Inverter Capabilities

[Joe Cannarozzi]

Running air conditioning off of the inverter-I have a question for those of you that have some experience here.

If you dedicate 1 inverter to a 15000 btu a/c and a couple of outlets what would be the smallest inverter you could safely, comfortably, get away with?

Question for all
Please explain the advantages of a 24 volt inverter over a 12 volt one.

[Jon Wehrenberg]

Charging voltage of the inverter is not related to the AC (alternating current) wattage rating. The charging voltage is just a number and it depends on how the inverter chose to set up the house electrically.

As a charger the inverter receives shore or generator power, which is sensed and used to power the charger portion of the inverter, and which is also passed through the inverter to circuits dedicated to the inverter. Those circuits are totally isolated in the 120 VAC electrical panel from all other circuits For a reason I will explain.

So when power is available to the inverters they are nothing more than chargers and they close the circuit to allow shore or generator power to bypass and go directly to the buss bar in the 120 VAC panel.

When power is not availble from shore or generator power the inverters no longer function as chargers, but instead take 12 or 24 volt DC power from the batteries, and convert that power to 120 Volt alternating current with a frequency of 60 HZ or 60 cycles per second. That power is supplied to the circuits dedicated solely to that inverter.

The reason the output circuits for an inverter must be separated is because the alternating current frequency of 60 cycles per second will never match exactly with the other inverter's output. Why is the frequency so important? Principly because all AC devices rely on that frequency for a variety of reasons and if the frequencies are mismatched it is bad news.

So to get to Joe's question specifically.....The answer is dependent upon what circuits are supplied by the inverter. If the inverter supplies only an air conditioning unit, and the unit draws 13 amps (typical) 2000 watts is probably ample. If the same inverter also has other circuits on its 120 VAC output such as TVs, a refrigerator, outlets, a microwave, etc. then those loads are also drawing power, possibly exceeding the inverter output.

Part of the answer is energy management. If an inverter has a limited output as all inverters do, then it is up to the person controlling the power to insure that output is not exceeded.

There is another consideration. Usually when inverters are used to power air conditioning units they do so while the bus engine is running. Let's play with some numbers to understand what is happening. On a 12 volt coach if it is set up so 2 air conditioners can run from inverters the inverters are supplying at least 13 amps per air conditioner at 120 volt AC. The bus may be set up with a 270 amp 12 volt alternator for house battery charging. At 12 volts the draw on the batteries per air conditioner is about 130 amps, times two which almost equals the rated output of the alternator, or 260 amps. Run anything else from the inverters and now the alternator is maxed out and the batteries are slowly discharging. The point here is that with big enough inverters almost any desired loads can be powered, but how much sense does it make to be driving down the highway discharging the batteries?

Maybe instead of big inverters it might make more sense to run the generator.

[Joe Cannarozzi]

I agree about running the Gen to do it.

It has also been suggested to me that since the bus in question only has duel 2500 watt inverters and is only capable of running 1 a/c even if it functions properly will not adequately cool the bus anyway.

Here is an 02 XLII with only prevo drivers air and one house a/c set up to run off inverter 2500 Watt inverters and he can not get the inverter to even start the a/c.

He is thinking his solution is to replace one inverter with a larger one but IMO just running the gen to cool the bus underway would be a better solution than all the expense to increase inverting capacity and he still is left with a 45ft bus with only dash air and 1 house a/c and still not have enough a/c to be comfortable.

Here is an example of really knowing what you need and what your getting when you buy.

I would still like to have it explained the different operating characteristics between a 12 volt inverter and a 24

[Jon Wehrenberg]

Joe, Generally designers select 24 volt over 12 volt because as the voltage increases, the amperage goes down, permitting lighter gauge wire to carry the loads

To express it in the context of a bus, a 120 V AC load such as the air conditioners being discussed is 13 amps. That same load, ignoring losses in the inverter requires 130 amps of 12 volt power, or 65 amps of 24 volt power. That not only reduces the size of the cables or wires if that was the only load, but our big alternators are usually 270 amps at 12 volts or 24 volts depending on how the converter set things up. Clearly running 2 air conditioners off a 24 volt DC circuit pulls 130 amps, leaving sufficient alternator capacity to do other work. If it was a 12 volt alternator then the load would be 260 amps.

With today's electrical loads for cars I am surprised that cars have not switched over to 24 volt. On the bus (which was designed for seated use) there are heavy lighting loads plus the loads of the air conditioning (OTR) evaporator and condensing fan motors.

Planes in the past were originally 12 volts but I think all of them now are 24.

[Petervs]

Joe,
A 12 Volt inverter is used to charge a 12 volt battery bank, and a 24 volt inverter is used to charge a 24 volt battery bank. The reverse also applies, the 12 or 24 volt bank provides the source of DC to make 120 volt AC. It is not more complicated than that.

My Marathon has house batteries wired as 24 volts. They send power to the inverters at 24 volts. There is a step down to provide 12 volt power to a few things from the house bank ( like the automotive stereos) but the bank feeds the inverters at 24 volts.

Marathon chose this set up so the 270 amp 24 volt alternator supplied by Prevost could charge the house bank and the chassis bank directly at the same time. The chassis bank is 24 volts to power the starter primarily. Using a higher voltage the wiring can be smaller for the same power draw.

Using Jon's example above, with a 24 volt alternator (270 amp) you can easily run 2 roof AC from the inverter and have power to spare. However, I still prefer to stay out of hot places where I need that!

[Joe Cannarozzi]

I thought I once heard that a 24 volt operates more efficiently than a 12 but do not know that to be true nor understand why that is, if it is true.

[Jon Wehrenberg]

You need to define efficiently.

Power companies send extremely high voltages across the country because the higher the voltage the lower the amps and thus the smaller the wire.

If the power lines were all at 120 volts we would need some huge cables to carry the loads, so in that sense higher voltage is more efficient.

[truk4u]

Joe,

To answer part of your original question, I think Marathon does it best with (2) 4000 watt inverters and they handle two A/C's very well, running or not. You do have to have the battery horsepower though to make it all work.

[rfoster]

I have to ask as I do not know the answer to this question-

What is the down side to a 24 volt system? Why do some coaches have 12 volt house lights and others a 24 volt system?

Having now owned both a 24 volt and now a 12 volt - I must admit the 24 volt is the more desirable with lighting (brightness) and for lack of a better adjective at this time the functions just seem to be "snappier" .

Since the 24 volt system according to previous comments does not require heavier wiring - is it possible to convert the 12 volt lighting to 24 volts and not blow the house computers? What would be involved in converting the lighting to 24 volt? Is it a big deal and something you don't want to fix if it ain't broke?

[dale farley]

Roger, I would think this could be a major problem without some significant programming. I would vote for leaving it as is.