# Motor for A/C – Can you drive a 5HP VAC from 140VDC?



## jaspersk (Jun 26, 2008)

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.


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## subcooledheatpump (Mar 5, 2012)

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.


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## jaspersk (Jun 26, 2008)

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/images/SD7V16_Performance.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.


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## subcooledheatpump (Mar 5, 2012)

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.


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## jaspersk (Jun 26, 2008)

subcooledheatpump said:


> 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?


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## subcooledheatpump (Mar 5, 2012)

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


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## jaspersk (Jun 26, 2008)

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.


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## jaspersk (Jun 26, 2008)

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.
> 
> ...


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.


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## subcooledheatpump (Mar 5, 2012)

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


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## jaspersk (Jun 26, 2008)

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.


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## subcooledheatpump (Mar 5, 2012)

Well yes, it's whatever you want. A smaller pulley which could mean more friction since a certain part of the belt is moving faster, or a motor with more poles which is a little bigger. Either way really.


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## dillond666 (Dec 27, 2010)

I have only seen an air conditioning compressor once in my life (Not often required in Scotland!) and it was on a Range Rover. It had a belt drive and an electromagnetic clutch to enable or disable the drive. Couldn't it just be driven from the motor tailshaft? The clutch could even be wired to the brake light to give A/C on braking, could be a useful way to capture some of the wasted braking energy?

Derek


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## jaspersk (Jun 26, 2008)

Attached is a picture of what a 3.8 CHP treadmill motor commutator looks like after trying to run an A/C compressor.


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## subcooledheatpump (Mar 5, 2012)

Thats why I only use brushless motors.... 

Yeah I'd say a little more power is required, something obviously went wrong


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## johntrhodes81 (Aug 8, 2010)

Any more thoughts about using a 5 HP motor? Could a generic industrial AC motor work running off an inverter?

John R.


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## PStechPaul (May 1, 2012)

I would think a 5 HP industrial three phase motor would work fine, but you would need about 300VDC for a 240 VAC VFD. Mine drops out below 200 VDC. It may be difficult to get an inverter rated at 5HP or 3kW, and they are not generally rated for driving motors. But I did rig up a 2kW inverter with a 12V battery and was able to run a sump pump with something like a 3/4 HP single phase motor.

Most inverters are not isolated from the battery, so you must be careful. You might be able to use two inverters and batteries, and rectify the outputs and run them in series to get about 320 VDC. Internally they have a 150-170 VDC supply which is then "modified" to appear as an equivalent sine wave. So if you can tap into that source you won't need to rectify it and you will have two DC sources you can connect in series. But the batteries will be floating well above ground, so you will have essentially an ungrounded system, or you can connect the DC link (-) or one of the batteries to ground.

But in your case, starting from 140 VDC, you would need to build a special inverter or doubler to use a standard VFD, or modify a VFD to work at a lower voltage. In that case you would only be able to run the motor at about 100 VAC, which would be half the rated voltage. You might be able to rewire the existing windings from star to delta and get 138 VAC nominal. Or you could use a 10HP motor and run at half voltage and half speed.

In any case, using the proper motor controller with internal overcurrent protection, and perhaps temperature sensors in the motor windings, should eliminate catastrophic failure as shown in the treadmill motor.


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## jaspersk (Jun 26, 2008)

Thanks for the reply. I do have a working system now. I found that the A/C compressor required a 5HP motor to work at the rated speed without overloading. I currently have an industrial 180VDC permanent magnet motor running on 144V pack. I used a starting resistor (ceramic heating coil which doubles as heater) and a time delay relay to bring it up to speed before putting it directly across the full battery voltage and another second after that, I engage the A/C clutch. It has been running for 3 months now with no issues and I am very pleased with how it is functioning.


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## jyanof (Nov 11, 2008)

Just wanted to share this here in this old thread. I've (mostly) successfully run the original A/C using a 5hp 230vac induction motor powered from a 144v pack using a homemade singlephase inverter.

More details are over on the ecomodder forum here.


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