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

If you want an automatic solution, Xantrex sells a transfer switch for this purpose: https://www.amazon.com/Xantrex-8080...mzn1.fos.fa474cd8-6dfc-4bad-a280-890f5a4e2f90

Just to define the challenge, three sources.

Inverter 1
Inverter 2
shore/gen

and what priority do you want?

ksanders said:

Just to define the challenge, three sources.

Inverter 1
Inverter 2
shore/gen

and what priority do you want?

Click to expand...

Just 2 sources. The second inverter has a transfer switch between the inverter and the shore power / generator.

Ted

rslifkin said:

If you want an automatic solution, Xantrex sells a transfer switch for this purpose: https://www.amazon.com/Xantrex-8080...mzn1.fos.fa474cd8-6dfc-4bad-a280-890f5a4e2f90

Click to expand...

Yes, saw that. Would prefer to have replaceable relay(s) and spares onboard.

Ted

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.

twistedtree said:

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.

Click to expand...

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

O C Diver said:

Just 2 sources. The second inverter has a transfer switch between the inverter and the shore power / generator.

Ted

Click to expand...

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

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.

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.


Jim

O C Diver said:

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

Click to expand...

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.

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

O C Diver said:

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

Click to expand...

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.

O C Diver said:

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

Click to expand...

Not from the way you described your setup

sbman said:

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.

Click to expand...

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

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

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.

Choices said:

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.

Click to expand...

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

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.

O C Diver said:

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

Click to expand...

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.

Could be done with a small PLC, but cost would jump to over $1k. And, you could not mechanically test it, abd need a laptop, sort of a black box thing.

With relays you could test it mechanically in the field.

If your serious PM me your number so we can talk and I can figure exactly what you want, and I can tell you my favorite rum...

I think everyone is over-complicating this. The transfer switch only needs to switch 2 sources based on the original description. It just needs to switch between what we'll call "ship's power" and "dedicated inverter". The switching for shore / gen / inverter on the "ship's power" side sounds like it's already handled upstream of the transfer switch.

Sorry Ted, reread this from the start and to answer your first question you need a 4 pole normally open reversing contactor with 120vac coils good for 10+ amps at 120-240vac. An IEC style SqrD/ Schneider contactor will work and is pretty small, roughly two ice cube relays.

The reversing contactor will mechanically choose the source, and mechanically lock out the other source. you can add a simple time delay block so it prefers ships source.

Aux contacts will allow logic to work and indicator lights. You can still have a manual switch to force a source.

rslifkin said:

I think everyone is over-complicating this. The transfer switch only needs to switch 2 sources based on the original description. It just needs to switch between what we'll call "ship's power" and "dedicated inverter". The switching for shore / gen / inverter on the "ship's power" side sounds like it's already handled upstream of the transfer switch.

Click to expand...

Exactly what I was alluding to in my previous post. Very simple really.

Help me learn. The purpose is to keep fridge running.
-main power supplied by shore power to 'panel load' to all AC devices.
-There an auto start for generator if shore power stops, + an Gen auto transfer switch to 'panel load'.
-Two inverters fed from 'panel load' charging batteries, both set on invert mode & ready, but not feeding power when on shore or Gen 'panel load'.
-an auto transfer switch when 'panel load' stops engages invert load.
-dedicated fridge inverter continues to supply fridge until batteries die and then another auto switch over to 2nd inverter.
-When shore/Gen power returns 'panel load' takes over & auto switches to feed fridge directly.
-auto switch to turn off Gen when Shore power available.

Sounds like a job for a computer program controlling relays

Choices said:

Sorry Ted, reread this from the start and to answer your first question you need a 4 pole normally open reversing contactor with 120vac coils good for 10+ amps at 120-240vac. An IEC style SqrD/ Schneider contactor will work and is pretty small, roughly two ice cube relays.

The reversing contactor will mechanically choose the source, and mechanically lock out the other source. you can add a simple time delay block so it prefers ships source.

Aux contacts will allow logic to work and indicator lights. You can still have a manual switch to force a source.

Click to expand...

even this seems overly complicated.
i would consider more of a master control relay sort of thing. two contacts set for open, two set for closed. one 120 volt coil. those contacts are usually rated at 20 amps, so if he wanted to run the little fridge inverter dc power through one side of it, there's plenty of headroom.

The problem with that is both sources can be closing/opening at the same time as relay changes state. Then the best thing that can happen is you trip the breaker as a fault. The worst is you take out an inverter.

A mechanical interlock stops that. But you can only have two sources. A third source requires control relays and timers to make sure you are not powering from two sources out of synch. If that happens Ted will have a black spot where the relay was. And in this case, every source will be out of synch.

Choices said:

The problem with that is both sources can be closing/opening at the same time as relay changes state. Then the best thing that can happen is you trip the breaker as a fault. The worst is you take out an inverter.

A mechanical interlock stops that. But you can only have two sources. A third source requires control relays and timers to make sure you are not powering from two sources out of synch. If that happens Ted will have a black spot where the relay was. And in this case, every source will be out of synch.

Click to expand...

i'll agree there's a benefit for the mechanical interlock. i will also say that the closed contacts are designed to de-energize a few milliseconds before the open ones begin to close. with proper circuit protection it would be perfectly safe.
this problem has almost an infinite number of ways to manage it. i will let myself out of the conversation. there's enough engineers on it without me.

What nobody seems o have mentioned is, the neutral and ground must be connected together at the "source" and nowhere else. When operating off an inverter, these two conductors must be connected together. When operating on shorepower, this connection must be broken and the neutral to ground connection is at the marina's service panel, not on the boat.

You have to not only switch the hot and neutral conductors, you have to switch the ground as well.

rwidman said:

What nobody seems o have mentioned is, the neutral and ground must be connected together at the "source" and nowhere else. When operating off an inverter, these two conductors must be connected together. When operating on shorepower, this connection must be broken and the neutral to ground connection is at the marina's service panel, not on the boat.

You have to not only switch the hot and neutral conductors, you have to switch the ground as well.

Click to expand...

Any good inverter with a transfer switch takes care of turning the neutral-ground bond on and off based on whether it's passing power through or on inverter power. Each inverter source needs its own neutral bus though, so the transfer switch needs to switch hot and neutral between ship's power and the dedicated inverter (which will only have its dedicated neutral bonded to ground). Grounds are all connected together, so no need to switch those.