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Has the geared motor had its day?

01 July 2009

The inventors of an innovative magnetic drive system believe that it is poised to challenge conventional practice, hitherto dominated by the geared motor

Magnomatics’ patented Pseudo Direct Drive (PDD) represents a significant advance over conventional motor technology, achieving a step-change in continuous torque density of up to eight times that of equivalently cooled permanent magnet machines. This dramatic increase in torque density allows the motor to drive directly those loads that would otherwise have required a motor and gearbox combination.
Reported via the DPA website in November 2007, PDD is a type of motor/generator configuration that is capable of achieving continuous torque densities far in excess of anything offered by existing electrical machines. The gearing effect is generated internally by permanent magnets with a level of efficiency considerably better than that of mechanical gearing. Further benefits include prevention of failure mechanisms such as jamming and reduced levels of noise and vibration. Several industrial applications are now realising the full potential of this technology, which challenges standard electromagnetic topologies such as motors, generators, and actuators.

In these applications, significant performance benefits are achieved by replacing the motor and gearbox with a direct drive motor. Primarily, efficiency is improved as there are no gearbox losses and, of course, gear wear is eliminated altogether. Additionally, there is no need for lubrication, reducing the maintenance burden and avoiding the possibility of oil leaks contaminating a production line or process.

In order to achieve high torque at low speeds in direct drive systems, brushless permanent magnet motors are typically used because they offer optimum torque density and torque-speed characteristics. However, due to limits on magnetic, electrical and thermal stresses, even when employing high-energy, rare-earth permanent magnets, the continuous torque output per-unit-volume/mass is limited. The resulting direct-drive motor is then often prohibitively large.

The PDD motor overcomes the torque density limitations of conventional direct drive permanent magnet motors, by exploiting a mechanically and magnetically integrated passive magnetic gear, which acts as a speed-reducing, torque-increasing transmission that requires no lubrication (see Figure 1). The resulting motor topography offers superior torque density, allowing drastic reductions in motor frame size to be achieved for a given load, while maintaining the practical advantages of a naturally cooled system.

An inner high-speed permanent magnet rotor and outer stator carrying windings act as a conventional permanent magnet brushless dc or ac motor. The rotation of the high-speed inner rotor is magnetically geared to give a high-torque, low-speed output on a second rotor. The key benefits of PDD may be summarised as follows:
* High torque density (hence compact machine for given torque)
* High efficiency across a wide operating range
* Removes need for mechanical gearbox
* Reduced servicing requirements
* Very low vibration and acoustic noise emission
* No (or minimal) cooling requirement
* High power factor operation
* Uses standard ‘off-the-shelf’ power electronics/drive technology

Torque density breakthrough
Magnomatics has successfully developed a totally enclosed, naturally ventilated PDD machine with a rated continuous torque capacity of 95Nm, a stator stack diameter of 180mm and length 45mm. The machine (Figure 2) has a rated output speed of 400rpm and an efficiency of 95% at rated load.

The active torque density is 83 Nm/m3 and is eight-times greater than a conventional naturally ventilated permanent magnet motor of similar size. It offers significant weight saving with an active mass of only 8kg, giving a continuous torque-to-weight ratio of 12Nm/kg. A typical figure for a standard direct drive permanent magnet machine with a similar diameter and cooling would be nearer 2Nm/kg.

Apart from its ability to dispense with gearboxes, its reliability and reduced servicing requirement, PDD does not incur the mass and volume penalties of a conventional direct drive motor, and remains a highly scalable system. It is an attractive technology for those considering implementing an energy-efficient, load matching variable speed drive installation, of which there are many examples, including fluid control valves; mixers and stirrers; grinders, crushers and mills; low speed drilling; material cutting and forming; extruders and injection moulding machines.

Profile: Magnomatics
A spin-out from the University of Sheffield, Magnomatics is currently developing technologies based on magnetic gearing, including passive magnetic gears with gear ratios of up to 1,000:1 as well as a magnetic continuously variable transmission. The magnetic gear technology can provide a non-contact, lubricant-free, geared transmission through a membrane removing the need for dynamic seals and preventing contamination, which is particularly attractive to food, chemical and pharmaceuticals manufacturers. The company is presently engaged on projects involving torque ratings ranging from a few Newton-metres up to MegaNewton-metres.

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