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Old 11-22-2022, 10:50 AM   #1
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Transfer switch fabrication

I'm planning to add a dedicated inverter for my 120VAC refrigerator. Would like to build a transfer switch that would switch the refrigerator between the 2 inverters and shore power / generator. The one inverter has a transfer switch to accomplish the shore power / generator switching.

A transfer switch is essentially one or two relays. In some transfer switches, a relay is used for each power source to prevent contact arcing. I'm looking at a maximum of 4 amps AC, and trying to determine whether I need 2 relays, 1 relay with two double pole double throw contacts, or if I can find it a 4 pole double throw relay.

Thoughts?

Ted
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Old 11-22-2022, 10:56 AM   #2
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If you want an automatic solution, Xantrex sells a transfer switch for this purpose: https://www.amazon.com/Xantrex-80809...0-890f5a4e2f90
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Old 11-22-2022, 11:13 AM   #3
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Just to define the challenge, three sources.

Inverter 1
Inverter 2
shore/gen

and what priority do you want?
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Old 11-22-2022, 11:48 AM   #4
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Just to define the challenge, three sources.

Inverter 1
Inverter 2
shore/gen

and what priority do you want?
Just 2 sources. The second inverter has a transfer switch between the inverter and the shore power / generator.

Ted
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Old 11-22-2022, 11:51 AM   #5
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Originally Posted by rslifkin View Post
If you want an automatic solution, Xantrex sells a transfer switch for this purpose: https://www.amazon.com/Xantrex-80809...0-890f5a4e2f90
Yes, saw that. Would prefer to have replaceable relay(s) and spares onboard.

Ted
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Old 11-22-2022, 12:06 PM   #6
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A double pole, double throw relay will do the job. You will need to decide how to control it, whether the control is 12VDC, 24VDC, 120VAC, or 240VAC, and which source should be selected when the relay is not energized. You could also use a double pole, double throw switch and control it manually.
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Old 11-22-2022, 12:32 PM   #7
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A double pole, double throw relay will do the job. You will need to decide how to control it, whether the control is 12VDC, 24VDC, 120VAC, or 240VAC, and which source should be selected when the relay is not energized. You could also use a double pole, double throw switch and control it manually.
I was planning on controlling it with power from the second inverter (relay energized). Obviously when the second inverter powers down and the relay drops out, there won't be any arcing issue when the contacts switch. When the relay energizes, the load is switching from one inverter to the other and was worried about momentary arcing between the load and two power supplying contacts.

Ted
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Old 11-22-2022, 12:52 PM   #8
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Just 2 sources. The second inverter has a transfer switch between the inverter and the shore power / generator.

Ted
OK, this is easy one double throw relay.

The common goes to the load.
The non preferred source is the normally closed contacts
The preferred source is the normally open contacts.

The coil for the relay is the preferred source.

Depending on your neutral arrangement you might want to switch the neutral as well, so a double pole double throw relay
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Old 11-22-2022, 01:05 PM   #9
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Do not quote me verbatim but if I remember correctly the use of an MOV , metal oxide varistor, will quell arcs. You MUST choose one that has a voltage rating somewhat above the actual operating peak voltage of the normal AC wave.

They come in varying amp capacities and voltage ratings.

IE. for a 120VAC circuit you would choose one that is rated at 170V or slightly more. The calculation is 1.42 x the rated AC voltage. So 120V x 1.42= 170.4 volts. better 175 or 180V. THis is to allow for the PEAK , not the nominal voltage, of the sign wave which is that 1.42 above what we all see.

Above that calculated voltage peak the MOV will conduct quelling the arc.

YOu will also need to allow for the amperage.

FOr this I would suggest contacting an electrician versed in controlls applications to do the actuall fine tuning calculations.

This also will allow for a smaller relay to be used than would otherwise be the case.

Just carry a few spares though as they will suffer the consequences of the arc suppression and need periodic replacement.

I have been out of this, retarded, for too long to have all the details in my head any longer. However, this was part of my job at one time.
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Old 11-22-2022, 01:20 PM   #10
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This is AC, not DC, so the potential arc is quenched 120 times per second. If you bought a relay rated at 10A from a quality manufacturer for a nominal 4A load, I would expect it to last for tens of thousands of operations. As an example, a Potter & Brumfield KA series with silver cadmium contacts is rated for 100,000 operations with a 10 amp load.


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Old 11-22-2022, 04:09 PM   #11
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I was planning on controlling it with power from the second inverter (relay energized). Obviously when the second inverter powers down and the relay drops out, there won't be any arcing issue when the contacts switch. When the relay energizes, the load is switching from one inverter to the other and was worried about momentary arcing between the load and two power supplying contacts.

Ted

Just get a relay that's rated sufficiently. I think you said the load is 4A, but the switch should be rated to match the fusing/breaker for the circuit that the fridge is on. So probably 15A. That shouldn't be hard to find, and they are designed to switch under load.
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Old 11-22-2022, 05:09 PM   #12
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There are a number of 4 pole relays available. Any value in having one pair of contacts for one inverter and the other pair for the second inverter?

Ted
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Old 11-22-2022, 05:41 PM   #13
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Originally Posted by O C Diver View Post
I'm planning to add a dedicated inverter for my 120VAC refrigerator. Would like to build a transfer switch that would switch the refrigerator between the 2 inverters and shore power / generator. The one inverter has a transfer switch to accomplish the shore power / generator switching.

A transfer switch is essentially one or two relays. In some transfer switches, a relay is used for each power source to prevent contact arcing. I'm looking at a maximum of 4 amps AC, and trying to determine whether I need 2 relays, 1 relay with two double pole double throw contacts, or if I can find it a 4 pole double throw relay.

Thoughts?

Ted
If it's a dedicated inverter for the fridge, why do the transfer switch at all? Just let it run on the dedicated inverter all the time. If you have sufficient battery charging already, the small load of the fridge's inverter shouldn't make much difference.

That's how I run my AC powered fridge 24x7. My Victron system will inform me via email if the inverter has a problem and they are cheap enough and simple enough to just carry a spare if you need a backup power source.
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Old 11-22-2022, 06:35 PM   #14
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There are a number of 4 pole relays available. Any value in having one pair of contacts for one inverter and the other pair for the second inverter?

Ted
Not from the way you described your setup
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Old 11-22-2022, 08:02 PM   #15
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If it's a dedicated inverter for the fridge, why do the transfer switch at all? Just let it run on the dedicated inverter all the time. If you have sufficient battery charging already, the small load of the fridge's inverter shouldn't make much difference.

That's how I run my AC powered fridge 24x7. My Victron system will inform me via email if the inverter has a problem and they are cheap enough and simple enough to just carry a spare if you need a backup power source.
I like options.
By building this simple transfer switch the refrigerator can:
Run off shore power though the Magnum Energy inverter.
It can run off the generator as above.
It can run off Magnum Energy inverter
It can run off the dedicated inverter if I shut everything else off to conserve the house battery.

All automatically.

Ted
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Old 11-22-2022, 09:25 PM   #16
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I would have a four position switch with each contact n/c at rest

Then I would use 4) 2 pole 15a relays, with ac120v coil, so it can not close without a source.

It can not pick it's own source unless you define a pecking order of sources.

I would then u
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Old 11-22-2022, 09:44 PM   #17
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If you can tell me the order you want the frig to choose from.... then you would need a Hand/Auto switch, and then a four position switch.
I think the rest of the logic can be contacts on the relays.
I would add push to test lights so you know where your powering from.
So you pick a source or it goes into automatic and goes through a preselected sequence.
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Old 11-22-2022, 10:35 PM   #18
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If you can tell me the order you want the frig to choose from.... then you would need a Hand/Auto switch, and then a four position switch.
I think the rest of the logic can be contacts on the relays.
I would add push to test lights so you know where your powering from.
So you pick a source or it goes into automatic and goes through a preselected sequence.
Think you're missing the point.

When there's shore power it switches to that.
When the generator is running, it chooses that.
When the Magnum Energy inverter is on, it chooses that.
When all of the above are off, it chooses the dedicated inverter.

If the dedicated inverter fails, I can turn on the Magnum Energy inverter, or the generator to keep the food from defrosting.

Ted
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Old 11-23-2022, 08:38 AM   #19
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I understand what you want to do, but what happens when your at the dock and everything is energized?

To work correctly, in auto mode, I would think:
Shore power
Generator
Inverter
Dedicated inverter

If shore power is not present relay can not energize so it goes to generator
If it drops out it goes back to Inverter, etc, etc.
You would need a few seconds time delay at all relays but the first, that will give each source a chance to settle.
Be aware this can be built pretty simply, but my guess your cost will be around $500 parts for this circuit. Maybe less if we can find a friendly supplier.

I think it would always go to the highest source when available. So you get to dock, plug in shore power, and it drops off inverter and goes to that. In Hand mode it stays where you select power there or not.
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Old 11-23-2022, 08:49 AM   #20
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Think you're missing the point.

When there's shore power it switches to that.
When the generator is running, it chooses that.
When the Magnum Energy inverter is on, it chooses that.
When all of the above are off, it chooses the dedicated inverter.

If the dedicated inverter fails, I can turn on the Magnum Energy inverter, or the generator to keep the food from defrosting.

Ted
It seems like what you’re saying is, if the ships ac system is powered up, the inverter is bypassed. Ac system off, inverter active. Correct?
Seems pretty simple.
You just need to decide if you’re going to cut the supply to the inverter or the output. I would choose supply side for less parasitic load. Means bigger relay though depending on how much current the inverter draws.
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