More efficient and environmentally friendly motors can be developed by focusing on the construction of the motor piece by piece to determine where new materials or constructions can be used to gain the greatest overall efficiency boost. Improving the efficiency of motor construction and operation starts with the most basic, but possibly the most important, component of the motor: the windings. Winding materials are often insulated wires wrapped tightly together in a dense coil, designed to produce a magnetic field in response to electric current. Superior electric motor windings may be the key to improved performance of electric motors in the future. Of course, the demand for efficiency and improved performance goes beyond electric motors and also applies to other devices requiring a tight coil of insulated wire such as generators, transformers, and electromagnets.
The ability to make motors lighter and smaller is necessary to achieve higher power efficiency ratings. In this article, we focus on the winding or magnet wire portion of the motor and consider the common or advanced materials that are used for these parts, or which may be used in the future to improve the weight, strength, flexibility, thermal/electrical conductivity, and cost of motor and generator winding constructions.
Copper is the most common magnet wire choice due to its high conductivity and relatively low cost. For most motors like the one shown below, we use copper with a very thin enamel coating and tightly wrap the wire to create the winding that will build the electromagnetic field to drive the motor.
The photo of a drone motor shown above gives us perspective of how much copper goes into the motor, and why material weight is important to increase the efficiency of the motor. If we were able to easily decrease the weight of all that copper on the motor and maintain its power output, this would dramatically decrease the amount of power required to fly the drone. Copper is a great choice for motor windings because of its high conductivity and relatively low cost, but it is also a highly dense and heavy material; this is even more of a problem for motors used in electric vehicles or aircraft, which need to be lightweight. Copper will work just fine for most motor applications, but when considering weight, strength, and stability in high temperature or other demanding environments, we should consider some other potentially better materials.
If we were considering weight alone, Aluminum wire would be a great choice for magnet wire construction. Aluminum is a commercially available magnet wire option, but because it is less conductive than copper, larger wire diameters and correspondingly larger motors would be necessary to create the same power outputs. Furthermore, Aluminum is more prone to flex fatigue and likely to break more easily after repeated movements. Another downside to aluminum is the increased potential for corrosion and the difficulty of keeping the contacts clean, causing a higher localized resistance and potential for connection point thermal failure. Improvements could be gained by using a combination of aluminum with other metals to increase the conductivity, maintaining the same physical motor size and the same power output as a motor with copper windings while still decreasing the weight.
Gold and Silver
Wires made from gold and silver offer a low resistance and are also more corrosion resistant than aluminum or copper; in fact, silver conducts electricity lightly better than copper itself. However, both gold and silver are substantially more expensive than copper. The increased cost and low availability of these materials would make it difficult for these materials to become mainstream magnet wires for electric vehicles and aircraft
Carbon Nanotubes (CNTs)
Carbon nanotube fibers and yarns have gained the attention of the electric motor and power generation industries due to the incredible combination of properties offered by CNT materials. Carbon nanotube fibers and yarns offer a highly flexible, strong, and lightweight option for motor winding constructions. Carbon nanotubes also offer higher conductivity than copper at the molecular level, although it has not yet been demonstrated that CNT yarns can achieve this level of conductivity on the scale of macroscopic fibers.
Current state-of-the-art CNT fibers have a conductivity that is 15 - 20 % that of copper; considering this, further improvement is needed before CNT fibers can be a competitive material for most types of magnet wire. There may be an advantage to be gained by using CNT fibers in motors that operate at higher frequencies, because copper's electrical performance is reduced at higher frequency operation compared to that of CNT fibers.
Flexibility of CNT fibers is vastly superior to copper, being more comparable to that of a textile thread with the ability to survive millions of flex cycles. Combined with its high strength, this level of flexibility can allow for an increase in packing efficiency of motor windings and enable faster, more reliable installation methods to create improved magnet wire constructions. CNT fibers and yarns are also by far the lightest option for a magnet wire, being 9 times lighter than copper wire and 3 times lighter than aluminum wire.
One of the primary drawbacks to using CNT yarns as motor windings is the cost of the material; these fibers are currently one of the higher priced alternatives to aluminum and copper, and are more expensive than gold and silver. As demand for carbon nanotubes fiber increase and advances in production techniques evolve, carbon nanotubes fibers may start to become more of a competitor in the magnet wire space when in terms of price per pound.
Material choice plays a large a role in determining a good magnet wire candidate, but changing the shape of the wire can also unlock more potential for efficiency. The shape and composition of each of the materials we have discussed so far can be modified to some extent; for example, most wiring materials are typically made with a round cross-section but can also be made into a film or ribbon shape. A major benefit of the ribbon shape is an increased packing density vs a round wire. The higher packing density can result in a more compact motor with the same power output; however, this construction does come with some drawbacks. Common problems with wire in the ribbon format include heat retention, flexibility, and installation complexity. With the right combination of insulating materials, the flexibility, thermal capacity, and strength of carbon nanotube films might make them an interesting option for flat magnet wiring.
Rather than considering just one material to improve magnet wiring, we should also consider that a combination of the right materials might yield the best result. Not all electric motors and generators are designed the same way, and not all motors and generators are trying to achieve the same job; when we compare the requirements of aircraft and the requirements of locomotives we see a large number of differences (one of them being how critical motor weight might be). The one requirement that is universal in every application is increasing the efficiency of power consumption. That said, designers of future motor tech should consider the needs of each different application and maintain an open mind towards the materials that can achieve a proper hybrid material to meet the desired goal.
One good example of a hybrid wire is a combination of copper and carbon nanotubes. This combination of materials can provide wires with a thermal stability much higher than copper alone. For motors that operate at higher frequencies and higher temperature ranges, we might see that a CNT-Cu composite could be the next version of commercialized copper as magnet wire to maintain efficiency for electric motors and generators running in harsh and demanding conditions.
In the video below video we provide a brief look at some of the experimental work that has been performed at DexMat to create CNT-Cu composite wires. Here, we use an electroplating process to coat carbon nanotube yarn with a layer of copper. This process results in a useful hybrid material, combining the conductivity of metallic copper with the strength and durability of lightweight carbon nanotube yarn.
The rapidly improving conductivity and excellent thermal properties of CNT yarns and films, combined with their light weight, high strength, flexibility, and ability to be combined with other materials may be the next big innovation in magnet wire for lightweight motors.