Inverter question?

[5B Steve]

   I thought I would throw out this question, Is it possible to run 2 roof top Ac's off a large inverter?  I want to run these without

running the generator while driving.  The 2 I have, one is 15,000 BTU (14.3 ) 13,500 (8.3 amps )  the alternator in my bus I

  Believe is rated at 270 amps? How big of an inverter do I need?

 
  Steve 5B.......

[TomC]

Just get the biggest one available-which I believe is 4000 watts using 24vdc.  You could run two 2800 watt inverters if they are on separate circuits.  Good Luck, TomC

[bevans6]

My understanding is that Sean can run two AC's using his Trace 4024 pure sine running from the bus alternator, but his system is also very sophisticated and can load share input from a ton of batteries.  I think, as long as you have some battery to help the alternator with surge loads and your cabling can support the current draw, that you could do this as long as you never end up trying to start the condenser motors at the same time.  I am in the process of getting a 3000W pure sine inverter to run one AC unit basically from the alternator backed up by the two 8D start batteries.

I find that my 5C is OK with one AC particularly if the door to the rear half of the bus is closed.  But then I only have one AC, so I don't have a comparison to make...  

Edit:  having invoked Sean above, I hopped over to read his blog and found this inference in his most recent post:  "It's already in the 80s here today,  and climbing, so in a few minutes we'll fire up the Detroit, turn on some air conditioners, and get under way. "
Synchronicity!

Brian

[BobBoyce]

I recommend going with a pair of smaller true sinewave inverters rather than a single larger inverter if you can afford to. Modified sinewave inverters will damage compressor and fan motors on A/C units. Having seperate inverters will mean that an inverter failure won't leave you totally without power and air conditioning.

Inverters will need to be sized to handle the surge of full LRA (Locked Rotor Amps) of a starting A/C unit, and a single inverter installation would need to handle a much higher surge rating to handle the simultanious LRA or both A/C units at the same time in case the compressors happen to kick in both at the same time. This means a much larger single inverter, if you choose to go that route. Splitting the loads up to a pair of inverters means that each inverter could be of a lower capacity rating. Please remember that the higher surge that an inverter must provide for, the larger the power source leads will be required to feed power to that inverter.

I have seen several inverter failures in off-grid systems that were suspected to be caused by excessive voltage drop of undersized power leads to the inverters. In an undervolt condition like that, most modern inverters will attempt to draw more current to compensate for lower input voltage. More current means even more voltage drop, ect. I use 3/0 wire between the 500 Amp rated DC panel and the 500 A/H 48V battery bank on my solar power system at home. The continuous 3.6 KW rated inverters themselves are fed with very short 1/0 power cables from the inverter breakers in that DC panel.

Bob

[Sean]

... Is it possible to run 2 roof top Ac's off a large inverter?  

Yes, as noted below, we do this all the time.  With our load-supporting inverter, we can even run three in a pinch; two is easily done with plenty of power to spare for charging the batteries from the big 50DN alternator.  Just make sure you have large enough cables, and use a true sine wave inverter.

There are several hundred posts on this exact subject in the archives.

My understanding is that Sean can run two AC's using his Trace 4024 pure sine running from the bus alternator, but his system is also very sophisticated and can load share input from a ton of batteries.  I think, as long as you have some battery to help the alternator with surge loads and your cabling can support the current draw, that you could do this as long as you never end up trying to start the condenser motors at the same time.  ...

Actually the sophistication of my system does not really come into play here, nor would we really need much battery to achieve this, as two roof airs is well within the capacity of the 50DN.  Remember, this alternator is rated at better than 6.5 kW -- lots of rigs have entire generators smaller than that, and in fact my old Fleetwood ran two roof airs (and everything else) on a 5kW genny.  I don't think the compressors (not condensers) starting together would even faze it.

I recommend going with a pair of smaller true sinewave inverters rather than a single larger inverter if you can afford to.

I disagree with this recommendation.  In addition to nearly doubling your costs (not only two inverters, but two of almost everything else, too), you will pay for this decision in efficiency.  a 2kW inverter operating at 90% capacity will be maybe 85% efficient, whereas a 4kW inverter running at 45% capacity will be closer to 95% efficient.  When you are running both A/C's at the same time, the efficiency of both methods would be the same, but when only one A/C is running (or none, just other loads) having a single larger inverter rather than multiple smaller ones will mean you will use less battery or alternator capacity.

Modified sinewave inverters will damage compressor and fan motors on A/C units.

This part is true and I have said this many times here over the years.

Having seperate inverters will mean that an inverter failure won't leave you totally without power and air conditioning.

Usually an inverter failure will not leave you without air conditioning anyway, since most of us also have an on-board generator.  So redundancy in this case would not be a cost-effective justification for multiple inverters.

Inverters will need to be sized to handle the surge of full LRA (Locked Rotor Amps) of a starting A/C unit, and a single inverter installation would need to handle a much higher surge rating to handle the simultanious LRA or both A/C units at the same time in case the compressors happen to kick in both at the same time. This means a much larger single inverter, if you choose to go that route. Splitting the loads up to a pair of inverters means that each inverter could be of a lower capacity rating.

While on the surface this argument is sound, in practice most RV inverters have a surge capacity that is double the continuous capacity.  So a 4kW inverter has an 8kW surge rating, whereas a 2kW inverter will have a 4kW surge rating.  So from a surge capacity standpoint, there is no difference between a single 4kW unit and a pair of 2kW units -- the combination still gives you 4kW of continuous power and another 4kW of surge capacity.

I have seen several inverter failures in off-grid systems that were suspected to be caused by excessive voltage drop of undersized power leads to the inverters. In an undervolt condition like that, most modern inverters will attempt to draw more current to compensate for lower input voltage. More current means even more voltage drop, ect. I use 3/0 wire between the 500 Amp rated DC panel and the 500 A/H 48V battery bank on my solar power system at home. The continuous 3.6 KW rated inverters themselves are fed with very short 1/0 power cables from the inverter breakers in that DC panel.

Here again, power is power is power, there is no free lunch.  You need the same amount of copper to provide 8kW (at a given voltage) whether it's one or two units.  In fact, it will generally be less expensive to run a single 4/0 cable to the larger unit than two separate 2/0 cables to the smaller ones.  Here again, at lower loads, the voltage drop on the single larger cable will also be much lower than on the smaller cables, magnifying the efficiency difference.

I would suggest to you, BTW, that your 3/0 cable is marginal for 500 amps.  4/0 is called for if the run is longer than a couple feet.  This is especially true in a bus conversion (as opposed to fixed residential use), where the cables are likely to be enclosed or otherwise poorly ventilated.

FWIW.

-Sean
http://OurOdyssey.BlogSpot.com

[JohnEd]

I never hear this mentioned any more and it surprises me:  There was a device on the market that was used to control the starting of the compressors in two AC units.  It prevented two from starting at the same time.  3.5 KW should run a couple 15KBTU roof tops.  Starting one with the other running would take that to 5.5KW....without putting too fine an edge on it.  Starting both simultaneously would take 7.2 Kw.  I would prefer to stay out of the inverters "over load" capacity as much as possible and I, as well, would not go any deeper into that OL capacity than absolutely needed.  Even though a 4KW would handle two AC units running uncoordinated I, personally, would control them.  Load your electronics as little as possible and a point or two of efficiency isn't my issue.  Like anything else, they work longer if maxed out less.  Heat is  killer.

So where is that load coordinating device?  It wired into the compressor start circuits and acted like an OR gate with a pause on the second.

John

[Sean]

...  There was a device on the market that was used to control the starting of the compressors in two AC units.  ...

I'll try to answer this in the other thread on the subject...

-Sean
http://OurOdyssey.BlogSpot.com