutting one phase at a time leading to un=balanced loads then the possibility of these "earth" currents being very high is significant.

 

[JohnW2]

A simply change over relay switching the fridge from one supply to the other will work.
BUT it may not be safe when both supplies are available. The two supplies may be on different phases which would have contacts in the same relay on different phases. That is NOT a good idea as the arc that would occur if the relay switched while the fridge was running could become an arc between two phases.
Much safer to have two contactors electrically ( and mechanically ) interlocked so that it is impossible for both to operate at the same time.

Indeed so, but even then the switching logic would have to prioritise one of the two supplies - otherwise one would have an undefined situation when both supplies were available. In other words, the logic would be that suuply A, if available, was always used but if it failed, the the load would be switched over to supply B (which may or may not be available) - i.e. only one of the supplies would be sensed.

I would also add that interlocking the two contactors is not necessarily quite as simple as it sounds, and may well involve additional relays.

Kind Regards, John

 

[dhutch]

Can see the argument for two relays and a control unit, but it would be a far more complex arrangment, and I would have thought that as long as the relay was make before break and rated for the between phase voltage, it would be ok,

That said, I cant say I would be confortable to make/use the preposed device, or suggest some else does, without having more experience/knowlage than I have currently!

How often does any one supply go down? Are we talking an hour or so, or half a day? (minutes, obviously, are not an issue in something like a fridge)


Daniel

 

[JohnW2]

Can see the argument for two relays and a control unit, but it would be a far more complex arrangment, and I would have thought that as long as the relay was make before break and rated for the between phase voltage, it would be ok,

I hope you meant 'break before make'! What you say is obviously theoretically true, but the capacity for disaster when one has both phases connected to the same set of changeover contacts would certainly make me very uncomfortable.

I have an example in my house (switching one load between two phases if the 'usual phase' fails) and engineering a changeover arrangement which did not invoke the discomfort mentioned above proved to be appreciably more challenging than I had expected.

Kind Regards, John

 

[dhutch]

Yes, sorry, a rather critical mistake that!

As said, I still favour the use of a standard UPS unit plugged into one of the two supplies. I feels quite smug to have it auto switch between the two, but equally, there has to be a lot said for keeping it simple.

Your not just ensuring you dont blow up the change over unit and fridge when your ensuring you dont touch the phases together, your also ensuring you dont create a fault where you are putting live from one supply back down the other isolated supply, which could be dangerous (lethal) for anyone working on it thinking it was dead.


Daniel

 

[JohnW2]

Yes, sorry, a rather critical mistake that! ... As said, I still favour the use of a standard UPS unit plugged into one of the two supplies. I feels quite smug to have it auto switch between the two, but equally, there has to be a lot said for keeping it simple. ... Your not just ensuring you dont blow up the change over unit and fridge when your ensuring you dont touch the phases together, your also ensuring you dont create a fault where you are putting live from one supply back down the other isolated supply, which could be dangerous (lethal) for anyone working on it thinking it was dead.

Yes, I agree in principle with all that. In practice, a problem might be in getting (and paying for) a meaty enough UPS for the job. Although the running current of a fridge is quite low, the start up current of the compressors can be very high. One sees a lot written about the fact that fridges often won't work (compressors won't start) when run from small generators with limited current-supplying capability, so I suspect that one might well have a similar problem with the inverter in a UPS, unless it was a very meaty (probably equates to expensive) one.

Of course, 12V and 24V DC fridges do exist. If one changed to one of those, it could be powered directly from the back-up battery(ies), withoutthe need for an inverter. Although that would represent an appreciable capital outlay, so might a meaty-enough UPS.

Having said all that, the changeover switching could be implemented pretty safely (although inevitably not as 'safe'as a UPS approach), if done carefully - as I said, I do have such a system in my house.

Kind Regards, John

 

[ban-all-sheds]

Unless the house is unoccupied for long periods, and therefore the fridge not accessed for ditto, I'd just have 2 sockets next to each other, and move the fridge plug into whichever one was live each time I went to open the door.

 

[JohnW2]

Unless the house is unoccupied for long periods, and therefore the fridge not accessed for ditto, I'd just have 2 sockets next to each other, and move the fridge plug into whichever one was live each time I went to open the door.

That certainly would be the simplest solution, but may well be regarded as rather tedious. In my situation, we are talking about equipment far more sensitive than fridges to modest periods without power, so it would not be a practical option. Indeed, even with a fridge, the periods of 'non-occupation' would not have to be that long to become a problem - e.g. going away for 'a long weekend'!

Kind Regards, John

 

[dhutch]

Valid point about the startup current, and I suppose I dont know the avilablity of UPS units to the OP, where I have had success in buying the odd cheap/used unit off ebay.

as I said, I do have such a system in my house.

Come on then, what do you use, presumably from you comments two contactors and various relays? Circuit diagram? Photos?


Daniel

 

[matmaxgeds]

Hi,

Thanks everybody for your replies so far. Sorry for my slow responses, internet access here is not that plentiful.

Trying to take the issues one-by-one:

I don't know how the different supplies are earthed so I will therefore anticipate switching the earths as well.

The rationing/cuts are typically for 6 to 7 hours at a time and a UPS of the size (plus the startup current issue) that implies would be extortionate/unnavailable here.

We currently use the tactic suggested of swapping the plugs to whichever is live at the time, however in the temperatures here, 8 hours with it in a dead socket overnight is enough to melt the butter.......and we only dream about icecream or frozen foods.

If I ignore the issue of arcing between phases then this means I can use a single 3 pole double throw relay.

If I don't ignore it, then either

a) I need to find a relay rated for the potential difference between the phases (presumably 240v+240v = 480v) at it's extreme?

Or b) I can use two relays,


With two relays, if I had a normally open relay for power supply A and a normally closed relay for power supply B, both triggered by power supply A, my thought was than when A had power, it's relay would close and B's relay would open. When A didn't have power, it would open and B would close. Where both had power, supply A would be preferred?

If it is prefferable to have most time spent in the 'normal' state, I could have supply A on a NC relay and supply B on a NO relay but in that case, they would both be triggered by power supply B and this means that when both had power, source B would be preffered.

If neither of those sounds sensible, with a controller, would a third relay suffice to select between the two sources?

Finally, how is this differing from a commercially available automatic transfer switch, is it just that I am working with lower currents so can use smaller/cheaper parts?

THanks so much for the tips so far, I promise to post a picture of the solution when implemented!

Matt

 

[stillp]

Finally, how is this differing from a commercially available automatic transfer switch, is it just that I am working with lower currents so can use smaller/cheaper parts?

No, it's not that simple. Commercial ATSE are designed to prevent foreseeable faults such as one relay sticking and causing an inter-phase short-circuit as well as incorporating overcurrent protection, timers, interlocks, etc.

 

[JohnW2]

Valid point about the startup current, and I suppose I dont know the avilablity of UPS units to the OP, where I have had success in buying the odd cheap/used unit off ebay.

I have no personal experience, but I've certainly often read people complaining that many fridge compressors won't start with the current available from a ~2kVA generator.

Come on then, what do you use, presumably from you comments two contactors and various relays? Circuit diagram? Photos?

I'll look out the circuit diagram. I remember that it was far from easy, and that I was not even totally comfortable with the end result. IIRC, the main problem occurs 'at energisation'. When no power is being supplied by either source, all relays/contactors are, by definition, in their unenergised states. That makes it quite difficult to dictate with certainty what will happen if both supplies are applied simultaneously, and to make sure that there cannot be momentary connection of both supplies to the load (hence supplies connected together) during that 'power up' period. The simple way around that involves time-delayed relays, but they are not so readily available - and I wanted to use 'standard components' that I was fairly confident were going to remain sourceable for the forseeable future.

Kind Regards, John