OTHER: Electrical - Motor - Cooling

• Heat decreases the efficiency of the motor.
• Excessively high heat will demagnetize the permanent magnets.
• 

• High temperature is one of the biggest limitations to higher power efficiencies.
• Losses are referred to as I²R losses. "A winding temperature rise from 25 to 155ºC increases wire resistance as much as 50%
• Do EVERYTHING that is reasonable to reduce the temperature rise.
• One way is the increase the efficiency of the motor.
• In PM brushless motors the power losses are practically all in the stator.

• During hover convection will want to cause the airflow to be upward.
• During forward flight the relative wind will probably want to cause the airflow to be downward.
• Considering the above, which direction should a cooling fan set the airflow that it produces.
• If the motor could be extremely efficient, would a cooling fan even be required?
• Idea: Consider extending the root of the airfoil all the way in so that it is very close to the O.D. of the rotorhub. The 'root loss' should rise up through the coils and airgap of the motor, and this should add to the upward convection caused by the coils heating the air.

• It appears that high temperature is one of the biggest limitations to higher power efficiencies. Since the motors on this site are for aircraft applications they should probably be air-cooled for lowest weight and greatest simplicity.

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• Since thin lamination iron cores are more efficient than coreless motors it appears that an attractive idea would be to provide the OD of the lamination stack, on an Inrunner, with a much larger surface area. Giving the odd laminations a larger OD than the even laminations might do this. This would create vanes and thereby a much greater cooling surface.
• The large OD laminations might also have alternating segments of their OD with large diameter and segments of their OD with a small diameter. A very thin walled aluminum tube might be shrink fitted over this and air be blown down inside this tube.

• A concern: Does the coating of the laminations reduce the transfer of heat to the convection airflow. Could these inward extending cooling tabs be plated to eliminate corrosion while allowing good heat transfer?
• For additional cooling due to evaporation consider having a wicking material bonded to these tabs and then have the tips of the 'wicks' inserted into a reservoir of water. The idea is to act as the water filled canvas bags that were placed on the front bumpers of cars.
• Consider modifying the furnace motor for testing this idea along with the winding and multiple phase ideas.

• If the end windings of the copper wire produce as much heat as the copper in the slots, it would appear that significantly reducing the end windings of the copper wire would reduce the production of heat within the motor. The reduction of end windings will also reduce the weight of the motor.
• " In PM brushless motors the power losses are practically all in the stator where heat can be easily transferred through the ribbed frame " ~ [Source ~ PMMT p.2]
• "The rotor gets hot due to heat rejected from the stator through the air gap and eddy current losses in PMs and ferromagnetic core. Although the rotor cooling systems are simpler, careful attention must be given to the temperature of PMs as their remnant, flux density B, and coercivity H, decreases, diminishing the machine performance. Table 8.1 summarizes some typical approaches to PM motors cooling." ~ [Source ~ PMMT p.2]
• "The end turns of a traditionally wound brushless motor do not provide additional power or torque, but rather make a motor less efficient and generate unnecessary heat. The end turns are also the most susceptible area to heat and voltage damage because they are surrounded by air - without a good thermal path for heat to escape. This damage can create a short in the windings, rendering the motor inoperable. Forming or compressing the end turns reduces their size, but also puts significant strain on the wire that can potentially lead to insulation breakdown."

• The Allied Megaflux Inrunner Motors appear to have clear areas the air gap end of the teeth for a reasonable volum of cooling air to pass through and thereby reduce the heat transfer to the magnets.

Just another wild idea.

• Cooling of the stator coils and laminations might be better if the air through the stator was moving upward. The reasons are;:
• The induced airflow is working in the same direction as natural convection current.
• The possibility of reducing the air density within the stator and there by causing a lowering of the air's temperature due to expansion.
• An upward airflow direction would be working with the fountain effect.
• Ideas for consideration:
• Extend a percentage of the stator laminations inward toward the centerline of the motor. These extensions might be tapered for weight/heat dissipation optimization. This might have the disadvantage allowing the laminations, and the bearings to shingle.
• Locate disks of wire mesh between holding rings that are press fit into 'mast'.

• Without Halbach Array:
• Use the steel flux bands, as on the F & P, as the only means of providing structural strength to the rotor.
• Have this steel ring fully exposed to the free air.
• Have 1 or 2 of these steel bands, which backup the permanent magnets, produced with a greater width so that this extended bare steel (upward, downward, or both) can provide additional interact with the free air to assist with the cooling of the magnets.
• In addition, consider notching and bending these extended band(s) and thereby have them serve as the fan blades to blow air over the exposed steel.
• Another advantage of the using steel (or aluminum) fan blades is that they move the air AND provide direct convection cooling.

• Consider installing an air compressor near the rotor. The small amount of air might be temporarily stored in a highly finned container. And then dispensed to orifices that directs the expanding air onto the coils. The expansion of the air might help in the cooling of the motor.
• Consider pre-cooling the motors before takeoffs.
• Can a cooling liquid suspend magnetic particles so that the movements of the rotor's magnets causes the collant to flow in a totally sealed system where there is no possibility of leakage?
• Perhaps do a Google and patent search.
• Sketch of thoughts on cooling.

• Conduction cooling: Consider using the blades as radiators for liquid cooling of the electric motor's stator. The fluid could flow out from the root end and then back in within the spar or adjacent to the spar.
• The cooling system might be totally enclosed without any moving part. The circulation of the fluid might be achieved by having a ferromagnetic material in the fluid and a linear micromotor located around a segment of the tube in the cooling loop.
• Convection cooling: Consider a slot opening in the leading edge and/or trailing edge at the root of the blade. This will force air into and/or draw air out of a cooling region about the stator of the motor.

• Blade Tip Cooling Air Exhaust: Consider passing the cooling air, which has passed through the motor, down the hollow spars of the blades to rear facing tip exhausts. Three potential advantages are;
• The free air velocity at the blade's tip may extract a reasonable volume of air.
• Centrifugal force may assist in moving the air down the spar from root to tip.
• The exiting air might be given a swirl that opposes the blade's tip vortex.
• Yaw Control: Would the blockage of air flow from the blades of one rotor result in a desired Yaw control?
• Temporary Cooling During Rotor Windup and Takeoff:
• Pre-compressed air that is exhausted and expands from a port that is an integral part of the stator laminations. This pots will provide conduction cooling to the laminations while the released air is also providing a convection cooling.
• Use of Precompressed Air and Venturi at every Coil:
• Consider releasing precompressed air that is stored from a filament wound bottle. This air would exit venturi that are directed at the end of every coil. Perhaps ambient air flowing upward into the slots between the coils will caused the cooled air to stay as a boundary layer on the coils.
• In other words, it must be possible to make modification to the air passage through the stator of the motor that will provide an optimum cooling.
• Mist :
• Could a mist be sprayed on the coils and the evaporation of this mist provide or assist with the cooling? The amount of water will probably be quite small. The water may have to be distilled.

For Cooling Idea: See; UniCopter ~ Synchronizing Gears Rootes blower type of concept.

• For centrifugal fans and pumps the power input is proportional to the cube of the speed, while the flow is proportional to the speed. Hence, a reduction to 80% of maximum speed (flow) will give a potential reduction in power consumption of 50%.

• Permeable Flux Carrying Materials:
• Values for some common materials
• Mu-metal
• Nickel
• Thermal Conductivity:
• In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann-Franz law
• Heat sinks are made from a good thermal conductor such as copper or aluminum alloy. Copper (401 W/(m·K) at 300 K) is significantly heavier and more expensive than aluminum (237 W/(m·K) at 300 K) but is also roughly twice as efficient as a thermal conductor.
• Kapon Have hard copy. pdf on Kapton from Dupont It was suggested as a good transfer of heat to be used on the stator between the steel laminations and the coils.
• Mica paper and Aramid paper are used between the copper winding and the laminations. It may be questionable as to how much heat can be extracted from the copper coils via the laminated iron core.
• Weight of Materials:
• Aluminum, solid: Specific gravity = 2.64. Pounds per cubic foot = 165.
• Steel, rolled: Specific gravity = 7.93. Pounds per cubic foot = 495.
• Copper, rolled: Specific gravity = 8.9, Pounds per cubic foot = 556.

• From another source; Aluminum is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical. DJ~ I assume that this is by comperable area.
• Welding Copper to Aluminum:
• http://www.esabna.com/euweb/mig_handbook/592mig8_3.htm
• http://www.esabna.com/euweb/mig_handbook/592mig8_13.htm
• Is it worth the expense and complexity to go from copper coils to aluminum buses to copper cable?

• Perhaps the optimal solution is to press fit an aluminum heat sink onto the stator laminate pack with thermal grease. However will the varnish be detrimental to the heat transfer and would extended tabs on the laminations be better.
• Strength of Materials: I.e. the steel can be thinner than the aluminum a this will reduce it's weight disadvantage.
• Aluminum:
• Steel:

• Since 1KW-H (kilowatt-hour) = 1000 Watt-Hours =3413 BTU
• Then 1 watt-Hr = 3413 BTU divided by 1000 = 3.413 BTU
• So... bottom line is 1 watt-hour is equal to 3.413 BTU
• Note: For Reference: 1 BTU is the amount of energy required to raise (or lower) 1 (one) pound of water 1 (one) degree

• See 'CSIRO/UTS Solar Car Motors'. Section 8 in 'ELECTROTOR - SloMo' binder or in 'Axial Flux Motors' binder.
• See paper 'Motor Calculations', Section 'Thermal Calculations'
• Access database - Form: Thermal

• The Best Heat Transfer Fluids for Liquid Cooling
• Permanent Magnet Motor Technology, Table 8.1 on page 367. Have hard copy.
• Book: Axial Flux Permanent Magnet Brushless Machines. Chapter 8. Specifically; 8.5.5 and table 8.1. Have hard copy.
• Thermistor:
• Motor Losses:
• One motor's specification page considers 'Ambient' temperature as being 40ºC and it says that the wind speed is 10 m/s.
• The induced velocity of a hovering ultralight helicopter is approximately 21 ft/sec.

Patent and Patent Application Search: Google ~ air cooling electric motor

• Nothing interesting.
• 20090152964 Method and Structure for Cooling an Electric Motor ~ No value
• 20090051230 Rotor for a Forced-Air-Cooled Electric Motor
• 3,878,809 Air-cooled electric outboard motor

• DESIGN: AeroVantage ~ Motor - Full-size - Cooling
• DESIGN ~ Electrotor-SloMo ~ Motor - Radial Flux - Stator - Cooling
• MAKE ~ Electrotor-SloMo ~ Motor - Radial Flux - Stator - F&P Development

Database ~ Access: Database ~ Motor; Form ~Thermal

• [Source ~ EMDPS p.60+]

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