Cold motors put out MORE POWER! (efficiently, too)

[safe]

With PV = nRT if the volume is fixed you can increase the number of moles of air and the equation could go either way where the temperature might adapt or the pressure might adapt. (or a little of both) It's unclear what actual behavior you get as far as temperature rise.

What we can say is that for every reduction in temperature in the condensor you will get a reduction of pressure in that same condensor. If a heat rise is in fact taking place (due to compression) you will get more air "compressed" into the condensor if it's cooled.

Turbochargers use intercoolers for this reason.

 

[safe]

Wikipedia to the rescue!

Charles's law says "when a gas is compressed, temperature is raised". There are three possible relationships between temperature and pressure in a volume of gas undergoing compression:

Isothermal - gas remains at constant temperature throughout the process. In this cycle, internal energy is removed from the system as heat at the same rate that it is added by the mechanical work of compression. Isothermal compression or expansion is favored by a large heat exchanging surface, a small gas volume, or a long time scale (i.e., a small power level). With practical devices, isothermal compression is usually not attainable. For example, even a bicycle tire-pump gets hot during use.

Adiabatic - In this process there is no heat transfer to or from the system, and all supplied work is added to the internal energy of the gas, resulting in increases of temperature and pressure. Theoretical temperature rise is T2 = T1·Rc((k-1)/k)), with T1 and T2 in degrees Rankine or kelvins, and k = ratio of specific heats (approximately 1.4 for air). The rise in air and temperature ratio means compression does not follow a simple pressure to volume ratio. This is less efficient, but quick. Adiabatic compression or expansion is favored by good insulation, a large gas volume, or a short time scale (i.e., a high power level). In practice there will always be a certain amount of heat flow, as to make a perfect adiabatic system would require perfect heat insulation of all parts of a machine.

Polytropic - This assumes that heat may enter or leave the system, and that input shaft work can appear as both increased pressure (usually useful work) and increased temperature above adiabatic (usually losses due to cycle efficiency). Cycle efficiency is then the ratio of temperature rise at theoretical 100 percent (adiabatic) vs. actual (polytropic).

http://en.wikipedia.org/wiki/Gas_compressor

 

[safe]

While the math is still eluding me (I have a low certainty factor) it does appear that we can safely assume that compressed air increases in temperature to some degree over regular air temperature. With this hot compressed air we run it through a radiator and cool it. With lower temperature you then get lower pressure given the same amount of air. When you release this air inside your electric motor it will expand and BOTH force the hot air out and also provide cooler air than the outside temperature to do it.

This seems like a real "win win" situation.

So what you need is:

1. Compressor

2. Radiator

3. Pressure Nozzle

...I'll update the other thread now.

 

[TylerDurden]

Anybody here remember ROVAC?



http://docs.lib.purdue.edu/dissertations/AAI8318488/

A ROTARY VANE OPEN REVERSED BRAYTON CYCLE AIR CONDITIONING AND REFRIGERATION SYSTEM
THOMAS CLAUDE EDWARDS

Date: 1970



Abstract


Results of an analytical and experimental investigation of the feasibility of using a combination compressor-expander-circulator rotary vane device in the open reversed Brayton cycle for the purpose of producing a simple and economic system of air conditioning and refrigeration of low to moderate capacity are reported in this thesis. The prototype system tested in this investigation is designed specifically to be competitive with conventional automotive air conditioning systems.

A mathematical model of each component was developed, taking into consideration a variety of real effects. This mathematical model was used to study performance and operating characteristics of the system as functions of physical and environmental variables. The prototype design was based on results of these studies.

The computer model predicted that a cooling capacity of 5000 BTU per hour with an operating coefficient of performance of 1.0 could be obtained in a compressor unit of 120 cubic inches, total internal displacements, operating at 2000 revolutions per minute.

Laboratory tests demonstrate that the prototype system performed much as expected and the accuracy of the mathematical model is generally verified.

This system delivers cooling power comparable with conventional automotive air conditioning systems while actually operating as efficiently. It is believed that a new and highly competitive system of air conditioning and refrigeration for a variety of applications is now available.

The general success of this work has resulted in the application for Letters Patent by the Purdue Research Foundation.


Subject Area
ENGINEERING, MECHANICAL (0548)

 

[Leeps]

Instead of going through all the trouble of compressing and releasing gasses, why not use a peltier device. No moving parts, one side gets cold the other side gets hot, mount the cold side on your hot stuff, heat sink on the other side. Theyre not consdiered efficient but im certain its much much more efficient than whatever pump your planning on using for decompressed gas (not sure what to call it) cooling. Liquid cooling is a good idea for semiconductors that are ok with 70c or so temperatures, i would want my motor to run cooler than this, getting good cooling depends on the temperature difference and thus your cooling capacity goes down if your target temperature is lower. The other cool thing is the peltier junction can get you to lower than ambient temperatures, your motor can be cooler than its environment.
downside is theyre expensive and might need a decent number of them if your trying to get rid of a decent amount of heat. Otherwise i would install an oversized liquid cooling system to try and get a lower temperature.

CO2 is seeming like a better and better idea to me though

Joe

 

[safe]

Peltier Junction

A very interesting idea... I didn't even know these things existed... wow... maybe run one of these at 20 Watts and a fan at less than that (10 Watts?) and you would do much better than a compressor it seems. All the compressors I've found seem to draw too much current... like closer to 100 Watts which is beyond consideration.

 

[safe]

A very "low tech" idea would be to do a "molding" of your motor in plastic and make something that could hold water. Then simply take the molding and place it into the freezer until the water is a solid hunk of ice. By the time the ice melts your batteries would be used up and you do it all over again the next day. (and if you got really thirsty you would have water to drink!) Sort of a "water bottle" with a practical purpose beyond drinking water...

 

[xyster]

http://www.electronickits.com/kit/complete/peltier/ck500.htm

50W, 12VDC, cools to a maximum of 25deg, has a fan and heatsink for $28.

Sounds pretty good, maybe also for cooling a controller?

 

[xyster]

A very "low tech" idea would be to do a "molding" of your motor in plastic and make something that could hold water. Then simply take the molding and place it into the freezer until the water is a solid hunk of ice. By the time the ice melts your batteries would be used up and you do it all over again the next day. (and if you got really thirsty you would have water to drink!) Sort of a "water bottle" with a practical purpose beyond drinking water...

So my lithium batteries will last longer than the standard three years, I lined their boxes in styrofoam (also to decrease vibration) and keep the bike in my 40F garage. During summer I'll move the bike into the air-conditioned inside. My first battery box was fully vented, using forced air cooling from wind while riding. Decided insulation in a closed box was the better way to keep the batteries cool because I only ride for an hour or two at a time, short enough time for the insulation to keep the cool in, and the heat from the sun out. I'd like to install a digital thermometer to test this however.

 

[safe]

What if the Peltier Junction concept for cooling was BUILT INTO the motor itself. Side by side with the coils you would have this Peltier Junction idea cooling the wires as quickly as they are used. In fact, cooling would increase at the very times you wanted it to increase (increasing amps) so it would be a superior motor design if they ever did such as thing...

 

[safe]

Stop the Presses

Bad news:

Thermoelectric junctions are generally only around 5-10% as efficient as the ideal refrigerator (Carnot cycle), compared with 40-60% achieved by conventional compression cycle systems

http://en.wikipedia.org/wiki/Thermoelectric_cooling

 

[TylerDurden]

Any cooling is still a kludge of sorts... continuous heat generated is many watts of power that could be better directed to the drivetrain.

For brief operation in the hot range, a motor could be cooled but still spend most of its time in the efficiency-zone where heat is a minimal byproduct.

If yer motor always runs hot, just imagine you are running a motor and an electric toaster off yer batteries. You might wanna get a bigger motor and chuck the toaster.

 

[xyster]

Posted: Sat Feb 24, 2007 3:36 pm Â   Post subject:

Any cooling is still a kludge of sorts... continuous heat generated is many watts of power that could be better directed to the drivetrain

Agreed.
To provide a net efficiency increase, obviously, the amount of energy spent cooling has to be less than the amount wasted by not cooling. This seems a tall order for small electrical systems -- as in, almost any cooling solution will cost more in energy than it saves by cooling.

Now if the idea is to run the motor or whatever at a level of power not otherwise possible without cooling, then cooling is more easily justifiable -- but not because it saves energy, but because it saves components from dying completely, or adds to their longevity. Aren't these the sole reasons we sometimes use forced air cooling for our motors? Not to gain efficiency, but to gain longevity and higher power capability.

 

[TylerDurden]

Aren't these the sole reasons we sometimes use forced air cooling for our motors? Not to gain efficiency, but to gain longevity and higher power capability.

Yep. I'll go along with that... given a proviso that a bigger motor is not an option.

If the bigger motor IS an option, there the chances are better for both longevity and higher power AND maintaining efficiency.

8)

 

[Lessss]

Actually I thought the reason for cooling is to keep the motor from burning out while under a heavy load like climbing a long hill, or on a really hot day.

 

[safe]

Actually I thought the reason for cooling is to keep the motor from burning out while under a heavy load like climbing a long hill, or on a really hot day.

But their point, which I agree with, is that if you selected the right sized motor and the right limit for a controller in the first place you would never have heat as a problem.

Heat is a byproduct of poor design... the right design never gets hot.

I'm starting to think the "frozen water bottle" approach is a good idea. Just make a mold of your motor and create a tight fitting water bottle and then simply freeze the water before you ride. My rides don't last for more than an hour anyway, so that's about how long it takes for the ice to melt.

"Low Tech" is sometimes the easiest... (and makes the most sense)

On second thought... the outside of the motor doesn't really matter all that much. It's the wires themselves that need to be kept cool, so I present and retract the idea in about the same posting..

 

[Lessss]

The right size motor.....

Well some locations have a lower legal limit on that than others. 750W US, 500W Canada, 250W Australia.

 

[safe]

The right size motor.....

Well some locations have a lower legal limit on that than others. 750W US, 500W Canada, 250W Australia.

Well, I hate to say it, but it's true... if you want to prevent heat you can always get gears. If you are in the correct gear all the time the motor never gets hot. Heat is an artifact of single speeds that try to pull at low rpms when they really shouldn't. The more gear options you have the less you have to worry about heat.

The "ideal" machine has a motor with a low current limit and gears. The motor size (if it's truly about Watts) is STILL going to be best if the voltage and amps are such that the "peak power" and "peak efficiency" are made to coincide.

The design process goes:

1. Accept Motor Watt Limitations.
2. Select appropriate voltage and current limit to hit the Watt Limit.
3. Use gears to expand torque across a wide range of speeds.

...and you can't do any better than that.

 

[xyster]

Actually I thought the reason for cooling is to keep the motor from burning out while under a heavy load like climbing a long hill, or on a really hot day.

This is one of two reasons contained in the statement, "Aren't these the sole reasons we sometimes use forced air cooling for our motors? Not to gain efficiency, but to gain longevity and higher power capability."

Once again, after rectifying misunderstandings, we agree to agree. Peachy!

 

[fechter]

I started using forced air cooling to keep my little Zappy motor from melting while going up a hill. I had no idea it was making it more efficient at the same time.

A larger motor for the Zappy was not really an option, since I did not have a welder, nor did I want to change the outward appearance (can you say sleeper?)

For a brushed motor, blowing air over the windings is the only practical approach to cooling. The idea is to extract heat from the copper, so anything on the outside of the motor is going to be fairly ineffective.

Without the air cooling, the windings got really really hot. Perhaps the improvement in efficiency by keeping them cooler might offset the power drain of the blower.

If we had superconducting motors, then there would be no heat to dissipate. Wouldn't that be nice. The more efficient the motor is, the less heat you have to blow off. I noticed a significantly less amount of heat from my BMC brushless motor compared to the stock Vego brushed motor running at the same power level.

 

[safe]

Gears... it's the cure for heat.

 

[Matt Gruber]

and a motor that runs cool like mine, doesn't need gears.

 

[Matt Gruber]

A very "low tech" idea would be to do a "molding" of your motor in plastic and make something that could hold water. Then simply take the molding and place it into the freezer until the water is a solid hunk of ice. By the time the ice melts your batteries would be used up and you do it all over again the next day. (and if you got really thirsty you would have water to drink!) Sort of a "water bottle" with a practical purpose beyond drinking water...
]

above motor use analuminum tray, w/plastic top. hook blower to top for fan cooling. ice cools magnets, fan sucks in ice cold air, cools armature.

 

[Matt Gruber]

Today i used this cold power knowledge for the 1st time!
knowing that a cold motor is way more efficient and ~40% more powerfull i hit the small hill just out my drive full amps ZZOOOM! and again on Saxon ZZZoom!
end of ride, back from store, motor 86F
Knowledge is POWER!

 

[Patrick]

Motor cooling can tremendously improve performance. The best example is probably the Predator:

http://www.icare-rc.com/plettenberg_predator.htm

which can develop 12 kW, and weighs less than 4 pounds. Being built for aircraft, it has natural flow through cooling.

For us ground dwellers, fluid is the way to go. Water has about 24 times the cooling power of air, and is preferred by the top vendors. Perm makes synchronous motors in both varieties; the water cooled units develop about 25% more power. See picture below.

Yes, you'll need a radiator and small pump. This may be practical only on a motorcycle size chassis; has anybody tried this?