Torque about direct drive technology

18 February, 2009

In high torque, low-speed applications an efficient high-speed electrical machine is typically used in conjunction with a mechanical gearbox. Despite precision machining and improvements in lubricant technology, however, gearboxes reduce system efficiency and introduce maintenance and reliability problems.

While moving to a direct-drive topology removes the mechanical gearbox, the much higher torque requirement results in larger and more expensive electrical machines. Magnomatics’ pseudo direct drive PDD technology delivers an exceptionally compact direct drive solution through the use of integral magnetic gearing.

Benefits

The PDD is a new type of motor/generator capable of achieving ground-breaking continuous torque densities far in excess of that offered by existing electrical machines. This step change in torque capability is achieved through gearing generated internally by permanent magnets with an efficiency considerably improved over that of mechanical gearing.

For a given torque, the PDD will typically be less than half the size of a conventional low speed permanent magnet machine. The low current density within its windings means that the PDD is highly efficient, typically operating without any forced air or liquid cooling. It also features passive protection from being overloaded as the magnet gear will slip before the windings burn out.

The technology also scales very well, with increasing benefit being realised as the torque requirement increases. This makes it particularly attractive for use in multi-megawatt applications such as wind turbine and other renewable energy generation, marine propulsion, industrial automation, large valve actuation and mining applications.

The technology

A PDD includes the two core components of a permanent magnet machine: a high-speed permanent magnet rotor and a wound stator. The windings interact with this inner rotor to produce torque. Within a larger than usual airgap between these components it also has a modulating rotor consisting of ferromagnetic segments (typically laminations or soft magnetic composite) and an additional larger array of magnets is affixed internally to the stator. The inner rotor and this static magnet ring and steel segments form the integral magnetic gear.

Magnetic gears are an advance on 1:1 magnetic couplings which have been widely used in industry for fifty years as trusted components within a wide range of industrial machines and processes. In a ‘synchronous’ magnetic coupling the fields of the two permanent magnet rotors which have the same number of poles interact to produce an alignment torque and, as one rotor rotates, it drags along the other rotor synchronously. In a magnetic gear, one rotor (low speed) has a high number of magnetic poles, the other (high speed) has a low pole number.

These two rotors when placed concentrically, as in a coupling, would not be able to transmit net torque. The key technology in the gear is the series of ferromagnetic segments between the two rotors that enable the two differing pole-number rotors to ‘couple’ with each other. This modulates the flux from one rotor to create the correct rotating field pattern which the other rotor can couple to. This field pattern travels at a speed that is different to the rotor producing it, and hence produces a gearing of speed and therefore torque. Using high energy rare-earth magnets, magnetic gears can be realised with continuous torque densities of similar magnitude to that of mechanical gears and, by appropriate choice of the number of rotor poles and pole pieces, a wide range of gear ratios can be achieved with negligible torque ripple.

If we consider the PDD in the mode of a motor, then the current through the windings will produce a rotation on the internal high speed rotor. This rotating field will be modulated by the ferromagnetic pole pieces and create a slower moving field that, as the outer array of magnets is fixed to the stator, will cause the modulating rotor to turn at a slower speed and with a higher torque than the free-spinning internal rotor.