Researching a 12-24v dc variable speed A/C compressor for a roof top

[Nick Badame Refrig/ACC]

I found a company who handels a compressor that fits our needs.

I wish I had more time right now to study this, but I'm jammed up with my customers.

I'll supply the link and the co.  Maybe one of you can do some reading on theese variable comps.

Then I can easily come up with a prototype.. It looks like theese compressors can run from 9 to 30v's DC. Thats great!

It looks like a 9400 comp will put out 14,000 btu's. and of course needs a controller.

http://www.masterflux.com/

Nick-

[Don4107]

About ten years ago I needed to replace the refer in my boat.  It was a typical 'cube' Norcold.  I stumbled onto a 12VDC only Dometic unit at the local RV parts dept when trying to order a replacement.  Some one had ordered it and then backed out.  Instead of the DC to AC inverter system that I thought all the boat units used, it has a 12VDC compressor.  Think it may be the one used by Sunfrost.  It has a control board and may be a brushless design.

If memory serves, the old inverter type used about 4.5 amps average.  This Dometic uses about 1.5 amps.  It made a big difference in how long we can anchor out without charging our batteries. One thing I had to do was wire around the converter directly to the batteries.  It would not start and run on the 12VDC converter.    This was pointed out in the manual.

Don't know if they still make a refer with this system.  Ours has been trouble free, but has not seen a whole lot of use since we started messing around with buses!

[Nick Badame Refrig/ACC]

Hi Don,

The Dometic refer you are referring to uses the 12v danfros compressor that is less then a 1/4 hp and that unit

has been around alot of years on boats. But too small for air conditioning.

Thanks
Nick-

[Nick Badame Refrig/ACC]

Here is a research report on class 8 tractors using electric dc air conditioning.

This study involved the us army and Masterflux

http://www.masterflux.com/userimages/2006-01-0165.pdf

Nick-

[Tim Strommen]

Danfoss makes most of the DC compressors I've seen in marine, RV, and solar "back-woods" refridgerators.

Their collection of DC compressors can be found here.

The typical size for boat or RV refers is the popular 35 or 50 (sometimes the 80 for larger boxes), however apparently due to demand for DC refridgeration, they introduced the 250 and 250-duo (two 250's attached to each other).

These are used with the Danfoss controllers which will do 12 or 24 volts DC and have variable speed functionality and/or fixed speed control to adapt to the compressor load.


I don't work for Danfoss, own any Danfoss products, but I've installed a few refers with these compressors in them, in a few cabins.

Cheers!

-Tim

[TomC]

Running a 14,000btu 12 or 24v DC air conditioner sounds good on paper.  But you'll have to run enormous cables to combat the voltage drop from the high amperage you'll run. If an AC powered air conditioner takes a 20 amp circut, then you'll have to run 100 amps at 24v or 200 amps at 12v.  While that works out on paper, thats a tremendous amount of amperage to be running on a continuous basis for air conditioning.  There is a real good reason why we don't run DC power at our homes (and it was the first type of electricity available)-and that is DC power does not travel well over any kind of distance-and at 12v or 24v, from front to the back of the bus is enough to make a difference.  Plus, DC can be more dangerous.  If you're shocked with DC, it will hold your muscle tensed, but AC will jolt you an usually can pull away.  Personally, I like to stick to the tried and trued.  AC roof top air conditioning is about as reliable as it comes.  And if a roof top A/C fails, it can be replaced in about an hour with a new unit.  Just my opinion-I realize everyone here is trying something new.  Good Luck, TomC

[oldmansax]

I'm guessing Nick is probably thinking about using 12V compressors on basement air not roof top. That would shorten the cables.

[Homegrowndiesel]

Hey Nick

I had not seen that company yet.
I have looked for dc driven ac units for quite some time.
Check www.solartechnologies.com.
That is one of the units I have found. They claim their "unit only consumes 25 % compared to it's AC counterpart".

I installed the 13,500 btu high efficiency 110volt rooftop unit that draws 10 amps @ 110 and 100 amps @ 12 volt. ( ok more because of the losses in the conversion in the inverter )

[Homegrowndiesel]

Wrong Button I guess. Windows is my Excuse!

Anyway, I am glad to see your research. I have seen plenty more but none that I have invested in YET.

There is an Australian unit that looks to be taking off also.

Keep up the alternative thinking Nick.

Those Danfoss units really work good for a freezer or refer but as noted have no capacity for a bus.

Direct drive, belt drive, are most efficient, but quite noisy when you want to sleep.
DC can be stored (Safely), no loss in conversion, I am with you.

I do not think this is the one, though I am probably Wrong.
If the $ are right, the amps equal about what an ac unit converts over to.

Selling millions versus hundreds =$$$

Time will tell.

Bill