How do Motors Work?
Motors are machines that convert electrical energy into mechanical energy using magnetic forces. When current is passed through a wire loop that lies in a magnetic field, a turning force, or torque, is created that causes the loop to rotate. In motors, this rotating motion is transmitted to a shaft. This rotational energy is then utilized for useful work in the form of mechanical power. Industrial motors are used for running pumps, fans, and compressors as well as in equipment involved in the handling and processing of manufactured materials. Marine propulsion motors are used to propel commercial vessels and warships.
The basic features of modern conventional AC (alternating current) and DC (direct current) electric motors were first designed in the 1890s, and the underlying technology has not changed significantly in the past fifty years. Despite the lengthy period of time in which motors have been in development, motors are still far from being perfectly efficient converters of electrical to mechanical energy. The principal causes of lost power in motors come from the electrical resistance of the wire and from mechanical friction.
Energy Consumption by Electric Motors
According to the U.S. Department of Energy, motors account for 70% of all energy consumed by the domestic manufacturing sector and use over 55% of the total electric energy generated in America. Large electric motors, those greater than 1000 horsepower, consume over 25% of the total generated electric energy. With some minor exceptions, nearly all cruise ships today are being built with electrical propulsion, and many other types of commercial vessels and warships are now also adopting marine motors as their primary source of motive power.
How do Superconducting Motors Differ from Conventional Motors?
Superconducting motors are new types of AC synchronous motors that employ HTS (high temperature superconductor) windings in place of conventional copper coils. Because HTS wire can carry significantly larger currents than can copper wire, these windings are capable of generating much more powerful magnetic fields in a given volume of space.
Advantages of Superconducting Motors
More Compact Size
Advances in coil design make it possible for a superconducting machine to match the power output of an equally rated conventional motor with as little as one-third the size and weight. The smaller size and compact nature of superconducting motors allows them to be manufactured at lower cost than equivalent conventional motors.
Additionally, the replacement of conventional copper rotor windings with non-resistant HTS coils results in sharply reduced electrical losses in the rotor. The increased electrical current in the motor’s rotor results ultimately in the increased power density (and hence smaller size). The HTS motor’s smaller size means it is significantly lighter, and therefore can be utilized in new and innovative ways. In addition, the motor’s increased efficiency results in lower operating costs than conventional motors.
HTS AC synchronous motors also have no iron teeth in the armature (stator windings), not only contributing to their smaller size and lighter weight, but also removing a significant source of motor noise.
Potential Energy Savings
As an example of the savings that can result from the use of large HTS electric motors, a 1% increase in the efficiency of a 5 megawatt (approximately 6,500 horsepower) electric motor, run 7 x 24, will result in the saving of over 430,000 kilowatt hours of electricity per year.
Where and How Will Superconducting Motors be Used?
HTS motors will compete in the large (1,000 hp and above) commercial motor market. These motors are ideal for use in pumps, fans, compressors, blowers, and belt drives deployed by utility and industrial customers, particularly those requiring continuous operation. They will be suitable for large process industries such as steel milling, pulp and paper processing, chemical, oil and gas refining, mining and other heavy-duty applications. An important and rapidly growing use for HTS motors will be in transportation applications, particularly naval and commercial ship propulsion, where size and weight savings will provide a key benefit by increasing design flexibility and opening up limited space for other uses. A marine engineering “revolution” is taking place today in the maritime industry, and more and more ships are being built with electrical motors as their primary means of propulsion as the benefits of electric propulsion are being verified and new types of innovative ship designs incorporating electric propulsion are introduced.
What is the Market Potential?
It is estimated that the worldwide addressable market for large industrial motors is over $1.2 billion annually. HTS motors will offer an attractive economic alternative to conventional motors by virtue of their lower first (acquisition) cost and their reduced ongoing (operating) cost. Electric marine propulsion is a market of approximately $250 million that is growing strongly. Some studies indicate that this market will quadruple over the next decade. Superconducting specialty motors will be particularly attractive for niche applications in which size and weight considerations come into play.
What are the Benefits?
• Lower first cost - Because of savings in materials and labour, HTS motors will cost less to manufacture than their conventional counterparts.
• Lower operating costs - An HTS synchronous motor has considerably reduced losses, yielding significant annual savings in electricity consumption. For continuous duty cycle motors, a one percent increase in efficiency can result in thousands of dollars in annual energy savings.
• Less vibration and noise - Elimination of iron teeth in the stator results in a lightweight motor that boasts quiet and smooth operation.
• Smaller size and weight - The compact design of HTS motors will facilitate placement in transportation applications where space and/or weight is at a premium, as well as in upgrades of industrial facilities where increased power requirements conflict with limited space availability.
• Increased stability - Superconducting motors are inherently more electrically stable during transients than conventional motors because they operate at small load angles (15 degrees vs. 70 degrees for a conventional motor) and have a much higher peak torque capability (~300%). As a result, the motor can withstand large transients or oscillatory torques without losing synchronous speed. The HTS machines do not require rapid field forcing during fast load changes or transients as is often the case with conventional machines.
• Faster delivery and installation - The smaller size of superconducting motors will enable them to be manufactured and shipped directly to the customer without costly disassembly and subsequent onsite re-assembly and testing. It would also be possible to build motors up to 5000 hp, 1800 rpm in NEMA (National Equipment Manufacturers’ Association) sizes on a continuous assembly line. Marine propulsion motors can be installed external to the hull of ships in “propulsion thrusters” (or pods, figure 1), which have the potential to dramatically lower shipbuilding costs and shorten construction schedules. These advantages will reduce delivery lead times and reduce overhead costs. Podded propulsion applications provide greater hydrodynamic efficiency and reduce operating expenses.
Figure 1. Pod-type motors for marine vessels.
Superconducting Motor Types
Superconducting technology will be employed in a variety of motor designs. Industrial HTS motors are designed for constant speed application. They will be operated at 1800 or 3600 rpm.
For marine propulsion, a high-torque, low-speed design is favoured. These motors will be designed to operate at variable speeds below 200 rpm.
Today’s motors are fundamentally similar to the electric motors designed over a century ago, and the motor manufacturing industry has seen only incremental improvement in product design over the past fifty years. The advent of high temperature superconductivity has created the opportunity for a quantum leap in the technology of large motors. The tremendous cost, size, weight and efficiency benefits of superconducting machines will significantly change the dynamics of the motor manufacturing industry and the motor end user market.