Saturday, February 24, 2018

Adding 240V Inverter Service

As discussed in more depth in this article, we wanted to be able to do laundry while underway without running our generator.  Laundry can take hours, and it's not really enough load by itself for the generator.  To run it off the main engine alternator required two things; 1) 240V inverter power, and 2) more alternator capacity to handle the steady increased load.  The alternator update was covered in this article, and this current article covers the inverter service upgrade.

I went around and around in circles on how to add 240V inverter service.  As a retrofit, I had to work within the context and constraints of the existing boat, creating the following considerations:
  • Any new equipment needs to fit somewhere, preferably with minimal relocation of existing equipment.
  • Minimize long, fat wire pulls.  This means any large DC cables need to be confined to the laz near the batteries.  And wire runs from the laz to the main AC breaker and distribution panel in the pilot house should be kept to a minimum.  It's a long, circuitous route with very little extra space for fat cables.
  • There should be minimal (preferably none) increase in idle background power loads.  There is a certain amount of wasted power in an inverter, and I wanted to keep that to a minimum.  Also, transformers consume power and I wanted to keep that to a minimum.  These losses are inevitable to some extent when actually powering a device, but when the device is turned off, I ideally wanted all those losses to stop completely.
  • Our existing 120V inverter system is capable of 7kw, so if there is a way to leverage that equipment, it would help solve a number of problems.
  • If new inverters were required, it was desirable to have them be the same make/model as our existing inverters.  This makes sparing easier, and utilizes the existing control panels.
  • The amount of rework of our breaker panel should be minimized.  We had a single 240V service to begin with, and the idea was to split it into two separate circuits, one powered only from shore power or generator, and the other powered by 240V inverter service.  Also part of this was dealing with the circuits that are 120/240 split phase, vs those that are pure 240V.
Like I said this sent me around in circles for a while considering different alternative.  Here are some of them:
  1. Add a separate, dedicated inverter for 240V service.  I could leave the existing 120V inverter service unchanged which was good.  It would also be an inverter that I could completely turn off when not used, so no background power drain.  But I would have to find space for the new inverter, necessitating fabrication of some sort of power panel in the laz.  Large DC cables would have to be run with fuses and disconnects.  And two 10/4 cables would have to be pulled from the laz to the pilot house (one for shore/gen input power, and one for output power), plus probably a control panel cable.
  2. Reconfigure my 120V inverters for 120/240 service.  This initially seemed very attractive.  I could reuse the existing inverters so no new equipment would be needed, and no new large DC cables would be needed.  But it created complications elsewhere.  In their 120V configuration, one of the inverters goes into standby when power loads are low, thereby reducing the background power loss.  When configured for 240V service, both inverters would be on all the time, roughly doubling the background power loss.  Also, both the AC input and output cables would have to be replaced.  The existing input cable didn't have a neutral conductor, and the output only had one line conductor.  But the worst part was the subsequent need to rearrange the 120V power panel to balance it across two split phases.
  3. Add an auto-transformer to create 240V off the existing 120V inverter service.  This seemed promising.  There would be additional power loss in the transformer which I wasn't thrilled about, but it would be limited to the 240V loads.  And as long as it could be turned off with a breaker, the losses would not be there all the time.  There would be no need to rewire the 120V panel, and if I could fit the transformer behind the pilot house console, there would be no need to pull wires back to the laz.  The only hang up was figuring out how to have breaker space for a master-disconnect for the auto-transformer and 240V service.
I ended up going with approach #3, using a 6kw auto-transformer from Outback Power, the same people who make my inverters.

Here is a before of the 240V breaker panel.  You can see all the 240V devices as part of a single service.

And below you can see the after picture.  The only things on the 240V Ships service are the water heater, battery charger, dive compressor, and input power to the inverter.  To the right is the 240V Inverter service with main disconnect at the top, and the load breakers below.  The washer, dryer, and oven can now be run while underway, or while on 50hz shore power.

And here's a wiring diagram.

So far we are really happy with the results.  It's great to be able to do laundry while underway without the generator, and the loads are comfortably within the capacity of the main alternators.

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