Originally Posted by joeblack5
The airco will be a very interesting project. I just found a nice 12 vdc 1hp permanent magnet motor in the scrapyard that I would like to hook up to the Ac compressor in our small bus. Please keep pushing that project since nobody has done that before..some real pioneering going on.
The A/C compressor in your bus is good for several ten thousand Btu/hr. Your 1hp electric motor is good for about 7000 Btu/hr.
So, your 1hp motor will stall if hooked directly to the existing compressor. If you use a pulley ratio that turns the compressor very slowly, then the system will not build enough high-side pressure since the thermostatic expansion valve (TXV) in the large evaporator is too big.
You could try to splice a smaller --preferably variable displacement-- compressor, receiver/drier and a smaller evaporator with matching TXV in parallel with your original compressor, receiver/drier and evaporator. This way you could at least use the original condenser with the parking A/C. I would add an accumulator before the small compressor to be safe and prevent any liquid refrigerant from entering the small compressor. Then add refrigerant to the system until the engine-driven circuit is back to specs while running. Prefill the small compressor with oil as recommended by the manufacturer, also taking into account the additional volume of lines and evaporator.
More straight forward would be a completely separate system driven by the electric motor but you would have to find space for the separate condenser. That's the route I am taking.
I chose a variable displacement compressor since the smallest, common automotive compressor with 89cc/rev is barely small enough for a 1hp (~750W) motor when turned at 1000rpm and I need 500W electrical input at most to keep my small space comfortable. See attached performance curve of the SD5H09 compressor.
The PXE16 variable compressor OTOH can adjust the displacement from 3.8cc/rev to 127.1cc/rev via a swash plate that gets tilted more or less by an internal piston. The actual piston position, swash plate angle, and displacement is determined by a solenoid valve which is triggered with a 12V PWM signal from a controller unit. 0% duty cycle (always OFF) will lead to minimum displacement and 100% (always ON) will give the max rated displacement shown in the performance curve.
In the simplest case you can obtain the desired displacement (and Btu/hr at a certain rpm) with a simple circuit that allows you to manually change the PWM duty cycle. Like an Arduino with a 12V Mosfet driver on one of the PWM outputs and a voltage divider with potentiometer on one of the analog inputs plus a few lines of code to relate the pot setting to the PWM duty cycle.
Internally controlled variable displacement compressors like the PXV
16 are going to hunt with a TXV. They are only useful with an expansion orifice. (It should be possible to hack them for a fixed, small displacement by blocking the swash plate tilting mechanism in the desired position.)