Replacing a diesel engine with an electric motor

[Jake Posch]

Hi all,

I have a little project on my hands and I will go into a little detail to help everyone better understand. I built a mobile boutique out of a shuttle bus for my fiancé to run her business out of. There is a large rooftop air conditioner on top of the roof for cooling and the compressor is ran of the diesel engine. In our application that doesn't work the best because the motor is usually of when the air conditioner is needed. I would like to power the air conditioner compressor with an electric motor. I already have a 12VDC inverter large enough to run all the air conditioner equipment except for the compressor. I would like some help sizing the electric motor needed to run just the compressor - i have attached a chart (via hyperlink) with what I think would be needed to size the motor. I know a couple things would important - RPMs, HP, and motor pulley size - the pulley size on the compressor is 5.3 inches in diameter.

https://postimg.org/image/ovrewjnk1/

 

[Drakkith]

Is the compressor usually powered electrically? If so, I don't understand the point of the electric motor.

 

[Jake Posch]

No it is normally powered off the diesel bus motor.

 

[sophiecentaur]

Does the bus engine do something else too? What?
Couldn't you just negotiate to have the engine run when you want A/C?

 

[Jake Posch]

The problem is when we are selling at vendor events we are not allowed to run a Diesel engine, we can only use shore power or in some cases a generator. Instead of adding a rv air conditioner i can plug in I would like to utilize what I have because it works so good, only problem is that you need the engine running which isn't allowed.

 

[anorlunda]

The problem is when we are selling at vendor events we are not allowed to run a Diesel engine, we can only use shore power or in some cases a generator. Instead of adding a rv air conditioner i can plug in I would like to utilize what I have because it works so good, only problem is that you need the engine running which isn't allowed.

The nameplate on the AC should tell you how much power it needs.

Your problem is going to be battery capacity, not inverter capacity.

 

[Jake Posch]

I won't need an inverter. I want to have an AC motor running the air conditioner compressor that I can directly plug into a generator or house power. The controls for the air conditioner can just stay ran off the bus batteries because there is very minimal draw. I just need an electric motor sized to turn the compressor.

Jake

 

[Drakkith]

What type of AC unit do you have? What are its specs?

 

[Jake Posch]

The compressor is a Sel-Tec TM-16HS. I attached a chart via a hyperlink to the first post with what I think would be helpful.

Jake

 

[anorlunda]

Come to think of it, it may be less cost and far simpler to get an ordinary RV roof air conditioner. They are already designed to be run on AC.

Leave the motor driven one alone; you can use that when driving.

 

[Jake Posch]

We don't need it while we drive. The cab of the bus has a factory installed one. I would like to eliminate another rooftop unit on the bus if possible. I would actually remove the one currently on it if need be but seems like such a waste when there is a perfectly good one on top. I would be looking at about 2 grand to add the rv style on top. I thought installing an electric motor would be a lot less expensive.

Jake

 

[sophiecentaur]

I won't need an inverter. I want to have an AC motor running the air conditioner compressor that I can directly plug into a generator or house power. The controls for the air conditioner can just stay ran off the bus batteries because there is very minimal draw. I just need an electric motor sized to turn the compressor.

Jake

Is there any point in using separate supplies for motor and controls? Just bite the bullet and buy a totally separate AC unit to work off the mains supply. I can see nothing but aggravation if you try to provide your own switchable drive for the vehicle's ac pump - two belts and pulleys with clutches?
You are talking in terms of several kW of power transfer.

 

[jim hardy]

When the bus is stationary , what moves cooling air through the condenser?
Is it not mounted in front of the radiator and cooled by the engine driven fan ?
Or is it in the rooftop ?

If condenser is in the rooftop you might have an airconditioner guy remove the engine driven compressor, plumb in an electric one from a household unit(R134?) and locate it someplace more convenient than the engine compartment..
Be aware though , your chart showed 5 horsepower which is nearly seven thousand watts.

The controls for the air conditioner can just stay ran off the bus batteries because there is very minimal draw.

Check the inside blower motor fuse. I wager it's twenty amps and the motor draws at least half that.

Good Luck !

 

[CWatters]

So if I've understood correctly the proposed set up is..

Existing battery - > Existing Inverter - > New AC motor -> Existing compressor -> Existing condenser unit

I tried to google for data on the compressor but couldn't find much. Is TM16 a standard size of compressor? If so then there is some performance data in here...

http://www.valeocompressors.com/media/brochures/TM13-15-16.pdf

The performance curve for TM16 suggests the compressor would consume between 1 and 5.5kW depending on the rpm. Picking a point in the middle somewhere gives:

3.3kW at 2000rpm

Your inverter might be able to deliver 3.3kW but how about your battery? If we assume it's a 24V system the current draw is going to be around 135-140A.

How long do you need it to run for? An 8 hour day?

137A * 8H = 1000AH but in practice you should probably double that to avoid deep discharging the battery. Big battery needed and better think how you will recharge it if you need to do the same next day.

As for the motor I'm thinking you would be looking for something like a >6HP (4.5KW) single phase 110V AC motor rated at 2000 rpm (or different rpm and suitable pulley ratio to produce around 2000rpm).

Motor may need a cooling fan if not fitted.

Given the cost of experimenting is quite high would it be better to buy a smaller complete portable AC unit intended for caravans?

No liability accepted for any errors in the above!

 

[sophiecentaur]

So if I've understood correctly the proposed set up is..

Existing battery - > Existing Inverter - > New AC motor -> Existing compressor -> Existing condenser unit

. . . . . . . .

Given the cost of experimenting is quite high would it be better to buy a smaller complete portable AC unit intended for caravans?

No liability accepted for any errors in the above!

That (along with all the other contributions) sums up the idea very well. It's a clear non starter. The performance and utility of an AC unit depends on many factors and just driving the compressor is only one of them. The cost of a cheap domestic AC unit is not all that high and, I'm afraid, it just goes with the territory, if someone wants to run a mobile shop. This is just one instance where initial planning of a project is essential. The AC on board a vehicle specifically makes use of mechanical power from the engine (a very efficient system when the engine is already being used) and there is no viable solution for using an alternative power source with the existing arrangement.

 

[jim hardy]

Most Non-Electrical-Oriented folks don't appreciate just how much power is involved in anything to do with heating or cooling.

Run the numbers like CWatters did,
then look at physical size of a 5hp single phase electric motor rated for continuous duty.

 

[sophiecentaur]

Most Non-Electrical-Oriented folks don't appreciate just how much power is involved in anything to do with heating or cooling.

Run the numbers like CWatters did,
then look at physical size of a 5hp single phase electric motor rated for continuous duty.

It's all that 4.2 Joules per Calorie; what we used to call "the Mechanical Equivalent of Heat"

 

[jim hardy]

It's all that 4.2 Joules per Calorie; what we used to call "the Mechanical Equivalent of Heat"

778 ft-lbs per BTU to me.

Airconditioning SEER mixes units, it's BTU's(Imperial) moved out of the room per watt hour(SI) used to do it.

 

[NTL2009]

So if I've understood correctly the proposed set up is..

Existing battery - > Existing Inverter - > New AC motor -> Existing compressor -> Existing condenser unit

I tried to google for data on the compressor but couldn't find much. Is TM16 a standard size of compressor? If so then there is some performance data in here...

http://www.valeocompressors.com/media/brochures/TM13-15-16.pdf

The performance curve for TM16 suggests the compressor would consume between 1 and 5.5kW depending on the rpm. Picking a point in the middle somewhere gives:

3.3kW at 2000rpm

Your inverter might be able to deliver 3.3kW but how about your battery?...

I don't think there are any batteries involved. In post #4, he mentions shore power.

For that case, the question becomes how much shore power can he draw, and how much will a ~5~6 HP motor cost? Plus the retrofit costs of getting that compressor out from under the hood so you can connect a motor to it, that jim hardy mentioned . Maybe it can be run at the low end near 1 kw, and provide adequate cooling? But that's still ~ 1.5 hp.

I like the idea of reusing what is there, but I'm skeptical that this can be done cheaper and easier than alternatives. Is there any salvage value to the present unit?

 

[Tom.G]

Not being well versed in refrigeration, I wonder if a second compressor with an electric motor could be plumbed in parallel with the existing compressor and mounted where convenient. There is still the problem of getting air thru the condenser though.

 

[jim hardy]

I like the idea of reusing what is there, but I'm skeptical that this can be done cheaper and easier than alternatives. Is there any salvage value to the present unit?

new one costs about $250 US

so a good used one ought to bring 50 to 75 bucks

Not being well versed in refrigeration, I wonder if a second compressor with an electric motor could be plumbed in parallel with the existing compressor and mounted where convenient. There is still the problem of getting air thru the condenser though.

They have check valves in the discharge side. It'd be an interesting experiment to mount one (and a motor) underneath and plumb it in.

Big commercial systems use parallel compressors.
http://engineeredrefrigeration.com/parallel-compressor-systems/

Still no answer to question where does the existing one reject its heat - in front of engine or up on the roof ?