Mega – watt?

By Carsten Paulun
Today’s world is unthinkable without electric motors. Whether in heaters, hair dryers, drills, or washing machines, or even in electric cars – electric motors can be found practically everywhere. Will that soon be the case increasingly often in aircraft and ships too?
© audioundwerbung/iStock

The capacity range of electric motors depends on the areas in which they’re used and extends from a few milliwatts for smaller actuators to several hundred kilowatts in electric cars. But above that? Are electric drives easily scalable? The keyword in that context is megawatt motors and they’re supposed to make aircraft, ships, trains, and large construction machines, etc. more climate-friendly and efficient.

Mega – watt? Experts describe a megawatt motor as a drive system that can convert electrical energy in the range of one megawatt into a motive energy. One megawatt (MW) is a unit of measure equaling one million watts or one thousand kilowatts. Those are motor capacities of the kind that industrial applications such as conveyor systems, mills, and construction equipment such as tunnel boring machines need as well.

“Today, the biggest megawatt machines in many cases are not used at all as motors but as generators. Unlike electric motors, they don’t convert electrical into mechanical energy but vice versa. In a wind turbine, for instance, the rotational motion of the rotor is converted into electric power,” explains Johannes Klötzl, Managing Director of Compact Dynamics, Schaeffler’s subsidiary specializing in electric drives.

20 MW

That’s the capacity of the currently most powerful wind turbine systems. Powerplant generators even top that. They have diameters of more than ten meters (33 feet) and capacities in the four-digit megawatt range.

© braverabbit/iStock

But even electric motors in the megawatt capacity range are already operating in many places. They score with high efficiency, ample torque, and comparatively simple technology. They’re most frequently used in the rail sector – and have been for decades. Even today, the BR 103 electric locomotive that was produced as a prototype in 1965 and as a serial type between 1970 and 1974 is deemed the world’s most powerful one-piece locomotive and most powerful vehicle ever to have been used in German line service. Its six-megawatt motors deliver a total output of 7,440 kW. Even clearly higher ratings can be found on ships.

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electric motors as tall as a man, each installed in propeller pods underneath the ship, power the Queen Mary 2 ocean liner. Each has a capacity of 21.5 megawatts. For comparison: if those motors were to be run as generators under full load, the energy they’d produce could supply 200,000 to 300,000 residents of a city with electric power.

© Cunard
Identical yet different

Even though small and large electric machines, whether they’re motors or generators, are identical in terms of their basic design principle – which has been established for nearly 200 years featuring a stator and a rotor and the rotational motion resulting from attractive and repulsive forces – there are differences due to the type of machine and its uses.

A key difference lies in the projected life span. “In the automotive sector, we expect 300,000 kilometers (186,000 miles) on the clock are expected to correspond to some 8,000 service hours. With an optimally positioned wind turbine that level may have been reached after just one year,” says Klötzl. “For service in these areas to be profitable, significantly higher mileages must be achieved."

Aviation as an outlier

Unlike ships, locomotives, or stationary systems such as wind turbines, aircraft are subjected to an altogether different set of load cycles. “You’ve got massive loads during takeoffs and reverse thrust while landing. Plus, the ambient temperatures to which aircraft are exposed change significantly several times a day. At cruising altitudes of 10,000 meters (33,000 feet) and more at the edge of the stratosphere temperatures of minus 55 degrees centigrade (–67 degrees Fahrenheit) prevail. These megawatt motors shall withstand all that – and do so for years,” Klötzl points out.

In 2035

aircraft manufacturer Airbus is planning to deploy large hydrogen-electric aircraft as part of its ZEROe project. The propulsion system called “Iron Pod” combines a 1.2-megawatt hydrogen fuel cell system, two electric motors, and the control and cooling components.

Smaller electric propeller-driven planes for short-haul service carrying up to 10 passengers are already on the market or on the way toward it. “In this case, electric motors running in parallel within the capacity range of a passenger car motor are sufficient,” says Klötzl. However, at Compact Dynamics, megawatt motors for medium-haul service are moving into focus as well.

Johannes Klötzl and his 90 colleagues at Schaeffler’s electric motor specialist Compact Dynamics know that the bigger the machines and motors the bigger the challenges. “In aviation, weight and design space are a much greater issue than with ships or wind turbines for example. The less of that is required by the technology the more passengers or payload an airplane can carry and the longer the distance it can fly. In a sector trimmed for maximum efficiency those are crucial factors,” Klötzl says based on many conversations with aircraft engineers, plus, “the trick is to keep the weight and design size of megawatt motors for aviation from skyrocketing despite their high output.”

One key to success is motor speed, “because power output equals motor speed times torque, but torque costs weight and design space,” says Kötzl, who has a degree in engineering. A megawatt motor in aviation must develop motor speed in the five-digit range to be able to deliver the requisite output in a unit with compact dimensions and weight. Yet increasing torque is no mean feat. At some point, centrifugal force sets a limit for the materials used. “The components must become lighter and lighter and smaller and smaller to keep them from blowing up in your face. But then the bearing support for drive shafts and the like becomes a challenge.” Transmissions can be of help but at the expense of design space and weight.

Mega – watt?
Johannes Klötzl, Managing Director of Compact Dynamics, Schaeffler’s subsidiary specializing in electric drives
© Privat

"Power output equals motor speed times torque, but torque costs weight and design space."

More output also means more current flow and therefore more heat, which takes us to another key item in the development specifications: thermal management. Additional components for cooling consume design space and add weight. At least the question of whether the current flows from a battery or from a fuel cell plays a subordinate role in that regard.

Limits to growth

In view of the complex harmony between power output on the one side and factors like weight, size, and stability on the other, Klötzl, especially with reference to electric aircraft motors in the multi-digit megawatt range, says, “At Compact Dynamics, we’re also achieving increasingly higher power density of our electric motors due to new manufacturing methods, enhanced materials, more efficient thermal management, and better control electronics, but at the level of today’s state of the art, output cannot be scaled up infinitely. Consequently, it just makes more sense to provide the required power output with several smaller units or to opt for other technologies like sustainable fuels for internal combustion engines instead of the motors becoming bigger and bigger.”

Besides the technical challenges there are regulatory and industry-specific ones as expert Klötzl cautions, “In aviation you must certify every bolt, every plug connector, every assembly component. The approval efforts are massive, which encourages a certain conservativism in technology development.”

Hopes pinned on superconductors

Especially in the field of electric motors in aviation, the research field of superconductors could close the gap between power output and weight. Klötzl shares the certainty that it could: “Superconductors can truly be a breakthrough technology that can massively advance electric motors both in terms of output and efficiency,” he says. The electrical resistance of these materials is near-zero, which means that clearly more electric power can be transported without enlarging the line cross sections than with today’s typically used copper wires. Consequently, megawatt motors could remain lightweight and compact units.

The big hurdle: For large-scale application, the technology is still far too costly and complex. Currently, superconductors function only at extremely low temperatures and/or under high pressures in the gigapascal range, i.e., more than one million times above the normal air pressure.

For wind turbine generators, the industry is currently testing so-called superconducting lines that are cooled down to minus 269 degrees centigrade (minus 452.2 degrees Fahrenheit) by means of helium. The advantage of this configuration is that the energy for cooling can be generated directly on-site. In the shipping sector, there are some initial and exceptionally impressive advances as well: U.S. company Northrop Grumman in 2022 successfully tested the world’s first 36.5-megawatt ship propulsion motor with a high-temperature superconductor, although the classification of “high-temperature” may be confusing because in this research field a chilling temperature of minus 243 degrees centigrade (–405 degrees Fahrenheit) is deemed “high-temperature.” The advantage is that thermal loads that especially powerful conventional electric motors are exposed to in normal operations are eliminated by this type of “deep freezing.” Without a doubt, the Northtrup Grumman behemoth is a powerful unit. In addition to the previously mentioned 36.5 MW (49,000 horsepower) it develops torque of 2.9 million Nm at 120 rpm.

The aim of this research is to find superconducting materials that work for production use in terms of price and technology. “That would raise electric motor development to a new level,” says Johannes Kötzl. But strides made in conventional technology provide megawatt motors with the potential to become a key pillar of industrial and environmental transformation.”

The electric specialists

Schaeffler’s subsidiary Compact Dynamics GmbH based in Starnberg is a development specialist in the field of innovative electric drive concepts. Whether in one-off production, small series up to 1,000 units, or in mass production in collaboration with Schaeffler; whether racing teams, automakers, or suppliers; whether visionaries for outstanding concepts or partners for research and predevelopment projects: for more than 25 years customers all over the world in the fields of motorsport, aviation, electric mobility, marine and industrials rely on the expertise of Compact Dynamics. Among other things, the company’s 90 employees have developed drive systems and generators/MGUs for Formula 1, Formula E, the World Rally Championship and for endurance races such as the Le Mans 24 Hours.

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