Explain vehicle / equipment top air conditions not on RVs.

Anyway, they all have large roof-top air conditioners. What do roof-top air conditioners run on for power? It must be 12VDC? I'm thinking of these vans, school buses, heavy equipment, etc.

Nearly all RV roof-top AC units run on 110VAC. You usually use a generator or shore power to operate them.

Steve

FWIW, our home's heat pump system has a variable speed blower fan and a variable speed compressor. The compressor type is a scroll compressor.

I don't know a lot about scroll compressor efficiency (although our system was state of the art in overall efficiency in 2009) ... and I never see any mention of this technology in the RV world.

I forgot to mention in my last post, that including the inverter as "part" of the compressor has a secondary benefit of simplifying the "controls" of the system. A 3 phase inverter very easily make variable frequency power so that the motor runs at whatever speed is required. No more compressor cycling on and off.

wa8yxm wrote:
There is no reason an electric motor . be it a blower motor or a compressor motor can not be made to run on 12 volts.. but since the compressor pulls over 1000 watts.. that's a whole lot of 12 volts.

Let get down to "brass tacks" ! An air conditioning compressor is almost identical to a refrigerator compressor. They are simply an electric motor drive a pump.

The most efficient electric motors use 3 phase power. So everyone is now saying, "Yeah, where do I get 3 phase power on an RV ?". Through the "magic" of modern electronics, you simply "make it" from what ever power source you have (actually works best if you have DC).

Danfoss is a long time manufacturer of HVAC compressors. The sell small compressors that take 12VDC (or 24VDC or 48VDC), convert it to 3 phase and send it to their motor. Don't get me wrong, these things STILL TAKE A FAIR AMOUNT OF POWER (500W+). Danfoss only sells the compressor so it is up to some one else to incorporate it into a product. They are most commonly used in small 12VDC refrigerators so they clearly do not have the capacity to cool and RV. AT least not one unit. Scaling up only requires better/most cost effective DC-to-3 phase AC inverters.

Danfoss was a Danish company, but was sold to a German company, Secop, in 2010. They then sold it to a Japanese company, Nidec, in 2017. Nidec has also acquired Embraco, a Brazilian compressor company. Nidec is probably not going to sit around and do nothing with these acquisition. With hybrid/electric cars coming there is a big demand for electric A/C. If they could get the power consumption down, auto makers would love to have a 12VDC air conditioner for non/mild hybrid vehicles !

I believe most of the 12 volt air conditioners actually have inverters built in, the super efficient ones have 3 phase inverters, variable speed..

And yeah, that Prevost didn’t hardly run the airs off the batteries, I mean it does, but not long before the 17 kw gen set kicks in.

There is no reason an electric motor . be it a blower motor or a compressor motor can not be made to run on 12 volts.. but since the compressor pulls over 1000 watts.. that's a whole lot of 12 volts.

I"m wondering if the Rooftops (I seem 'em on diesel Semi's too) do not use an engine driven compressor and then just a 12 volt blower.. that is how I'd do it more than likely. Else I'd use an inverter and feed 'em `120 v.

Or a Generator.

A friend has a bus company, they have several small shuttle buses (25 passenger?) built on a Ford F550 chassis. He says that it keeps up just fine on the hottest of days, and bear in mind, 25 adults would be the equivalent of 2,500 watts of electric heat running over and above just cooling the space.

The rear roof-mounted a/c runs off a second engine run compressor. IIRC the largest Ford a/c compressor setup runs 3.5 pounds of refrigerant so it would have a theoretical capacity of nearly 5 tons.

My bus conversion ran 110V Roof air from engine alternator. A 225 amp oil cooled alternator. There were 2 8d batteries or 4 group 27's.

The batteries did not run the AC they were there as a buffer during low idle conditions.

I have seen the roof condensers on commercial vehicles and wondered about them, I see nothing on the net??

2oldman wrote:

Chris Bryant wrote:
The last Prevost I worked on had 4 rooftop air conditioners, all run off 24 volt 4kw inverters.

Wow. What kind of batteries we talking about?

IIRC, it had 4 8D batteries for the inverters. Had an oil cooled alternator also.

pnichols wrote:

2oldman wrote:

Trackrig wrote:
Why don't manufacturers install this type of A/C on top of large DPs (where money doesn't really seem to be a concern) for going down the road?

I don't know of any type that runs on 12vdc. That would require wires the size of your fist, and a lot of batteries.

I run mine on a big inverter and an Li battery and solar, but not for hours.

Here you are - 12 volt powered air conditioners for boats and vehicles - other manufactuers offer them too:
http://www.cruisencomfortusa.com/

46 amp nominal draw equals a helluva battery bank for 8000 btu rated capacity.