# What watt motor to run A/C?



## Ball (Jun 24, 2016)

Hi. 

I don't have an electric car, but I have a problem which you guys seem to have tackled before. 

I want to run an A/C compressor with a 12V motor via a flex shaft. The idea is to run A/C (optionally) via the engine while driving, but can also be run via solar panels and a thermostat (this vehicle is far too large to run via electric motor so please don't even start on that tack).

I'd need to know how many amps at 12v the motor would be rated for, and possibly the ideal motor and controller to use. It would probably have to be a brushless motor. It would have to spin freely when not engaged.

Thank you for your technical knowledge!


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## major (Apr 4, 2008)

Just a guess: 1500-2500 Watts. You'll not be able to find a suitable 12 Volt motor. Even if you did, it'd drain your battery in minutes so you'd need the engine running the alternator. Better off powering the compressor directly from the engine. 

major

ps. Take a look at what long haul truckers do where they can't idle overnight.


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## Ball (Jun 24, 2016)

What about an auxillary 12v air conditioner


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## major (Apr 4, 2008)

What is that?


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## Ball (Jun 24, 2016)

A small air conditioner specifically made to run on 12v. 

I just looked and they're very expensive specialty items. 

There are 12v swamp coolers. I may have to just drive to cooler locales.


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## major (Apr 4, 2008)

Got big doubts about effectiveness of things like that. If they did work, still suck your battery dead on short order.


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## bigmotherwhale (Apr 15, 2011)

what you want is a scroll compressor with an integrated motor and controller like the ones used on the prius, you will also need a very large high voltage solar array, around 1000w minimum. its going to be quite costly and large, might work on a truck however as you have the space for the panels. 

You will also need a controller to tell the compressor to turn on and off at what power setting and temperature, you might also benefit from a small high voltage battery pack as an energy buffer, so it doesn't power down immediately when its dark. 

it is entirely possible but quite expensive and moderately difficult, you will need to be able to program or pay someone who can.


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## miscrms (Sep 25, 2013)

Yup, you are looking at at least a couple of kW. Maybe 4kW to be safe, particularly if you are trying to drive a traditional pulley driven compressor? Really depends on the size and efficiency of the compressor though, as well as the efficiency of the motor/controller and matching the rpms for both well. Trying to run it off 12V that's going to be many hundreds of amps. 48V or so might be more reasonable.

Doing this with solar might be feasible, but challenging. My 5.5kW array takes up about 400sqft, and cost about $5000 for just the panels. The problem is it only puts out close to its rated power under ideal conditions of clear sky, cool temperature, and direct sun at the proper angle in the middle of the day. In Phoenix it generates roughly 20-33kWh per day over the course of the year. Spring and Fall are actually the highest, as high temps reduce efficiency in summer. If you used a 48V solar charge controller to maintain a 48V battery bank to even out the electrical supply/demand, that might get you 5-7 hrs a day of AC run time at 4kW. But that assumes its sunny, and you are in full sun all day. Ideally with the panels angled toward the sun.

Here's an example of a 100A/48V MPPT Solar charge controller:
http://www.wholesalesolar.com/29500...ollers/magnum-energy-pt-100-charge-controller

On the motor/controller side one approach might be to look at components targeted toward electric bikes / scooters / karts in the range of 4kW or so. Here's an example of a 4kW 48V controller:
http://kellycontroller.com/keb48401x24v-48v280a4kw-bike-brushless-controller-regen-p-271.html

This controller also includes regen which might give you the option to charge your batteries using the motor as a generator off the gas engine if you are connecting everything through the drive belts/pulleys.

Here's an example of a continuous duty 100A/48V motor that works with that controller:
http://kellycontroller.com/mars-0907etek-comparable-pmsmbldc-motor-p-29.html

Not sure how feasible all that is practically or economically, but hope it helps.

Rob


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## Ball (Jun 24, 2016)

1KW would be doable. 4KW would be pushing the limits.

(This is on an ambulance, BTW)

Looking into swamp cooling it. That only requires a fan if I wet the material using capillary action (or use a small 100psi water pump with a sprayer).


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## miscrms (Sep 25, 2013)

Part of the problem is Automotive AC units are usually much bigger than you would think. The volume they are cooling is small, but the heat load due to poor insulation and all those windows can be huge. I've read they can end up being as big as 5 tons. My heat pump at home is 5 tons and runs in the neighborhood of 5-6kW on 3-phase 240V AC.

A high efficiency electric compressor unit off something like a Leaf might only be 2kW or less, but then you need to deal with high voltage (~390V DC) which creates all kinds of other challenges mating it to solar.

Rob


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## miscrms (Sep 25, 2013)

If you are in a very dry area like the SW US (most summers anyway, just not so much this summer) evaporative cooling can be fantastic. We run one on our house most of the spring/fall, and even most of the summer until the humidity gets too high during the monsoons, and can usually maintain high 60s to mid 70s inside. Over 10% or so and it doesn't cut it any more though, at least when its also 110 outside.

Rob


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## Ball (Jun 24, 2016)

Oh, I'm a definite believer. For years I would keep my food in one of those small Coleman coolers for sodas that has the flat top with a rim. I put a towel on the cooler and an upside-down nalgene (or whatever) on top and my food would keep cool for an entire day even in 90° weather so long it was dry. 

Didn't worth worth a poop in humid weather, however.


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## Ball (Jun 24, 2016)

BTW the box is insulated with small windows which I think are double-pane. 

I may end up just running the rear A/C for a few minutes before sleeping to take the edge off. Id would be interesting to see just how many watts I'd need to run the compressor, but as it's installed right now it would be difficult to test. I'm thinking I'd have to remove the clutch and pulley then attach a flex shaft to a 3-phase motor (I have a few...) then measure RMS. 

I like the swamp cooler idea, however, and will be looking into designs.


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## bigmotherwhale (Apr 15, 2011)

there is a much simple option you could connect a solar array to a decent inverter and use that to power something like this

http://www.appliancesdirect.co.uk/p...ce=apadwords&gclid=CJqeq6Pkwc0CFcluGwodbcEJJQ

this would require little effort to get it running and its quite low power. 
go for maximum solar panel voltage, dont try and use a 12v or 24 system and the efficiency shouldn't be too bad. 

i would suggest painting the vehicle with some kind of infra red reflective paint too.

I have no idea if this will provide enough cooling or how well it will work.


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## mons2b (Nov 17, 2015)

Ball said:


> Hi.
> 
> I don't have an electric car, but I have a problem which you guys seem to have tackled before.
> 
> ...


Forget the 12 volt idea. Go to ebay. Find a 1.5 to 2.0 HP DC treadmill motor. The revs need to be be around 2000-3000 rpm approx. Remove its flywheel. Remove clutch from existing AC compressor. Match up two love joy type connectors. Wire to drive batteries. Enjoy cool air. 
Tips from me. Find a motor with an intact mount it will save some hassle. Im currently trying to get this sorted myself. One problem is that the compressors have a teeny weeny shaft once you remove the surplus clutch. Dooable just takes some thinking to find a connector. I need to find a way to reduce the drive voltage down to the 90 to 100volts the treadmill motor would be happy with . PM me if you want to know more.


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## dcb (Dec 5, 2009)

fyi, observations on my 2004 prius using el-cheapo bluetooth obd and torque on my android phone (and custom prius pids).

the pack voltage varies from 200-230 ish

the pack current draw is increased by about 3 amps when the ac compressor is on (engine off).

so ~650 watts for the prius ac compressor.

the fan on high adds another amp or ~220 watts.

This is of course a well considered system, the compressor design and operating conditions are matched to the motor torque curve.

a 1+hp treadmill motor might be a good fit for ~100v system (or a junkyard prius compressor/motor for a ~200v, or a leaf for ~400v)


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## Caps18 (Jun 8, 2008)

https://www.amazon.com/Frigidaire-W...2&sr=8-2&keywords=mini+window+air+conditioner

One of the questions asked about this unit was how much power it takes, and they said 400-450 Watts when starting and 50-55 Watts when running. I'm not sure how well it will do with vibrations and rain, but the price isn't too bad. I have no idea if it would work though for a vehicle.


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## samwichse (Jan 28, 2012)

Car air conditioners are usually 16-20k BTU...

That might work if you can pre-cool for a while while plugged in, but cooling a hot car down to a comfortable temp is going to be real slow (think running your AC on fan speed 1 of 4 only).

Sam


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## Caps18 (Jun 8, 2008)

I think if the car is in a garage or by adding window tint it could help reduce the heat load at the beginning. I also had the idea to use a fan or vents to circulate the air when the car is parked so that it doesn't get quite as hot. But, I wasn't sure what rating a car A/C had. All I know is that I will have to roll down the windows to cool off in my truck.

All of this goes out the window in Arizona or the southwest this time of year though. A/C is the only thing that works on the hottest days there.


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