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Old 09-08-2009, 11:32   #1
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s/v Jedi's Avatar

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Join Date: Feb 2009
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Boat: Sundeer 64
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A Different Setup for AC System Aboard

We are halfway through a project installing a new AC system aboard Jedi. The setup we have chosen is different from the regular but with many advantages, so I decided to post this. If you are thinking about re-doing the AC system, this can be interesting for you!

This setup is not for a 30' daysailer. Let's say from a 42' live aboard boat up. It also assumes you will be spending some time at docks/marina's around the world in addition to being anchored.

First, our old setup:

shore power: a 120V 30A inlet connects to a double-pole 30A breaker, on to a 3600W isolation transformer, which connects to the familiar rotary switch in the cockpit with Gen-Shore-Off positions.

genset: a 120V 50A Northern Lights genset connects to a double-pole 50A breaker and on to the same switch in the cockpit.

boat: from the cockpit switch wiring runs to a double pole, 50A breaker in the main switch panel. From there to a Xantrex 3000W inverter charger and on to the distribution switches in the main switch panel again.

exception: the watermaker is a big AC unit and located in the lazarette. It is wired with breakers that connect to the output of the rotary switch in the cockpit, not back to the main switch panel.

This is a fairly standard setup. What I don't like about it is:

- The exception for the watermaker. I want all power off when I flip the main breaker at the switch panel.

- The big start-up surge from watermaker and A/C which means that you need to switch consumers on in a specific order to avoid problems.

Other, specific to us, problems:

- We blew some diodes in the inverter/charger. As I never liked the unit ever since I bought it, I wanted to replace it with something else instead of repairing it.

- The isolation transformer belongs in a museum (really big and heavy) and isn't really an isolation transformer.

- While replacing the washer/drier years ago, we looked at better units but they were all 240V 60Hz. We decided to keep it to the simple 120V model because balancing loads over the two hot wires for the generator is so much trouble.

While researching the inverter/charger market and all other available options, this is what we came up with:

shore power: Same inlet but using self-build adapters for ashore that fit any/most outlets in use, preferring a 240V outlet instead of the current 120V (which can be used if 240V isn't available). You need to build these adapters yourself as you take the two hots and not the neutral like all standard adapters do and feed that through a 30A 3 conductor cord (protective ground is 3rd conductor). You must have an transformer to do this safely. Good reason to buy one if not already on board.

Once aboard, the same double pole 30A breaker is used. After this, it is fed into a new 3600W isolation transformer from Victron. It's input windings are configured to match the source ashore (120V or 240V). It's output windings are always configured for 240V. The output is connected directly to the main switch panel, i.e. the cockpit selector switch isn't used anymore. The Victron transformer has breakers that limit the current to 32A at 120V or 16A at 240V. Both output conductor are hot, floating, so not connected to ground. The ground from shore stops inside the transformer (like with any real isolation transformer) and the boat ground connects to the transformer housing.

genset: The current economy allowed us to buy a brand new Northern Lights 6 kW genset for a great price ($7,600 instead of $13,000). It's the same model as the old one that has 5,200 hours on it without any trouble. It will be scavenged for parts. The only difference I found is the air intake and air filter; all the rest is identical.

The genset will be configured to output 240V 25A at 60 Hz. Only hot1 and hot2 are connected, the neutral isn't used. This means it's load will always be perfectly balanced over the windings. The two output conductors are both hot, floating, so not connected to ground. The engine block is grounded to boat ground. The output connects to a new double pole 25A breaker and from there to the main switch panel. The rotary switch in the cockpit isn't used anymore.

boat: The main switch panel already had a selector mechanism for shore or generator. The double pole breakers for that are replaced with 25A for genset- and 20A for shore power versions. There is still no neutral and both hots are floating.

Each input has it's own Blue Sea Systems digital AC multimeter that shows voltage, current, frequency and wattage. They do 240V even though some shops list them as 120V only.

The output of the "selector breakers" is fed into a new Victron 3000W inverter charger. Now, this is where it gets interesting. You need a European version for this. These units can be configured for both 50Hz and 60 Hz and for several different voltages, incl. 120V and 240V. In our setup, it will be configured for 240V 60 Hz. Also, all inverter/chargers have an internal relay that connects the output neutral to ground when the unit switches to inverter-mode. This relay must be disabled (= configuration option) because we still only have two floating hots and no neutral.

The output of the inverter/charger is connected to two hot bus bars. From there to the distribution panel where several double-pole breakers are installed to feed 240V consumers.

To the same hot bus bars, a Victron auto transformer is connected. An auto transformer is a single winding with a center-tap. It's a "small" device. The winding is connected to the two hot conductors with 240V across them and the center tap becomes a new neutral. The voltage between the center tap and any hot is 120V. The interesting thing about an auto transformer is that only the imbalance part of the current runs through the winding. Example: loads connected between hot1 and neutral draw 10A, while loads between hot2 and neutral draw 20 A. The current through the auto transformer is only the difference, 10A. The reason for this gets to technical for this post so I will explain it in a simplistic way: it's much like when you connect two 6V batteries in series to create 12V. Here, we connect two 120V AC devices in series to a 240V source. Also, the two hot's are in counter-phase (a 180 degree phase shift, just forget this if you don't understand it because it's not important enough to worry about). If there's a lot of interest in this, I can write a separate post about how this exactly works, much like I did for the isolation transformer.

So, now we have a brand new, self made, neutral. However, code's require that it must be grounded, so it's connected to the boat's ground.

The neutral is also connected to a bus bar at the main switch panel and all 120V consumers are roughly distributed over the two hot leads and the neutral, like for standard 120/240V systems.

The big motor of the watermaker can be jumpered for either 120V/208V/240V so it's being reconfigured to 240V service. A neutral is still needed because I don't feel like replacing the 120V boost pump.

The 120V A/C unit knew it's days were counted because it blew all of it's refrigerant into the raw water side of the heat exchanger. We have a nice 240V 18kBTU CruisAir on the way (not so good price unfortunately, it seems A/C unit sales are not much influenced by the current economy!)

We have another, smallish 50A Xantrex battery charger than can do 240V (auto-sensing). The washer/drier is still going strong but will be replaced with a 240V model when it's time has come.

These high load consumers running at 240V instead of 120V means that they use only half the current, run somewhat cooler and the start-up surge is much less. But I have saved the best thing on that front for last:

The Victron inverter/chargers have a feature that can ADD power to genset or shore power when it is needed. So, if you are using 25A on a 30A shore power connection, you can just switch on another 15A device. This is not just that the inverter/charger reduces the battery-charging like most units can do... no, it will actually take power from the batteries and add that to the feed from shore or genset. It keeps it's inverter on-line and synchronized to the incoming AC and just switches it into "the grid" when more power is needed. This means that with a 3,600W shore power connection you can run 6,600W load (3,000W from inverter added) and with a 6,000W genset you can run 9,000W !! This takes all worries about start-up surges away.

Another consideration for cruisers with an AC watermaker: what happens when the genset breaks, can you still make water? When you have an inverter big enough to supply power to the watermaker plus the alternator(s) on the main engine(s) that can produce at least the same amount of power that is consumed by the watermaker, you have a good backup. The inverter takes it's input power from the alternators in that case, instead of from the batteries (as always, the source with the highest voltage supplies all the power when multiple sources are connected in parallel)

Hope this is of interest or even help for anyone reading this!

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