In 2012, the total energy consumption of the United States was estimated to be 96 quadrillion BTUs (96 x 10^15). Figure 1 illustrates how this usage breaks down by sector. At the time of this study, energy consumed by the transportation sector was almost entirely from gasoline or diesel fuels. In the remaining residential, commercial and industrial sectors, it is estimated that approximately 35% of the energy is consumed by electric motor systems.
Figure 1. Total primary energy usage by sector in the United States
Looking within the residential sector (21% in Figure 1), Figure 2 shows the breakdown of energy usage by specific household electrical motor devices. What is most notable here is that approximately 87% of residential electrical motor energy is consumed by HVAC (heat pumps, air conditioning, HVAC fans and refrigeration). Thus, approximately 30% of the residential energy consumption is used for HVAC and refrigeration. Figure 3 breaks down energy usage in the commercial sector. Similarly to the residential sector, about 23.6% of energy is consumed by electric motor systems. What is also notable is these devices typically have electric motors rated at under 5 horsepower. Table 1 shows the full load efficiency of various motor sizes.
Figure 2. Residential energy usage by household devices
Figure 3. Commercial energy usage by type
Table 1. Full-Load Efficiencies for General Purpose Electric Motors [Subtype I] (DOE, 2012f)
These relatively small electric motors have, at best, a full load efficiency ranging from 85% to 88%. If the energy efficiency of this motor class can be increased by approximately 5%, the potential savings of energy can be roughly 870 x 10^12 BTU’s—or about 0.93% of the total energy usage in the United States—just in residential HVAC and refrigeration. Similarly, in the commercial sector, the potential energy savings are in order of about 500 x 10^12 BTU’s. Although the combined total seems small, they represent greater than 1% of the total energy consumption in the United States.
It should be noted that the potential energy savings cited above is calculated based on a 100% conversion to higher efficiency motors. This is unrealistic because of the cost—and sometimes the impossibility—of replacing the various motors. However, this potential savings can be realized as older HVAC and refrigeration systems and other electric motor drive systems are replaced, provided that the equipment is replaced with higher efficiency designs. In many cases, these higher efficiency motors will represent a higher initial cost to purchase; however, the payback on this higher initial investment can be a short as 1 year for refrigeration and freezers to about 5 years on certain HVAC systems. Thus, the reduced operational cost will offset the initial investment with the added benefit of lower utility bills in the long term.
The question is, “How can these small horsepower motors be improved to produce these greater efficiencies?” The answer includes variable frequency drives and rethinking the basic design of the motor itself. At present, the vast majority of commercial and residential motors are induction style motors. These are a proven design with long term reliability, yet there is room for improvement. Axial flux motors, transverse flux motors, and even a rethinking of the basic design of the radial flux motor can provide the necessary torque with lower energy usage. Let’s take a look at each of these new motor types and a new class of electrical materials (soft magnetic composites, SMCs) to evaluate their potential.
Axial Flux Motors
Axial flux motors (AFM) have been around almost as long as the traditional radial flux motor. The basic principle of the axial flux motor is that the magnetic flux is parallel to the axis of rotation of the motor. This reorienting of the motor windings affords the opportunity to simplify the motor winding process. A recent study demonstrated that these motors can have efficiencies up to nearly 96%, but, just as importantly, have higher efficiencies under lower load conditions. Specifically, for a 2.2 kW variable speed application, the axial flux design can provide nearly 90% efficiency at 50% of the rated maximum load.
SMC Axial Flux Stator Design
Yokeless Axial Flux Motors
Taking the axial flux motor a significant step further is the concept of the yokeless axial flux design. In this design, the individual poles of the AFM are not attached on either the top or bottom. This offers the possibility of having permanent magnets on the top and bottom of the rotating element, thus producing more torque. As reported by YASA , this increase in torque can be up to 400% of that of the same radial flux design with the same higher operational efficiencies but in a smaller and lighter package.
SMC Yokeless Axial Flux Stator Tooth
Overall, the advantages of the AFM design are
- Lighter weight and smaller overall sizes for the same output
- Simplified winding with the possibility of greater utilization of the copper (no end turns)
- Potential to offer material savings via the use of SMCs (discussed below).
Transverse Flux Motors
Transverse flux motors (TFM) were first envisioned in the late 1890s. TFMs have high torque and power density, and these are unique merits for direct-drive systems such as wind turbines and in-wheel traction. The basic TFM structure consists of a ring-shaped coil that embraces stator U-cores, which guide the magnetic flux from one rotor permanent magnet to another. Due to the “transverse” magnetic plane, it is possible to increase the number of pole pairs while keeping the machine diameter constant. Theoretically, torque increases in proportion to the number of pole pairs. However, cogging torque, efficiency, power factor, and manufacturing of TFMs should still be improved. The efficiency of the TFM is a high as 92%.
Figure 4. Schematic of transverse flux motor winding
The main advantage of the TFM is high torque density, especially at low speeds. Some of the disadvantages of the FM are
- Torque ripple
- Complex design
- Nonlinear dynamic performance
Trapezoidal Radial Flux Motor
Recently, the radial flux motor has been reimagined as the trapezoidal radial flux motor. In this design, reliance on expensive rare earth magnets is reduced or eliminated via the use of ferrite magnets or a unique pre-wound trapezoidal tooth that gives superior performance. These pre-wound trapezoidal coils have high fill factors and eliminate the end turns of conventional radial flux motors. The end result is a high torque, energy-efficient motor. For example, a 17-kW motor that weighs only 6 kgs offers up twice the power and torque density per liter as the RFM equivalent.
How does SMC technology fit into these nontraditional motor concepts?
Soft magnetic composites (SMCs) are an exciting subset of the powder metal industry. Essentially, they are iron powder particles individually coated with an electrically insulating material to prevent excessive eddy currents from forming in the component. SMC components can be produced via standard powder metallurgy compaction techniques. This implies high material utilization, dimensional precision, and low losses particularly at higher operating frequencies (typically >500 Hz). SMCs are the preferred alternative manufacturing method for each of the three motors discussed earlier. In addition to the benefits listed above, SMCs also provide 3D shape making and 3D flux carrying capabilities.
Can you produce these motor types with laminations? The answer is yes. However, this faces you with a difficult assembly process and the need to either weld or mechanically join the laminated pieces. Shown below is the negative effect of welding laminations relative to the core loss of the device. It is apparent that welding effectively creates electrical shorts between the laminations, producing excessive eddy current heating and a loss in both performance and efficiency.
Figure 5. Effect of welding laminations on the total core loss.
You might think that you could utilize a relatively low-cost induction motor. Depending on the operational characteristics of the total system, however, the simple induction motor may not give the starting torque required. You would need to utilize a capacitor start design that incorporates an additional winding plus the high voltage capacitor. (Even a simple garage door opener has a capacitor start design.) This could mean up to 30% higher productions costs. So, the potential savings are not a great as once thought.
The electric motor market is undergoing a revolution, emphasizing unique motor shapes that enable higher efficiencies while reducing cost, weight, and demand on critical resources such as lamination steels, magnet materials, and copper.
How can you be part of this transition? When used in innovative motor designs, soft magnetic composites can help you build a greener future.
