Magnetic-geared propulsion: a sea change in efficiency and reliability

Future marine propulsion systems face considerable demands. Aside from providing vessels with the necessary power and control for speed and manoeuvrability, they need to be energy-efficient, lightweight, durable, safe and have low maintenance requirements. Additionally, with growing concern about the impact of marine transport on the environment, vessels will need to be greener to comply with international regulations.

From as far back as 1903, the marine industry has wavered in its favour between electric and conventional propulsion systems. What held electric propulsion back, despite its superior efficiency and other benefits, were its cost, complexity and size. Magnetic gears, first patented in 1968, offered the potential to address these issues, but it was not until new magnetic materials and analysis techniques were introduced early this century that a feasible solution was found.

Compared to conventional marine propulsion motors, such as permanent-magnet and wound-field electrical machines, magnetically geared systems are far more effective in delivering the needs of modern vessels. Regarding size, weight and cost, traditionally the use of a mechanical gearbox has allowed a high-speed electrical motor to drive a low-speed propeller, enabling the motor to be more compact and less costly than a direct drive equivalent. However, these advantages are lost through the need for a high-torque gearbox.

A magnetic gear system overcomes this issue. By coupling a high-speed permanent magnet motor with a magnetic gear, the result is a low-speed, high-torque output which is optimised to the propeller performance curve. Moreover, with a thruster smaller than an equivalent direct-drive motor and with the integrated magnetic gear removing the need for a conventional gearbox, high speed can be achieved with a design that is significantly smaller, lighter, less complex and less costly to produce.

“The modulated magnetic coupling allows the two rotors to turn at different speeds, creating the gear effect”

What makes this possible is the magnetic motor system’s innovative engineering and design. Similar in concept to an epicyclic gear, the latest high-powered magnetic gears feature a central high-speed sun gear that is coupled by connecting planets to a low-speed outside ring gear. Instead of having mechanical teeth that are prone to wear, the interlock is provided by a series of North/South magnetic pole pairs. The rotating planets, meanwhile, are replaced by a series of steel segments that modulate the magnetic fields so that the sun and ring rotors can be coupled. As a result, the modulated magnetic coupling allows the two rotors to turn at different speeds, creating the gear effect.

The outcome of this design approach is a compact, high-speed, low-load electric motor that is ideally suited to the needs of propellers and which overcomes the issues associated with conventional gear systems by reducing size, complexity and cost. Moreover, by simulating the characteristics of a direct drive, these magnetic systems offer high torque at low speeds and by gearing up the inner rotor torque between 5:1 and 10:1 in the novel polepiece rotor, deliver exceptional efficiency.