Report 3 - Motors and Optimization Strategies

 

Report 3 – Motors and Optimization Strategies

By: Armando Moncada, Certified Energy Manager

(Please Note: The United States Patent Office prohibits patenting work that is already part of published material. If it is not your work you cannot patent it. These reports are brief white papers not patent ideas. They are part of the public domain.

Secondly, these ideas are being shared in an open-source manner with everyone including designers, manufacturers and researchers to discover effective means of reducing energy costs. We encourage individuals, facility owners, businesses and research centers to explore the ideas and develop them even further. This idea pool is available to the public to directly address energy efficiency within product design, in an effort to globally update our technology, reduce CO2 emissions and reduce energy consumption. Good luck, be safe and have fun!)

The theme of this report includes motors, generators, bearings, and gear teeth. The subthemes include cooling of parts during operation, minimizing surface contact, the role of ceramics in reducing frictional heating, strategies for increasing motor inductance, and simplifying designs by introducing generator-motor pairs as a replacement for gears.

 

1.      Motor Bearings with Self-Cooling

In any rotating cylinder, air pressure will drop proportionally to the velocity of the moving surface. Bernoulli’s theorem quantifies the balance between air pressure and velocity. In short, the higher the velocity of the moving air, the lower the pressure of that air stream.

If bearing design can incorporate the movement of high-velocity-low-pressure air through any of its components, one can then assume that the internal air pressure of the bearing will drop. If the pressure drops so does the temperature and the net desired effect is that the resistivity in the copper windings is lowered. Lowering the resistance equates to a more efficient motor, with less waste heat and less energy consumption. The best method for lowering the air pressure in motors may be very simple.

Simply having a hollow ceramic cylinder enclosed by a metal cylinder to minimize the dimensions and weight of the bearing, one can incorporate air flow through cylindrical, roller, and needle bearings. If a ceramic hollow tube is to replace a solid metal core, then the compressive strength of the ceramic must be an equal match to the compressive strength of the metal being removed. Lastly, the interior of the ceramic cylinder may have miniscule surface veins like an axial fan to initiate air flow in one direction. Another strategy may involve pock marks like golf balls on the interior cylinder to drive the pressure to a lower state and allow it to mix with the surrounding air by its own means.

Another strategy for bearing cooling involves using four protective sleeves per bearing instead of two and overlapping them enough to form a thin shared boundary layer. When the air is sheared between the two rotating discs, the pressure drops, and the temperature follows.

Typical bearings have protective sleeves to keep out dust and to keep in oil. In our case, the two protective sleeves on each side, because of a reduced pressure within the air gap, will require reinforcement to avoid contact. Making the protective sleeve out of metal C-clips instead of a rubbery plastic solves this problem. Lastly, because the air is dependent upon sticking to the surface at the “no slip” layer, it would be effective to sand the surfaces to a rough finish to increase the stickiness of the air and the drop in pressure.

 

Figure 1. A pair of overlapping C-Clips form a bearing cooling mechanism via a low-pressure boundary layer in a state of shear.  

2.      Weight Reduction and Surface Contact Minimization

Ball bearings are typically uniform in the material used for the design. A reason why is because they are so small that it is common to keep manufacturing costs low and make them out of a singular material. With 3D printing and laser sintering it is possible to create a slightly more complex bearing, made of two materials, a metal and a ceramic, with a void in the center to allow for the material to flex more easily and to reduce weight.

The manufacture of such a bearing would include a method for weight balancing, smoothing out ripples from the 3d-print, (if required), enabling a good connection between the metal and the ceramic surface, spray coating the last layer for ultimate smoothness, and an annealing process to create internal tension in the metal as the two materials cool and contract at different rates. Once annealed the metal is in a state of tension.

 It is not too different than a prestressed beam. The metal works in tension and the ceramic works in compression to create a residual stress that improves performance. This also assists the metal in having a higher hardness as it is in tension, allowing for a smaller deformation area which in turn reduces frictional heating. A reduction in frictional heating means a longer lasting bearing, since heat leads to accelerated wear of the material.

A more lightweight bearing enables savings in motors that start and stop frequently, although the savings are minor, they exist. The main reason for creating a bearing with a composite material design is that the bearing could last for a very long time, saving replacement costs and avoiding bearing failure. Hollow bearings also have the added benefit that oil can be placed in the central void to assist in lubrication.

The amount of heat that develops in a bearing is proportional to the surface contact area. Surface-to-surface interfacing increases under significant loads as materials always deform slightly, (assume highest load conditions during design). By creating a bearing which has a slightly elliptical raceway, we can hope to minimize drag between the two surfaces. We have to be very careful and keep in mind that the raceway also supports the bearing and transfers the structural load. Perhaps we can imagine that the addition of a ceramic insert inside of the ball bearing causes it to deform less, and the raceway to ball bearing surface contact area is also lessened. Surface to surface abrasion will also decrease between the ball bearing and the raceway.

If we closely analyze the bearing spacing mechanism, we quickly realize that there exists an opportunity to minimize the spacer-to-ball bearing contact surface area as well. Spacer to surface contacts do not have to be anything more than the minimum number of points to secure the ball bearing in place. They may even be flexible to assist a faster assembly time or made of materials like a metal frame with a carbon ceramic contact pin to further reduce friction. In larger bearings this may be very relevant. In Figure 2, below it is shown how the contact points for the spacer sum to a total of four – two indentations on the top and bottom of each ball two points of contact on the right and left of each ball. Ideally, there should be a small space between the points of contact and the ball to enable oscillation of the ball between contact points to further reduce friction. Perhaps MEMS or magnets can enable the design of a contactless spacer.

Figure 2. Bearings Cages with Surface Contact Minimization via Concave Nodes and Silicon Carbide Ceramic Contacts at Bridge.

Figure 3. Cut Away of Bearing Assembly showing Pill Shaped Ceramic Spacers.

3.      Motors with Air Flow Pathways Designed for Cooling

Resistivity, the inverse of conductivity, in metal wiring is proportional to wire temperature.   A strategy like the airflow through a ceramic cylindrical bearing described above can be applied to a rotor-stator assembly to keep the temperature of the air surrounding the wiring very low. If the wire temperature jumps by a factor of 200% in Kelvin, the resistivity also increases by a similar factor. In any electrical system, the energy consumed is proportional to the amps multiplied by the voltage. The voltage is equal to the amps times the resistance. So, the watts will equal resistance times amps squared. If we can lower the resistance via temperature the amps stay the same, but the voltage begins to drop.

The best design for creating a significant pressure drop around the copper wiring might be as simple as equipping the bearing to rotor connection with angled fan blades as the main support structure between the outer surface of the bearing and the end surfaces of the rotor. If air flow is slight via small perforations in the motor housing, a significant pressure drop is achievable, and the temperature will also drop. See Figure 4 below.

Figure 4. Rotor cooling is achieved via fan blades with tiny apertures along the equator of the motor casing. Fan blades direct air from the perforated middle to each end of the motor.

In some mechanical rooms, there exists “waste cooling”. Waste heat comes from equipment such as a water heater using a compressor to generate heat. In such cases, discharge air can be directed to motors using a cooling jacket.

Nearly all buildings have hot water demand. It would be effective to utilize the waste cooling from the hybrid water heaters to cool the motors. Ducting the air directly to the motors may be awkward, but if the heat pump runs off compressed air in the first place, the ducted air could be as simple as a pipe. In Figure 5, below, I demonstrate how waste cooling can be directed to cooling down a motor using the system already mentioned above in Figure 4. It is an open loop system, where the heat of the motor recirculates in the mechanical room so it works very well. It is important make certain that the air flow from the water heater works perfectly well even when the motor is not in operation, and that the cooling system for the motor works when the water heater fan is not in operation. Cool air may create condensation which is also a concern, therefore it may be important to design humidity controls. In general it would be effective to provide a bypass for both the motor air flow and the water heater air flow, in case one system is turned off.

Figure 5. Waste Cooling Routed to Perforated Motor Enclosure with Internal Negative Pressurization via Fans.

4.      Wire Dimensions, Inductance and Power Density

Inductance in a solenoid is measured by the formula: I = N²*A*µ₀/L. I is the inductance, N is the number of windings or turns, A is the cross-sectional area, µ₀ is the permittivity of free space and L is the length of the solenoid. It makes sense to optimize the number of windings per length of coil because the net effect is multiplied twice. There are two ways to optimize the number or turns. One is to make the wire rectilinear which creates tighter packing. The other is to make the wire rectangular and to attempt to maximize the height to width ratio. Height to width ratios that are easily attainable include - 1.5 to 1.

 

5.      Axial Motors vs. Radial Motors for Power Density

One of the reasons why axial motors became so popular for lightweight applications is that the inductance in a motor is proportional to the cross-sectional area of the solenoid, as described in the formula above, which is a fraction of the radius times the circumference. In radial motors it would be the circumference times the width of the motor that defines the cross-sectional area of the solenoid. Axial motors tend to be more coin shaped and have a higher power density. As the scale of the axial motor expands, the solenoid cross sectional area also expands, without having to add width to the axial motor, (the height of the cylinder defined by the motor). See figures 2 and 3 below.

 

Figure 6. Axial vs. Radial Motors – Orientation of Copper Windings in Solenoid with Respect to the Center of the Motor.

In the case of motors that have variable speed controls, an axial motor is recommended to lower the inertia of the motor. Every time the motor starts and stops, speeds up or slows down, energy is required to change the rotational inertia, which translates to energy costs. It is very effective to select motors with a lower mass.

Similarly, axial motors can increase their torque by increasing the radius of the design, but if the central area of the motor opens up and allows air to flow freely, this further facilitates the cooling of the coils, which furthers the energy savings. Axial motors work well with designs that attempt to incorporate air flows for cooling. For example, the double sleeve cooling strategy that we described for bearings could also be incorporated into axial motors with a medium to large radius very effectively. See Figure 7A below.

 

6.      Axial Motors with Two Stators and One Rotor

Radial motors have one advantage over axial motors - the central axis of the magnetic field of both solenoids align. It can be observed that the attractive force between two magnets is greater if the axis of both magnetic fields is aligned.

Axial motors with an additional stator enable the motor designer to create a shorter copper wire winding path per path line of copper, which is the first advantage to adding a second stator. The path is shorter by default because the magnetic field is created in two places within the stator not just one, allowing for a reduction in resistivity due to a shorter wire path.

The second advantage is that motors designed this way also have amplified magnetic field interaction. By splitting the stator in half the rotor solenoid can travel directly through the space shared by the two stator solenoids to the degree that the axis of the magnetic fields align completely. From the perspective of the rotor solenoid, at least 2 out of 6 surfaces of the trapezoidal-cube-like winding are exposed to the magnetic field of the stator windings.

In both examples the power density increases. See figures 7A and 7B below.

 

Figure 7A. Cooling plates that counter rotate to provide cooling via depressurization in shared boundary layer. The two exterior plates move with the stator and two interior plates move with the rotor, providing motor cooling.

Figure 7B. Shows a schematic layout of two stator rings, one rotor ring and air-cooling plates with spiral perforations. Structure along with thermal bridging within structure is not shown.

 

7.      Shorter Wire Paths to Increase Amperage via Central Conductive Rings

The wiring configuration in a motor should always be designed to shorten the wire path as much as possible, or order to maximize amperage through the wires. Shorter wires are capable of higher amperages for the same diameter without overheating. In Figure 3, presented is a method for shortening the wire path in motors with multiple solenoids arranged along the circumference. This is intended for any type of motor, axial or radial, but the central ring shortens the wire path for each solenoid significantly. The same strategy can also be employed on the stator. Conductivity to and from the rings can be achieved in numerous ways, some motors use carbon ceramic contacts and springs for conducting power in and out of the rotating axle and rotor. If it was my design, I would use a conductive bearing made of graphite and steel applied with spring loaded lever, placed along the conductive ring.

Figure 8. Axial Motor Winding with Central Conductive Ring.

 8.      Conductive Bearings for Power Access to Central Conductive Rings

Conductive bearings are a viable alternative to spring loaded carbon ceramic contacts or brush wire connections. Brush wires create significant drag and require maintenance. With conductive bearings, one could use copper, but copper is a soft material. With time the risk of deformation under heat may present itself.

An advanced solution might be if graphite is combined with electrical steel to make a conductive bearing, and a continuous flow of electricity can be achieved. If surface deformations vary the amperage, and one wishes to avoid noise in the power to the motor, one can add capacitors on both sides of the rotor wiring to engage in noise filtration. This in turn delivers cleaner power to the motor.

 

9.       Wires with Highly Conductive Elements like Graphene for Increased Amperage

Graphene displays properties of conductivity much higher than copper. Graphene can be removed from graphite, as a layer that already exists within the graphite. A topic worth researching may be the effects of adding graphite or other undiscovered material powders to wiring materials such as copper, steel, aluminum or stainless steel to increase the conductivity of the material. PNNL, Pacific Northwest National Laboratory, has shown in a recent study 1, (2023) that adding as little as 18 parts per million of graphene to copper resulted in 11% reduction in electrical resistivity and a decrease in temperature.

As a side note: it is possible to combine graphite with dielectric materials such as glass or ceramics, to create ceramic fibers that are conductive. This may be useful if one requires the wiring to have a high melting temperature or requires that wiring withstand high compressive stresses as in the case of a stress gauge. If the outer layer of the wiring is left without graphite, one can use the outer skin of the ceramic as the dielectric insulator.

In motors, strategies for mixing highly conductive graphite into the wiring are relevant if one seeks to simply increase the amperage, reduce the temperature and reduce the wire gauge for the same output.

  

10. Magnets as Replacements for Copper Solenoids to Reduce Heat Load and Cost

Replacing copper windings with permanent magnets in the stator of motors enables a reduction in motor heat discharge, which affects the temperature of the remaining copper windings. Reducing heat in motors is critical to motor longevity and maintaining low resistivity. Input power consumed is proportional to the resistance in the wire, which is defined by the temperature of the wire. Furthermore, the resistance in an electrified wire creates heat which increases the resistance, which in turn creates more heat. The net result is like a cascade effect, which we should seek to counterbalance with cooling strategies.

 

Figure 9. Resistivity vs. Temperature of Copper, Silver and Gold.

 

Magnets made of iron save money when compared to Neodymium magnets which are more expensive. One can also cast their own magnets if the powder version of the magnetic material is available and the resins needed are available at a non-cost prohibitive price.

Magnets made of abundant elements are currently available now. New chemical formulas for magnetic materials have been developed recently that are currently available; one such formula is Fe16N2. The cost is less than Neodymium magnets and the magnetic field strength is higher. Fe16N2 magnets have a field strength of 2.4 Teslas, neodymium (NdFeB) magnets have a strength of 1.5 Teslas, while iron magnets stand at 1.8 Teslas.

 

11. Generator-Motors pairs that Replace Gear Teeth. Energy Storage is Optional

A generator–motor pair is the concept that an input torque can drive a generator to operate at a specific rotational velocity and force and then wiring is sent to a motor elsewhere and the output torque is nearly the same but slightly less due to minor inefficiencies. Furthermore, just as gears are designed to vary force and rotational velocity, a generator-motor pair can be designed to do the same thing. The output force can increase while the rate of rotation decreases, so long as the torque remains constant, in other words, force times distance must remain the same.

In mechanical systems, it is advantageous to diminish the amount of times torque is transferred through gearing, because each transfer creates gear to gear surface contact which represents a reduction in efficiency. The average gear efficiency varies depending on the type of gear, but most gears operate with an efficiency between 90 to 99%.

In a motor to generator pair, the ratio of output force to output RPM can be varied a motor control circuit. A very simple example may be a transmission replacement in a vehicle or gear system in a bike.

In the bike example, if one coordinates the system to include energy storage where the energy produced from pedaling is evenly inputted across a 30-minute voyage but the energy output varies because of steep inclines mixed with flat ground, one could smooth out the journey by equilibrating the torque of all of the input strokes so that the bike ride is more enjoyable. It would require quantifying the elevation change and the duration of the journey first, to estimate the average torque input per stroke. If left over energy remains it can be used on the next trip.

It can also be used for force magnification analogous to a hydraulic system. In hydraulics the constant is volume expressed as cross-sectional area vs. distance that the piston travels. Force magnification is created by varying the cross-sectional area. In a generator-motor pair that requires additional force, but the distance must remain the same, a repetitive input action can create the required work via stored energy. It is analogous but what remains constant is work multiplied by time equal to energy.

Generator-motor pairs allow for a generator to run at a constant RPM which improves efficiency if energy storage is included. Energy storage should resemble a hybrid system with both capacitors and batteries. Batteries are less expensive, but capacitors can receive charge more effectively in short time intervals. Therefore, in some systems, input power could go directly to a small bank of capacitors and then the energy would drain via a circuit to a bank of batteries and finally be used again for power at the motor.

12. Heat Sinks in Motors and Cubic Boron Nitride as a Thermally Conductive Additive to Plastic and Metal Parts

In motor design, removing heat from the motor is critical. The lower the temperature of the copper wiring the lower the resistivity. Each material is different, but copper has a linear relationship between thermal resistivity and temperature. Similar to semi-conductor wiring copper is capable of very low resistance at very low temperatures.

An effective way to remove heat from metal parts, bearings, heat sinks and necessary thermal bridges would be to add ceramics that have a very high thermal conductivity to the plastic and metal parts being cast. Cubic Boron Nitride and Silicon Carbide have a higher thermal conductivity than copper along with a much higher compressive strength. In some cases, the parts can be substituted altogether with the ceramic material, in other cases they can be joined to create a thermal conductive bridge. The design of composites is beyond the scope of this report, but one example worth mentioning would be thermal bridges added onto the squirrel cage of the motor which allow for a direct passage of heat to a heat sink or heat sink fin. Simply by lowering the temperature of the heat sink the flow of heat out of the motor can be maximized as well, as demonstrated by depressurizing the air internal to the motor with a fan between the bearing casing and the rotor interior.

The thermal conductivity of Cubic Boron Nitride is 220-420 W/m-K. It is moderately priced and its compressive strength is 225 ksi. The thermal conductivity of Silicon Carbide is 120-250 W/m-K and its compressive strength is listed as 130 ksi but is closer to 60 ksi if sintered from powders.

 

 13. Circumferential Motors

Motors that have been discussed so far include Axial and Radial, but in today’s motor design culture very few people ever mention a third option: a design which is circumferential in its nature. Please see Figure 10 below.

Figure 10. Circumferential Motor Rotor with Two Wire Paths per Solenoid and a Centrally Conductive Ring.

Motors that have windings where the magnetic field travels along the motor circumference means that the wire paths can be shortened to an extreme degree, they could even be singular windings if manufacturing costs allow. See a cut away of a motor with 360 independent windings across 360 degrees in Figure 11 below. The rotor is placed inside of the stator to enable a nearly complete “sharing” of the magnetic field.

Figure 11. Cut Away Section of Motor with Singularly wound Wire Paths. Coils are shown in front cut away; structure is shown in back cut away.