[jaspersk]

This weekend, I attempted to run my A/C compressor from a treadmill motor. I studied the compressor curves and came to the conclusion that it would pull under 3HP so I thought a 3.8 CHP treadmill motor might work. I guess I should have believed the folks that said treadmill motors are not correctly rated. Everything looked great until I started to charge the system with refrigerant and engaged the compressor clutch. I quickly turned the motor into a boat anchor. I am back to the drawing board.

I need to have a smaller footprint motor but most of the small motors in the EV world are rated for under 50V. I’d love to use a TEFC AC motor, however, I can’t figure out a good way to drive it from my DC pack. Most common bus AC drives need higher voltage. Has anyone successfully utilized a 3-5HP AC motor to drive accessories (short of building own controller)? Any other thoughts for driving the compressor? I have a dual shaft main motor but it looks like a nightmare to try and mount my compressor in a manner that would allow belting to the main drive. I am only going to do that as a last resort.

[subcooledheatpump]

You'd really need a VFD and a battery of atleast 340 volts DC. Even if you had a drive that worked at a lower voltage, the motor couldn't produce peak torque with a voltage that low.

You can always use a seperate compressor with the motor built in, they are made for EVs and they are available on ebay. (the small purple ones, etischer has one)

Also, bear in mind the power of a cars air conditioner at full speed, it's more than 3 HP. You probably won't be able to get the full cooling power with a smaller compressor/motor.

[jaspersk]

The treadmill motor was 120V DC. I brought it up to speed using a starting resistor. Getting it up to speed wasn't a problem. It was when I added a load by introducing refrigerant that everything went bad. Unfortunately, I wasn't monitoring amps at the time to see what the load really was.

The curve for my compressor is on-line:
http://www.sanden.com/originals/imag...erformance.pdf
It uses 2-5HP depending on RPM. Using the correct pulley size, I was able to ensure it should stay well under 3HP. That said, my estimation for the treadmill motor RPM might have been inaccurate and driven it to a higher HP than expected.

[subcooledheatpump]

It's as I suspected, the graph confirms you'd need 10 HP for the compressor to really work. Any less and you're kidding yourself.

You can use it at a lower speed but the pressures on the compressor are going to be really high. Even if you do manage to run and charge it, there will be a hot day when the refrigerant is under a higher pressure and you won't be able to start it.

Just my opinion, I'd get a bigger motor.

[jaspersk]

Originally Posted by subcooledheatpump

It's as I suspected, the graph confirms you'd need 10 HP for the compressor to really work. Any less and you're kidding yourself.

You can use it at a lower speed but the pressures on the compressor are going to be really high. Even if you do manage to run and charge it, there will be a hot day when the refrigerant is under a higher pressure and you won't be able to start it.

Just my opinion, I'd get a bigger motor.

I am confused. The curve shows that at 2000 RPM the compressor will pull 3KW or 4HP. That is exactly half the maximum allowed RPM of that compressor so I assume it is well above idle speed of an ICE engine. If I put the appropriate pulley so that the compressor doesn’t exceed this speed for the expected speed of the motor at the operating voltage, I should be able to limit the HP, right? What leads you to believe a 10HP motor is required?

[subcooledheatpump]

You can use a smaller pulley and you can use it at a lower speed, but it won't be like it was originally. The torque required to start it will still be unpredictable as the pressures change.

It might run and start sometimes, but other times it might not, especially during high temperatures.

Torque and speed are interchangeable, but that doesn't mean the compressor doesn't need torque to start. 10 HP motor will output a certain torque at a certain speed, and a 3 HP will output a totally different torque at a certain speed.

You need constant torque over a wide range of speeds, and a smaller motor won't output the same constant torque as a 10 HP motor.


Thats why you can spin it up with no refrigerant, but as soon as the refrigerant is introduced, the compressor stalls. Again you can use a smaller pulley, but the cooling power will be greatly diminished.

So bottom line, you need more actual power for cooling to happen, and for the compressor to start

[jaspersk]

I saw someone on the evalbum that had a large DC motor driving their compressor. I sent him an email asking about it and this is what he said:

I experienced many similar motor burns so I did select +20HP motor (1 hour rating) and run very slow around 1200 rpm.
Total power consumption is around 1KW.
I built my own controller with 300A IGBT + big heatsink to vary speed on this system.

Select very large motor (my unit is +25kg compound wound) with low RPM, and you will get reliable system.

This agrees with your point. It might not take much power to run the compressor since in his case he is using 1.3HP but it certainly must take a lot to start it since he went with 20HP motor after burning up several small ones.

For those reading this thread, I searched all of the EV Album for "Air Conditioning" and "A/C". I only found two people that were running their original compressor from it's own dedicated motor. Overall, there are not nearly as many successful A/C installations as one might think. Excluding mass production EV's (Solectra, Leaf, etc), I saw under 20 cars that had successfully implemented A/C. I didn't keep count but it seemed about half were running their original compressor off of the main motor tail shaft and another half were running a dedicated compressor, most often the Electrolux.

[jaspersk]

I got a note from the one other person that had an auxiliary motor running an OEM compressor directly. This is what he said;

I don't remember how many amps it pulled, but it was a 1.5 HP motor... and that was barely sufficient. The thing blew a 10Amp fuse once, so I changed to one that was rated for 15Amps.

The way it was rigged up, the motor started when I turned the whole vehicle on. Pretty crude, I know.

Most of the time, I just left one end of the fuse pulled out from the holder, which broke the circuit and left the AC off.

Good luck with your project, though. AC is nice to have when it's ridiculously hot outside.

He has a 156V system so 10 amps would be around 2 HP. Of course, that could have actually been a much larger current draw if he had a slow blow fuse. I should note that this was an industrial 1.5HP motor. I am still feeling like a 5HP should have a good chance of working. I am not sure if I am confident enough to spend the money to try it though.

[subcooledheatpump]

A 5 HP motor probably would work, but you'd still need a VFD and a high voltage battery to drive it.(or some kind of boost converter) Those are the only problems I really see with using an industrial induction motor.

Also, if you do decide to get an induction motor, get a 4 pole (1800 RPM) or 6 pole (1200 RPM)

A 2 pole (3600 RPM) will not make enough torque to turn the compressor

[jaspersk]

I appreciate the help and guidance. I think you are correct that an induction motor won't work since I can't get a 5HP inverter that would work off of 144VDC source.

I have a specific industrial 5 HP, DC motor in mind to try next but I am hesitant after the failed treadmill motor experiment. It has an 180V rated commutator and 104 ohm shunt field. It is rated 1750/2050 RPM for 1.38/.94 field amps. My pack is 144V so a direct connection would provide full field (144/104 ohms=1.38A). At full field, the motor is rated for 1750RPM at 180V so 144 volts should produce ~1400 RPM at full load. (It also has maximum torque at full field).

I wouldn't bother with speed control. I'd just use a starting resistor (ie. my ceramic heater which is just about right) and a time delay relay to start the motor. I can get just about any size serpentine pulley that will work well with this speed range from utterpower.com to get the compressor speed I want.

I am not sure I agree or understand your comment about wanting a slower speed motor though. I get that HP is a function of torque and speed so faster speed means lower torque but I think the size of pulley can counteract that since torque on the belt is proportional to the diameter of the pulley. A 3600 RPM motor would need half the size of pulley to create the same compressor speed.