Magnetic gearing enables compact electric propulsion motors
Magnomatics’ PDD motor drastically reduces the size of direct drive electric motors.
Increasing demands for better fuel economy, lower emissions, and quieter operation, has led builders of a wide range of vessels to consider hybrid electric propulsion systems.
Series-hybrid diesel-electric systems are frequently employed in cruise ships, ferries and naval vessels (often with additional gas turbine systems). The engine drives a generator which produces electrical power for the propulsion motors and hotel load. This architecture enables engines to run at optimum efficiency and allows flexible layouts. In smaller craft, batteries can enable low-noise, zero-emission, all-electric running in harbours and environmentally sensitive areas, while the diesel engine allows long range motoring. The alternative parallel-hybrid system simply enables the engine to be down-sized, with a power boost provided by an electric motor mounted on the same shaft.
To directly drive a propeller requires large and expensive electric motors and for this reason, it is usual to combine a high speed motor with a mechanical gearbox. However, mechanical gearboxes reduce efficiency and reliability, while increasing noise and maintenance requirements. The transmission oil is also a potential fouling hazard.
Sheffield UK based Magnomatics’ patented Pseudo Direct Drive PDD motor drastically reduces the size of direct drive electric motors, by integrating a revolutionary magnetic gear within a permanent magnet brushless motor. The high internal speed of the PDD motor output is reduced by the integral magnetic gear, giving a torque output that is up to six times greater than that of any traditional electric motor of a similar size. The non-contacting magnetic gear element is quiet and has no friction, thus enabling superior system efficiency over the entire operating speed and load range.
Benefits of the PDD are ultra high torque density, allowing a smaller motor for a given torque; lower diameter for concentric shaft mounting; the motor can be placed further towards the stern, reducing shaft length; and it provides optimum hydrodynamic performance for podded propulsion. It delivers high efficiency across a wide operating range, including low loads; reduces fuel consumption, emissions and operating costs; and requires only a minimal cooling system. Mechanical gearboxes are eliminated, reducing servicing requirements and also resulting in very low vibration and acoustic noise.
Propulsion power for large vessels is in the megawatt (MW) range with typical propeller speeds of less than 200 rpm. PDD motors at this power rating are commensurate in size and efficiency with similarly rated superconducting machines, but without the requirement for complex and expensive cryogenic systems. The potential advantages for large vessels are such that the UK Ministry of Defence has recently awarded Magnomatics a six figure contract to initiate development of magnetically geared propulsion motors at power levels of up to 15 megawatts for Integrated Full Electric Propulsion Systems for future frigates and submarines.
Magnomatics is developing a range of magnetic gears and PDD motors incorporating magnetic gearing. Magnetic gears can provide a non-contact, lubricant free transmission through a membrane, removing the need for dynamic seals and preventing contamination. Magnetic gears and PDD motors are scalable and Magnomatics is currently developing solutions with torque ratings from a few Newton-metres up to mega-Newton-metres.
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