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Author Topic: How to power a Carrier AirV Roof top AC  (Read 5399 times)
ojgetaway32
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« on: August 20, 2009, 11:20:27 AM »

Okay guys, I am reluctant to ask as I am sure this topic has been beat to death, but alas, here goes.  I have one Carrier AirV 13,500 BTU Roof Air that is brand new in the box.  I mounted it, hooked it up to an extension cord off my shop and it cools wonderfully, you could hang meat inside now.  However, I cannot get it to run off of either a 1575 watt inverter or a brand new 2500 watt Cobra Inverter I just bought at the truck stop.  The fan will blow, but when it tries to kick the AC on, the inverter trips and shows power overload.  This is wired to a 12V sytem with a bank of four batteries.  I heard someone mention a hard start capacitor.  Is this an option?  Do I need a bigger inverter?  A genset is not feasible as this is all that I want to run for now.  I don't know what modified sine wave and pure sine wave is either.  What can I say, I am not educated on such matters.  Please help.

Thank you,

Josh Miller
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Josh Miller, Attorney, hockey player, son, brother, friend and busnut...
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« Reply #1 on: August 20, 2009, 11:34:23 AM »

You need a 3,000 watt Inverter minimum just to get the compressor to start
and then keep it running for the 15 minutes until the batteries are down to minimum
level and the inverter shuts down.

When I say 3,000 watts, I don't mean the cheap ones that have outragious claims either. More like a Heart/Xantrex/Prosine or similar.

You need to be able to handle 15 to 18 amp loads continous to run a 13,500 btu
a/c unit. Preferably a Pure Sine Wave ( Expen$ive ).

The inverters of that kind cost almost as much as a generator.

I doubt the cheapie inverters will even be worth the effort. Maybe a 2,500 running a small 5,000 btu window unit would work for a little while until the smoke escapes.
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ojgetaway32
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« Reply #2 on: August 20, 2009, 11:42:47 AM »

Fiftenn minutes?  But if the engine is running, the inverter wont shut down right?
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Josh Miller, Attorney, hockey player, son, brother, friend and busnut...
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« Reply #3 on: August 20, 2009, 11:45:09 AM »

Josh,

I am not an electrical guru, like Sean or somebody, but it does sound like your inverter is too small. If you got a bigger inverter, then that would probably do it. We are only planning on running one or two of our roof airs off of our two inverters. We have to 4000 watt Trace inverters, that are Pure Sine wave. We will mostly be running our roof airs off of the generator, so not to tax our batteries and inverters, but I am sure that what you are trying to do will work.

BTW, are your four batteries the ones that your bus starts off of, or are they dedicated house? What size batteries are they?

God bless,

John
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« Reply #4 on: August 20, 2009, 12:03:33 PM »

Josh,
    Most ACs require a 20 amp circuit, that is 2400 watts (120 volts X 20 amps).  2400 watts divided by 12 volts equals 200 amps, so you will need to maintain 200 amps output from your alternator and this does not allow for inverter ineffeciencies or less than optimal alternator speed. If your coach is 24 volt and you install a 24 volt inverter, you would need a 100 amp alternator.  
    Wires from the inverter to the batteries should be 3-0 or 4-0 to allow enough power to flow from the batteries to the inverter.  Jack
« Last Edit: August 20, 2009, 12:07:38 PM by JackConrad » Logged

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« Reply #5 on: August 20, 2009, 12:22:29 PM »

Modified sine wave has a wave form that is very square in steps, creating less then perfect A/C output.  Pure sine wave, while still have square steps in the sine wave, have the steps much smaller and finer closer duplicating a true since wave form.
Call Dick Wright at Wrico International and he'll most likely suggest the Magnum inverter/charger pure sine wave.  They make a 2800 which has good surge motor starting capability.
And yes you could get a hard start kit that includes a capacitor to ease with starting the compressor.  Good Luck, TomC
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« Reply #6 on: August 20, 2009, 12:28:04 PM »

Josh,

What everyone has said.  The cheap inverters are rated by lawyers, not engineers.  What lies can we get away with and which ones will cost us.  That inverter is probably rated at 2500 watts into a resistive load like a toaster, not an inductive load like an air conditioner. They will never give you what you want.

The alternator (assuming a 50DN or equivalent) will work fine at road speed but idling through a 30 minute traffic jam will kill the batteries in no time.

Getting educated by buying the wrong stuff is very expensive.  You could ask any one of us how we know that.
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« Reply #7 on: August 20, 2009, 12:42:59 PM »

In layman's terms, motors and generators are almost the same thing.  When a motor is running a normal speed, it's using some electricity, but it's also generating a lot of electricity that it sends back at the source.  That is called counter electro-motice force, and the only reason it's interesting is that when the motor isn't turning, like when you first turn it on, or if it stalls, the CEMF isn't there and the motor the draws a TON of current, like three times normal.  If your source can't supply that, the motor never gets going, and the breaker trips off.  the capacitor kind of stores up some electricity and uses it like a little battery to help get the motor turning - as soon as it starts to turn, things get to normal really quickly.

the thing about modified sine wave and sine wave is that a lot of things take the electricity and immediately do something with it, like a TV or a radio puts it into a power supply that changes it to something else.  They tend to run OK on modified sine wave output.  A motor uses it directly as it comes.  Since it is developing it's own electricity as CEMF, and for some other reasons, it runs really poorly on the square wave electricity that a modified sine wave inverter puts out and so it uses more  electricity and can actually cause the motor to overheat and damage itself.

hopefully that made sense, it's been a lot of years since electric circuits 101...

Brian
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« Reply #8 on: August 20, 2009, 12:57:06 PM »

The Carrier A/C units use a lot less than 20 amps of power.  I would have to look it up for sure, but I believe my 15K BTU Carriers are rated at 13 or 14 amps.  They need to be on a 20 amp circuit, but that is more for proper breaker and wire size.  A 20 amp circuit should really have a sustained load of 16 amps.

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« Reply #9 on: August 20, 2009, 03:16:07 PM »

LEN SILVA - you wrote -->
 "...The alternator (assuming a 50DN or equivalent) will work fine at road speed but idling through a 30 minute traffic jam will kill the batteries in no time......"

Len - you need to do a little resarch before you make BLANKET statements like this

The typical gear driven 50DN is geared at 2.93 to 1 -

at an engine speed of 600 RPMs - a 50DN spins at approx 1800rpm and can produce a max REGULTED output of approx 4500 watts ( 165 amps at 28 volts)
at an engine speed of 1000RPMs - the max REGULATED output jumps to almost 7000 watts

even at IDLE - that's more than enough to overcome any inverter loses / still charge the bus batteries / and run a standard 13kbtu-15kbtu rooftop air conditioner

I'm just getting damn tired of all the MISINFORMATION


and yes I can back this up with my maintenance manual spec for the 50DN


Pete RTS/Daytona
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« Reply #10 on: August 20, 2009, 05:17:43 PM »

Pete,

My Bad.

That was my experience with one 12 volt Eagle Entertainer, and I never got to fully check it out before it went out on another trip.  Could have been a bad alternator, regulator, loose belts etc.  Driver said he was stuck in traffic for an hour and the inverter kicked out for low bat.  Came back when he got back up to speed.

So, I'm sorry for any mis-information I put out.

I don't know what kind of bus Josh has but he did say 12 volts.

Len
« Last Edit: August 20, 2009, 05:21:04 PM by Len Silva » Logged


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« Reply #11 on: August 20, 2009, 06:46:35 PM »

Come on Guy's...

The original poster said he was new to this and didn't know anything about inverters.

He asked if it would work. Which it didn't for reasons that the rest of us are aware.

He didn't specify if it was a bus, truck or golf cart for that matter.

He did say he bought off the display inverters which mostly are 12 volts.

Nobody needed to jump in and start a war over a 50DN alternator or why or how
something should/shouldn't, what output or anything else that I saw.

The bottom line is simple: He is using 12 volts, He needs enough battery amperage
to handle a 2400 to 3000 watt inverter to run his A/C ( Theoretically ).

A standard Automotive type alternator might not have enough output to run
with a large load. Since he didn't say what he has and how big the alternator is
we are in a blind spot here.
A Bus alternator or big truck alternator may be of
sufficient capacity to operate his rig, But there are way too many other things
that must be considered.

So, That said ... Have a Nice Day and Be Nicer Please....
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« Reply #12 on: August 20, 2009, 07:49:19 PM »

Hi ojgetaway32,

Welcome!

Just to add to all the great replys you recieved,, you also have to remember that these units use more current/amps as the outside ambiant

temp rises.. 14 amps is normal at 80 degrees. Here in N.J. today, it's 92 degrees and my 1- 15,000 btu unit is drawing 18 amps. Two days ago it

was 96 degrees and it drew 19 amps. Just doubble that for start up power..  This should give you a good idea what your power needs should be.

Good Luck
Nick-

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« Reply #13 on: August 20, 2009, 08:12:58 PM »

Just to set the record straight - Dave - I responded to some misinformation about a typical 50DN - and that's the way I framed my response
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« Reply #14 on: August 20, 2009, 08:39:17 PM »

I doubt it will pan out but I am looking for a surplus rotary aircraft inverter that might do the job. I think that most of them were 400hz. I will see what I can come up with. I sure hope you can get it working I have had my share of troubles in the AC dept, mostly self inflicted  Grin
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