On 26/02/2016 18:16, David wrote:
On Fri, 26 Feb 2016 17:03:13 +0000, Fredxxx wrote:

On 26/02/2016 10:59, David wrote:
This first post is a place holder in part, so that I don't forget to
ask the question, as I usually do once I get involved in stuff during
the day.

The Motor Home has mains powered habitation air conditioning - that is
an ELectrolux slug on the roof.

As soon as we know the spec, it might be possible to assist.

So for it to work you need to be on a site with mains electricity.

There is also the issue of the power surge on start-up compared to the
power demand on normal running.

What I would like to be able to do:

(1) Run the A/C whilst driving - that would I assume involve an
inverter which could take power through the 12V electrics buffered by
the habitation batteries (unless the demand needs a direct connection
to the alternator charging circuit instead of via the charge controller
which charges the habitation batteries). This also allows starting the
engine, firing up A/C and then stopping the engine and letting the A/C
run on using the habitation batteries.

For a narrow boat I installed a 3kW inverter (with surge capability),
150A alternator and 4 100Ah batteries. It can be done though
challenging. I doubt you'll have the space to fit a second alternator.

(2) Run the A/C when away from mains power - using a small Honda
generator which might be able to meet most of the demand apart from
start-up. I am envisaging perhaps the generator pushing power into the
system whilst the A/C takes power out so that the use of habitation
battery charge is slowed. This probably equates to running a UPS (that
is, power in to UPS for charging, power always out of UPS for running
the device) and using a small generator to keep feeding some power into
the UPS during a power cut. The generator may not fully meet the power
demand but it slows the rate of discharge.

Again without knowing the rating of the air con, and the size of your
portable gen, we're in the dark.

This does seem to demand a lot of inefficiency, though, potentially
with the generator input being converted from 240V to 12V then back
again. It would be nicer if the battery 12V power could be used to
boost the 240V input from the generator so that most of the power comes
directly in at 240V.


Bottom line is that one way may require blending two 240V inputs into a
single output, with obvious (I think) requirements to lock the wave
forms of the 240V together.

You cannot "blend" 2 AC voltages.

Can nobody see my post of 12:29 today?
This contains the details, but I keep getting "as soon as we know the
specs" responses much later in the day.

My apologies.

That sort of power is doable; 4 a 100Ah batteries at 12V at 750W, taking
into inverter efficiencies and battery characteristics might give you 2
or 3 hours at most. Your biggest problem is how to charge them back up!
I believe a generator would be a better solution.

With regards to blending two AC voltages, how does solar from the roof
blend into the UK grid, then? Thought it was pushed out of an inverter and
synced with mains - depending on how much you are drawing at the time you
should get all inverter (plus some pushed back up the wires) or part mains
and part solar. Or all mains if it is night time!

You are converting the DC power from the PVs into AC using an inverter.
They are purpose built. You cannot just join 2 AC sources, it either
requires some form of inverter or "manual" synchronisation of a
generator to a source. I doubt the inverter will survive as it is most
likely a modified sine wave type.

Or am I missing something about how solar power is delivered from the
rooftop?

Cheers

Dave R